relation that may originate in a clustering process. Waiting times longer than a correlation time exhibited an exponential distribution characteristic of a Poisson process. Bandyopadhyay et al. (2020b) studied the association of SEP events with intermittent magnetic structures, finding a suggestion of positive correlation (see also §7.5.2). The association of intermittency with enhanced plasma heating (measured via ion temperature) was studied by Qudsi et al. (2020); their results support the notion that coherent non-homogeneous magnetic structures play a role in plasma heating mechanisms. These series of studies indicate that intermittent structures in the young solar wind observed by PSP have certain properties that are similar to those observed in near-Earth turbulence.

In addition to the statistical properties described in the previous paragraphs, some attention has also been given to the identification of structures associated with intermittency, such as magnetic flux tubes and ropes. Zhao et al. (2020b) used wavelet analysis to evaluate magnetic helicity, cross helicity, and residual energy in PSP observations between 22 Oct. 2018 and 21 Nov. 2018. Based on these parameters they identified 40 flux ropes with durations ranging from 10 to 300 minutes. Chen et al. (2020b) used a Grad-Shafranov approach to identify flux ropes during the first two PSP Encs., and compared this method with the Zhao et al. (2020b) approach, finding some consistency. Pecora et al. (2021a) developed a novel method for flux rope detection based on a real-space evaluation of magnetic helicity, and, focusing on the first PSP orbit, found some consistency with the previously mentioned approaches. A subsequent work applied this method to orbit 5, presenting evidence that flux tubes act as transport boundaries for energetic particles (Pecora et al. 2021b). The characteristics of so-called interplanetary discontinuities (IDs) observed during PSP's 4th and 5th orbits were studied by Liu et al. (2021), who found that the occurrence rate of IDs decreases from 200 events per day at 0.13 AU to 1 event per day at 0.9 AU, with a sharper decrease observed in RDs as compared with TDs. While the general decrease in occurrence rate may be attributed to radial wind expansion and discontinuity thickening, the authors infer that the sharper decrease in RDs implies a decay channel for these in the young solar wind.

We close this section by noting that the studies reviewed above employ a remarkable variety of intermittency diagnostics, including occurrence rate of structures, intensities of the associated gradients, and their fractal properties. The richness of the dataset that PSP is expected to accumulate over its lifetime will offer unprecedented opportunities to probe the relationships between these various diagnostics and their evolution in the inner heliosphere.

# 7.5 Interaction Between Turbulence and Other Processes

## 7.5.1 Turbulence Characteristics Within Switchbacks

The precise definition of magnetic field reversal (switchbacks) is still ambiguous in the heliophysics community, but the common picture of switchbacks is that they incarnate the S-shaped folding of the magnetic field. Switchbacks are found to be followed by Alvénic fluctuations (and often by strahl electrons) and they are associated with an increase of the solar wind bulk velocity as observed recently by PSP near 0.16 AU (Kasper et al. 2019; Bale et al. 2019; Dudok de Wit et al. 2020; McManus et al. 2020; Bourouaine et al. 2020; Wu et al. 2021; Whittlesey et al. 2020). Switchbacks have been previously observed in the fast solar-wind streams near 0.3 AU (Horbury et al. 2018) and near or beyond 1 AU (Balogh et al. 1999). The switchback intervals are found to carry turbulence, and the characteristics of that turbulence have been investigated in a number of works using PSP data. Dudok de Wit et al. (2020) studied the spectral properties of inertial range turbulence within intervals containing switchback structures in the first PSP S/C. In their analysis they introduced the normalized
relation that may originate in a clustering process. Waiting times longer than a correlation time exhibited an exponential distribution characteristic of a Poisson process. Bandyopadhyay et al. (2020b) studied the association of SEP events with intermittent magnetic structures, finding a suggestion of positive correlation (see also §7.5.2). The association of intermittency with enhanced plasma heating (measured via ion temperature) was studied by Qudsi et al. (2020); their results support the notion that coherent non-homogeneous magnetic structures play a role in plasma heating mechanisms. These series of studies indicate that intermittent structures in the young solar wind observed by PSP have certain properties that are similar to those observed in near-Earth turbulence.

In addition to the statistical properties described in the previous paragraphs, some attention has also been given to the identification of structures associated with intermittency, such as magnetic flux tubes and ropes. Zhao et al. (2020b) used wavelet analysis to evaluate magnetic helicity, cross helicity, and residual energy in PSP observations between 22 Oct. 2018 and 21 Nov. 2018. Based on these parameters they identified 40 flux ropes with durations ranging from 10 to 300 minutes. Chen et al. (2020b) used a Grad-Shafranov approach to identify flux ropes during the first two PSP Encs., and compared this method with the Zhao et al. (2020b) approach, finding some consistency. Pecora et al. (2021a) developed a novel method for flux rope detection based on a real-space evaluation of magnetic helicity, and, focusing on the first PSP orbit, found some consistency with the previously mentioned approaches. A subsequent work applied this method to orbit 5, presenting evidence that flux tubes act as transport boundaries for energetic particles (Pecora et al. 2021b). The characteristics of so-called interplanetary discontinuities (IDs) observed during PSP's 4th and 5th orbits were studied by Liu et al. (2021), who found that the occurrence rate of IDs decreases from 200 events per day at 0.13 AU to 1 event per day at 0.9 AU, with a sharper decrease observed in RDs as compared with TDs. While the general decrease in occurrence rate may be attributed to radial wind expansion and discontinuity thickening, the authors infer that the sharper decrease in RDs implies a decay channel for these in the young solar wind.

We close this section by noting that the studies reviewed above employ a remarkable variety of intermittency diagnostics, including occurrence rate of structures, intensities of the associated gradients, and their fractal properties. The richness of the dataset that PSP is expected to accumulate over its lifetime will offer unprecedented opportunities to probe the relationships between these various diagnostics and their evolution in the inner heliosphere.

# 7.5 Interaction Between Turbulence and Other Processes

## 7.5.1 Turbulence Characteristics Within Switchbacks

The precise definition of magnetic field reversal (switchbacks) is still ambiguous in the heliophysics community, but the common picture of switchbacks is that they incarnate the S-shaped folding of the magnetic field. Switchbacks are found to be followed by Alvénic fluctuations (and often by strahl electrons) and they are associated with an increase of the solar wind bulk velocity as observed recently by PSP near 0.16 AU (Kasper et al. 2019; Bale et al. 2019; Dudok de Wit et al. 2020; McManus et al. 2020; Bourouaine et al. 2020; Wu et al. 2021; Whittlesey et al. 2020). Switchbacks have been previously observed in the fast solar-wind streams near 0.3 AU (Horbury et al. 2018) and near or beyond 1 AU (Balogh et al. 1999). The switchback intervals are found to carry turbulence, and the characteristics of that turbulence have been investigated in a number of works using PSP data. Dudok de Wit et al. (2020) studied the spectral properties of inertial range turbulence within intervals containing switchback structures in the first PSP S/C. In their analysis they introduced the normalized
<figure><img src="image_25.png" /><figcaption>**Fig. 23** Power spectrum of radial magnetic field fluctuations for quiescent times ($z < 0.05$) and for the entire interval (all $z$), during a period near perihelion of Enc. I. The quiescent times show a lower overall amplitude, and a $1/f$ break at higher frequencies, suggestive of a less evolved turbulence. Figure adapted from Dudok de Wit et al. (2020)</figcaption></figure>

parameter $z = \frac{1}{2}(1 - \cos \alpha)$ (where $\alpha$ is the angle between the instantaneous magnetic field, **B**, and the prevalent field $\langle \mathbf{B} \rangle$) to determine the deflection of the field. Switchbacks were defined as a magnetic field deflection that exceeds the threshold value of $z = 0.05$. They estimated the power spectral density of the radial component $B_R$ of the magnetic field for quiescent ($z < 0.05$) and active (all $z$) regimes.

Figure 23 shows the results of both power spectra. They found that the properties in active conditions are typical for MHD turbulence, with an inertial range whose spectral index is close to -3/2 and preceded by 1/f range (consistent with the observation by Chen et al. (2020a). Also, the break frequency (at the lower frequency part) was found to be located around 0.001 Hz. For the quiescent power spectrum, the break frequency moves up to 0.05 Hz showing a difference from the active power spectrum although both power spectra have similar spectral index (about -3/2) between 0.05 Hz and 1 Hz. The authors suggest that in the quiescent regime the turbulent cascade has only had time to develop a short inertial range, showing signature of a more pristine solar wind.

McManus et al. (2020) have studied the turbulent quantities such as the normalized residual energy, $\sigma_r(s, t)$, and cross helicity, $\sigma_c(s, t)$, during one day of PSP first S/C as a function of wavelet scale $s$. Their study encompasses switchback field intervals. Overall, they found that the observed features of these turbulent quantities are similar to previous observations made by *Helios* in slow solar wind (Bruno et al. 2007), namely highly-correlated and Alfvénic fluctuations with ($\sigma_c \sim 0.9$ and $\sigma_r \sim -0.3$). However, a negative normalized cross helicity is found within switchback intervals, indicating that MHD fluctuations are following the local magnetic field inside switchbacks, i.e., the predominantly outward propagating Alfvénic fluctuations outside the switchback intervals become inward propagating during the field reversal. This signature is interpreted as that the field reversals are local kinks in the magnetic field and not due to small regions of opposite polarity of the field.

The turbulence characteristics associated with switchbacks have also been studied by Bourouaine et al. (2020) using 10 days of PSP data during the first S/C. The authors used a technique that is based on the conditioned correlation functions to investigate the correlation times and the power spectra of the field $q$ that represents the magnetic field **B**, the fluid velocity $V$ and the Elsasser fields $z^{\pm}$, inside and outside switchback intervals. In their study, the authors defined switchbacks as field reversals that are deflected by angles that are larger than 90° w.r.t. the Parker spiral (the prevalent magnetic field). This work confirms that the dominant Alfvénic fluctuations follow the field reversal. Moreover, the authors found that, in switchback intervals, the correlation time is about 2 minutes for all fields, but in non-switchback intervals, the correlation time of the sunward propagating Alfvénic fluctuations
<figure><img src="image_26.png" /><figcaption>**Fig. 23** Power spectrum of radial magnetic field fluctuations for quiescent times ($z < 0.05$) and for the entire interval (all $z$), during a period near perihelion of Enc. I. The quiescent times show a lower overall amplitude, and a $1/f$ break at higher frequencies, suggestive of a less evolved turbulence. Figure adapted from Dudok de Wit et al. (2020)</figcaption></figure>

parameter $z = \frac{1}{2}(1 - \cos \alpha)$ (where $\alpha$ is the angle between the instantaneous magnetic field, **B**, and the prevalent field $\langle \mathbf{B} \rangle$) to determine the deflection of the field. Switchbacks were defined as a magnetic field deflection that exceeds the threshold value of $z = 0.05$. They estimated the power spectral density of the radial component $B_R$ of the magnetic field for quiescent ($z < 0.05$) and active (all $z$) regimes.

Figure 23 shows the results of both power spectra. They found that the properties in active conditions are typical for MHD turbulence, with an inertial range whose spectral index is close to -3/2 and preceded by 1/f range (consistent with the observation by Chen et al. (2020a). Also, the break frequency (at the lower frequency part) was found to be located around 0.001 Hz. For the quiescent power spectrum, the break frequency moves up to 0.05 Hz showing a difference from the active power spectrum although both power spectra have similar spectral index (about -3/2) between 0.05 Hz and 1 Hz. The authors suggest that in the quiescent regime the turbulent cascade has only had time to develop a short inertial range, showing signature of a more pristine solar wind.

McManus et al. (2020) have studied the turbulent quantities such as the normalized residual energy, $\sigma_r(s, t)$, and cross helicity, $\sigma_c(s, t)$, during one day of PSP first S/C as a function of wavelet scale $s$. Their study encompasses switchback field intervals. Overall, they found that the observed features of these turbulent quantities are similar to previous observations made by *Helios* in slow solar wind (Bruno et al. 2007), namely highly-correlated and Alfvénic fluctuations with ($\sigma_c \sim 0.9$ and $\sigma_r \sim -0.3$). However, a negative normalized cross helicity is found within switchback intervals, indicating that MHD fluctuations are following the local magnetic field inside switchbacks, i.e., the predominantly outward propagating Alfvénic fluctuations outside the switchback intervals become inward propagating during the field reversal. This signature is interpreted as that the field reversals are local kinks in the magnetic field and not due to small regions of opposite polarity of the field.

The turbulence characteristics associated with switchbacks have also been studied by Bourouaine et al. (2020) using 10 days of PSP data during the first S/C. The authors used a technique that is based on the conditioned correlation functions to investigate the correlation times and the power spectra of the field $q$ that represents the magnetic field **B**, the fluid velocity $V$ and the Elsasser fields $z^{\pm}$, inside and outside switchback intervals. In their study, the authors defined switchbacks as field reversals that are deflected by angles that are larger than 90° w.r.t. the Parker spiral (the prevalent magnetic field). This work confirms that the dominant Alfvénic fluctuations follow the field reversal. Moreover, the authors found that, in switchback intervals, the correlation time is about 2 minutes for all fields, but in non-switchback intervals, the correlation time of the sunward propagating Alfvénic fluctuations
<figure><img src="image_26.png" /><figcaption>**Fig. 24** Power spectra of Elsasser variables (upper panels) and velocity/magnetic fluctuations (lower panels) for periods of switchbacks (SB; left panels) and periods not within switchbacks (NSB; right panels). Magnetic spectra are multiplied by a factor of 10. Power law fits are also indicated. There are notable differences in both the amplitudes and shape of the spectra between SB and NSB intervals. Figure adapted from Bourouaine et al. (2020)</figcaption></figure>

(the minor Elsasser field) is about 3 hr and longer than the ones of the other fields. This result seems to be consistent with previous 1 AU measurements (Chen et al. 2013; Bowen et al. 2018). Furthermore, the authors estimated the power spectra of the corresponding fields (Fig. 24), and found that the magnetic power spectrum in switchback intervals is steeper (with spectral index close to -5/3) than in switchback intervals, which have a spectral index close to -3/2. The analysis also shows that the turbulence is found to be less imbalanced with higher residual energy in switchback intervals.

Wu et al. (2021) has investigated the turbulent quantities such as the normalized cross helicity and the residual energy in switchbacks using the first four Encs. of PSP data. In their analysis they considered separate intervals of 100 s duration that satisfies the conditions of $B_R > 0°$ and $B_R > 160°$ for the switchbacks and non-switchbacks, respectively. Although, the analysis focuses on the time scale of 100 s, their findings seems to be consistent with the results of Bourouaine et al. (2020) (for that time scale), i.e., the switchback intervals and non-switchback intervals have distinct residual energy and similar normalized cross helicity suggesting that switchbacks have a different Alfvénic characteristics.

In another study, Martinović et al. (2021) have investigated the spectral index and the stochastic heating rate at the inertial range inside and outside switchback intervals and found a fair similar behavior in both intervals. However, at the kinetic range, the kinetic properties, such as the characteristic break scale (frequency that separates the inertial and the dissipation ranges) and the level of intermittency differ inside and outside switchback intervals. The authors found that inside the switchbacks the level of intermittency is higher, which might be a signature of magnetic field and velocity shears observed at the edges.

### 7.5.2 Impact of Turbulence and Switchbacks on Energetic Particles and CMEs

Strahl electrons are observed to follow the reversed field within the switchbacks, however, it is not yet understood whether higher energy energetic particles reverse at the switchbacks as well. Recently, Bandyopadhyay et al. (2021) examined the radial anisotropy of the energetic particles measured by the EPI-Lo (instrument of the IS○IS suite) in connection to magnetic switchbacks. The authors investigated switchback intervals with $|σ_c| > 0.5$ and $z ≤ 0.5$, respectively. The ratio $r = (F_{away} - F_{toward})/(F_{away} + F_{outward})$ has been used to determine the dominant flux direction of the energetic particles. Here “away” and “toward”
<figure><img src="image_27.png" /><figcaption>**Fig. 24** Power spectra of Elsasser variables (upper panels) and velocity/magnetic fluctuations (lower panels) for periods of switchbacks (SB; left panels) and periods not within switchbacks (NSB; right panels). Magnetic spectra are multiplied by a factor of 10. Power law fits are also indicated. There are notable differences in both the amplitudes and shape of the spectra between SB and NSB intervals. Figure adapted from Bourouaine et al. (2020)</figcaption></figure>

(the minor Elsasser field) is about 3 hr and longer than the ones of the other fields. This result seems to be consistent with previous 1 AU measurements (Chen et al. 2013; Bowen et al. 2018). Furthermore, the authors estimated the power spectra of the corresponding fields (Fig. 24), and found that the magnetic power spectrum in switchback intervals is steeper (with spectral index close to -5/3) than in switchback intervals, which have a spectral index close to -3/2. The analysis also shows that the turbulence is found to be less imbalanced with higher residual energy in switchback intervals.

Wu et al. (2021) has investigated the turbulent quantities such as the normalized cross helicity and the residual energy in switchbacks using the first four Encs. of PSP data. In their analysis they considered separate intervals of 100 s duration that satisfies the conditions of $B_R > 0°$ and $B_R > 160°$ for the switchbacks and non-switchbacks, respectively. Although, the analysis focuses on the time scale of 100 s, their findings seems to be consistent with the results of Bourouaine et al. (2020) (for that time scale), i.e., the switchback intervals and non-switchback intervals have distinct residual energy and similar normalized cross helicity suggesting that switchbacks have a different Alfvénic characteristics.

In another study, Martinović et al. (2021) have investigated the spectral index and the stochastic heating rate at the inertial range inside and outside switchback intervals and found a fair similar behavior in both intervals. However, at the kinetic range, the kinetic properties, such as the characteristic break scale (frequency that separates the inertial and the dissipation ranges) and the level of intermittency differ inside and outside switchback intervals. The authors found that inside the switchbacks the level of intermittency is higher, which might be a signature of magnetic field and velocity shears observed at the edges.

### 7.5.2 Impact of Turbulence and Switchbacks on Energetic Particles and CMEs

Strahl electrons are observed to follow the reversed field within the switchbacks, however, it is not yet understood whether higher energy energetic particles reverse at the switchbacks as well. Recently, Bandyopadhyay et al. (2021) examined the radial anisotropy of the energetic particles measured by the EPI-Lo (instrument of the IS○IS suite) in connection to magnetic switchbacks. The authors investigated switchback intervals with $|σ_c| > 0.5$ and $z ≤ 0.5$, respectively. The ratio $r = (F_{away} - F_{toward})/(F_{away} + F_{outward})$ has been used to determine the dominant flux direction of the energetic particles. Here “away” and “toward”
refer to the direction of the measured radial particle fluxes (F) in the selected energy range. Figure 3 of Bandyopadhyay et al. (2021) displays the scattering points that correspond to the measurements of the first five Encs. plotted as a function of the z parameter and the ratio r. The analysis shows that 80–200 keV energetic ions almost never reverse direction when the magnetic field polarity reverses in switchbacks. One of the reason is that particles with smaller gyroradii, such as strahl electrons, can reverse direction by following the magnetic field in switchbacks, but that larger gyroradii particles likely cannot. Therefore, from this analysis one can expect that particles with higher energies than those detectable by EPI-Lo will likely not get reversed in switchbacks.

Bandyopadhyay et al. (2020a) studied the connection between the enhanced of the population of energetic particles (measured using IS○IS) and the intermittent structures (using FIELDS/MAG) near the sun using PSP data. Intermittent structures are generated naturally by turbulence, and the PVI method was proposed previously to identify these structures (Greco et al. 2018) For single S/C measurements, this method relies on the evaluation of the temporal increment in the magnetic field such as $|\Delta B(\tau, t)| = |B(t + \tau) - B(t)|$, and thus the so-called the PVI(t) for a given τ index is defined as

$$PVI(t) = \sqrt{\frac{|\Delta(\tau, t)|^2}{|\langle\Delta(\tau, t)|^2|}} \quad (3)$$

where (...) denotes a time average computed along the time series. The analysis given in Bandyopadhyay et al. (2020a) examined the conditionally averaged energetic-particle count rates and its connection to the intermittent structures using the PVI method. The results from the first two PSP orbits seem to support the idea that SEPs are likely correlated with coherent magnetic structures. The outcomes from this analysis may suggest that energetic particles are concentrated near magnetic flux tube boundaries.

Magnetic field line topology and flux tube structure may influence energetic particles and their transport in other ways. A consequence of the tendency of particles to follow field lines is that when turbulence is present the particle paths can be influenced by field line random walk. While this idea is familiar in the context of perpendicular diffusive transport, a recent study of SEP path lengths observed by PSP (Chhiber et al. 2021b) suggested that random walking of field lines or flux tubes may account for apparently increased path of SEP path lengths. This is further discussed in §10.1.

The inertial-range turbulent properties such as the normalized cross helicity, $\sigma_c$, and residual energy, $\sigma_r$, have been examined in magnetic clouds (MCs) using PSP data by Good et al. (2020). MCs are considered to be large-scale transient twisted magnetic structures that propagate in the solar wind having features of low plasma β and low-amplitude magnetic field fluctuations. The analysis presented in Good et al. (2020) shows low $|\sigma_c|$ value in the cloud core while the cloud’s outer layers displays higher $|\sigma_c|$ and small residual energy. This study indicates that more balanced turbulence resides in the cloud core, and large-amplitude Alfvénic fluctuations characterize the cloud’s outer layers. These obtained properties suggest that low $|\sigma_c|$ is likely a common feature of magnetic clouds that have a typical closed field structures.

## 7.6 Implications for Large-Scale and Global Dynamics

As well as providing information about the fundamental nature of turbulence, and its interaction with the various structures in the solar wind, PSP has allowed us to study how turbulence contributes to the solar wind at the largest scales. Some of the main goals of PSP
refer to the direction of the measured radial particle fluxes (F) in the selected energy range. Figure 3 of Bandyopadhyay et al. (2021) displays the scattering points that correspond to the measurements of the first five Encs. plotted as a function of the z parameter and the ratio r. The analysis shows that 80–200 keV energetic ions almost never reverse direction when the magnetic field polarity reverses in switchbacks. One of the reason is that particles with smaller gyroradii, such as strahl electrons, can reverse direction by following the magnetic field in switchbacks, but that larger gyroradii particles likely cannot. Therefore, from this analysis one can expect that particles with higher energies than those detectable by EPI-Lo will likely not get reversed in switchbacks.

Bandyopadhyay et al. (2020a) studied the connection between the enhanced of the population of energetic particles (measured using IS○IS) and the intermittent structures (using FIELDS/MAG) near the sun using PSP data. Intermittent structures are generated naturally by turbulence, and the PVI method was proposed previously to identify these structures (Greco et al. 2018) For single S/C measurements, this method relies on the evaluation of the temporal increment in the magnetic field such as $|\Delta B(\tau, t)| = |B(t + \tau) - B(t)|$, and thus the so-called the PVI(t) for a given τ index is defined as

$$PVI(t) = \sqrt{\frac{|\Delta(\tau, t)|^2}{|\langle\Delta(\tau, t)|^2|}} \quad (3)$$

where (...) denotes a time average computed along the time series. The analysis given in Bandyopadhyay et al. (2020a) examined the conditionally averaged energetic-particle count rates and its connection to the intermittent structures using the PVI method. The results from the first two PSP orbits seem to support the idea that SEPs are likely correlated with coherent magnetic structures. The outcomes from this analysis may suggest that energetic particles are concentrated near magnetic flux tube boundaries.

Magnetic field line topology and flux tube structure may influence energetic particles and their transport in other ways. A consequence of the tendency of particles to follow field lines is that when turbulence is present the particle paths can be influenced by field line random walk. While this idea is familiar in the context of perpendicular diffusive transport, a recent study of SEP path lengths observed by PSP (Chhiber et al. 2021b) suggested that random walking of field lines or flux tubes may account for apparently increased path of SEP path lengths. This is further discussed in §10.1.

The inertial-range turbulent properties such as the normalized cross helicity, $\sigma_c$, and residual energy, $\sigma_r$, have been examined in magnetic clouds (MCs) using PSP data by Good et al. (2020). MCs are considered to be large-scale transient twisted magnetic structures that propagate in the solar wind having features of low plasma β and low-amplitude magnetic field fluctuations. The analysis presented in Good et al. (2020) shows low $|\sigma_c|$ value in the cloud core while the cloud’s outer layers displays higher $|\sigma_c|$ and small residual energy. This study indicates that more balanced turbulence resides in the cloud core, and large-amplitude Alfvénic fluctuations characterize the cloud’s outer layers. These obtained properties suggest that low $|\sigma_c|$ is likely a common feature of magnetic clouds that have a typical closed field structures.

## 7.6 Implications for Large-Scale and Global Dynamics

As well as providing information about the fundamental nature of turbulence, and its interaction with the various structures in the solar wind, PSP has allowed us to study how turbulence contributes to the solar wind at the largest scales. Some of the main goals of PSP
<figure><img src="image_27.png" /><figcaption>**Fig. 25** (a) Ratio of outward-propagating Alfvénic energy flux, $F_A$, to solar wind bulk kinetic energy flux, $F_k$, as a function of heliocentric distance, $r$. (b) The same ratio as a function of solar wind radial Alfvén Mach number, $M_A$. In both plots, the black solid line is a power law fit, the red/green dashed lines are solutions to the Chandran et al. (2011) model, the data points are colored by solar wind speed, $v$, and crosses mark times during connection to the coronal hole in Enc. 1. Figure adapted from Chen et al. (2020a)</figcaption></figure>

are to understand how the solar wind is accelerated to the high speeds observed and how it is heated to the high temperatures seen, both close to the Sun and further out (Fox et al. 2016). PSP's orbits getting increasingly closer to the Sun are allowing us to measure the radial trends of turbulence properties, and directly test models of solar wind heating and acceleration.

To test the basic physics of a turbulence driven wind, Chen et al. (2020a) compared PSP measurements from the first two orbits to the 1D model of Chandran et al. (2011). In particular, they calculated the ratio of energy flux in the Alfvénic turbulence to the bulk kinetic energy flux of the solar wind (Fig. 25). This ratio was found to increase towards the Sun, as expected, reaching about 10% at 0.17 AU. The radial variation of this ratio was also found to be consistent with the model, leading to the conclusion that the wind during these first orbits could be explained by a scenario in which the Sun drives AWs that reflect from the Alfvén speed gradient, driving a turbulence cascade that heats and accelerates the wind. Consistent with this picture, Chen et al. (2020a) also found that the inward Alfvénic fluctuation component grew at a rate consistent with the measured Alfvén speed gradient.

To see if such physics can explain the 3D structure of the solar wind, the PSP observations have also been compared to 3D turbulence-driven solar wind models. Bandyopadhyay et al. (2020a) calculated the turbulent heating rates from PSP using two methods: from the third-order laws for energy transfer through the MHD inertial range and from the von Kármán decay laws based on correlation scale quantities. These were both found to increase going closer to the Sun, taking values at 0.17 AU about 100 times higher than typical values at 1 AU. These were compared to those from the model of Usmanov et al. (2018), under two different inner boundary conditions – an untitled dipole and a magnetogram from the time of the S/C. The heating rates from both models were found to be in broad agreement with those determined from the PSP measurements, although the magnetogram version provided a slightly better fit overall. Chhiber et al. (2021c) later performed a comparison of the first five orbits to a similar 3D turbulence solar wind model (which captures the coupling
<figure><img src="image_28.png" /><figcaption>**Fig. 25 (a)** Ratio of outward-propagating Alfvénic energy flux, $F_A$, to solar wind bulk kinetic energy flux, $F_k$, as a function of heliocentric distance, $r$. **(b)** The same ratio as a function of solar wind radial Alfvén Mach number, $M_A$. In both plots, the black solid line is a power law fit, the red/green dashed lines are solutions to the Chandran et al. (2011) model, the data points are colored by solar wind speed, $v$, and crosses mark times during connection to the coronal hole in Enc. 1. Figure adapted from Chen et al. (2020a)</figcaption></figure>

are to understand how the solar wind is accelerated to the high speeds observed and how it is heated to the high temperatures seen, both close to the Sun and further out (Fox et al. 2016). PSP's orbits getting increasingly closer to the Sun are allowing us to measure the radial trends of turbulence properties, and directly test models of solar wind heating and acceleration.

To test the basic physics of a turbulence driven wind, Chen et al. (2020a) compared PSP measurements from the first two orbits to the 1D model of Chandran et al. (2011). In particular, they calculated the ratio of energy flux in the Alfvénic turbulence to the bulk kinetic energy flux of the solar wind (Fig. 25). This ratio was found to increase towards the Sun, as expected, reaching about 10% at 0.17 AU. The radial variation of this ratio was also found to be consistent with the model, leading to the conclusion that the wind during these first orbits could be explained by a scenario in which the Sun drives AWs that reflect from the Alfvén speed gradient, driving a turbulence cascade that heats and accelerates the wind. Consistent with this picture, Chen et al. (2020a) also found that the inward Alfvénic fluctuation component grew at a rate consistent with the measured Alfvén speed gradient.

To see if such physics can explain the 3D structure of the solar wind, the PSP observations have also been compared to 3D turbulence-driven solar wind models. Bandyopadhyay et al. (2020a) calculated the turbulent heating rates from PSP using two methods: from the third-order laws for energy transfer through the MHD inertial range and from the von Kármán decay laws based on correlation scale quantities. These were both found to increase going closer to the Sun, taking values at 0.17 AU about 100 times higher than typical values at 1 AU. These were compared to those from the model of Usmanov et al. (2018), under two different inner boundary conditions – an untitled dipole and a magnetogram from the time of the S/C. The heating rates from both models were found to be in broad agreement with those determined from the PSP measurements, although the magnetogram version provided a slightly better fit overall. Chhiber et al. (2021c) later performed a comparison of the first five orbits to a similar 3D turbulence solar wind model (which captures the coupling
<figure><img src="image_28.png" /><figcaption>**Fig. 26** Blue '+' symbols show PSP data from orbit 1, plotted at 1-hour cadence except $\lambda$, for which daily values are shown. Red curve shows results from the model, sampled along a synthetic PSP trajectory. Quantities shown are mean radial velocity of ions ($V_R$), mean radial magnetic field $B_R$, mean ion density $n_p$, mean ion temperature $T_p$, mean turbulence energy $Z^2$, correlation length of magnetic fluctuations $\lambda$, and normalized cross helicity $\sigma_c$. The shading in the top four panels marks an envelope obtained by adding and subtracting the local turbulence amplitude from the model to the mean value from the model (see the text for details). The vertical black line marks perihelion. The model uses ADAPT map with central meridian time 6 Nov. 2018, at 12:00 UT (Run I). Minor ticks on the time axis correspond to 1 day. Figure adapted from Chhiber et al. (2021c)</figcaption></figure>

between the solar wind flow and small-scale fluctuations), examining both mean-flow parameters such as density, temperature and wind speed, as well as turbulence properties such as fluctuation amplitudes, correlation lengths and cross-helicity. In general, the mean flow properties displayed better agreement with PSP observations than the turbulence parameters, indicating that aspects of the turbulent heating were possibly being captured, even if some details of the turbulence were not fully present in the model. A comparison between the model and observations for orbit 1 is shown in Fig. 26.

Adhikari et al. (2020b) first compared the PSP plasma observations to results from their turbulence transport model based on the nearly incompressible MHD description. They concluded that there was generally a good match for quantities such as fluctuating kinetic energy, correlation lengths, density fluctuations, and proton temperature. Later, Adhikari et al. (2020a) developed a model that couples equations for the large scale dynamics to the turbulence transport equations to produce a turbulence-driven solar wind. Again, they concluded
<figure><img src="image_29.png" /><figcaption>**Fig. 26** Blue '+' symbols show PSP data from orbit 1, plotted at 1-hour cadence except $\lambda$, for which daily values are shown. Red curve shows results from the model, sampled along a synthetic PSP trajectory. Quantities shown are mean radial velocity of ions ($V_R$), mean radial magnetic field $B_R$, mean ion density $n_p$, mean ion temperature $T_p$, mean turbulence energy $Z^2$, correlation length of magnetic fluctuations $\lambda$, and normalized cross helicity $\sigma_c$. The shading in the top four panels marks an envelope obtained by adding and subtracting the local turbulence amplitude from the model to the mean value from the model (see the text for details). The vertical black line marks perihelion. The model uses ADAPT map with central meridian time 6 Nov. 2018, at 12:00 UT (Run I). Minor ticks on the time axis correspond to 1 day. Figure adapted from Chhiber et al. (2021c)</figcaption></figure>

between the solar wind flow and small-scale fluctuations), examining both mean-flow parameters such as density, temperature and wind speed, as well as turbulence properties such as fluctuation amplitudes, correlation lengths and cross-helicity. In general, the mean flow properties displayed better agreement with PSP observations than the turbulence parameters, indicating that aspects of the turbulent heating were possibly being captured, even if some details of the turbulence were not fully present in the model. A comparison between the model and observations for orbit 1 is shown in Fig. 26.

Adhikari et al. (2020b) first compared the PSP plasma observations to results from their turbulence transport model based on the nearly incompressible MHD description. They concluded that there was generally a good match for quantities such as fluctuating kinetic energy, correlation lengths, density fluctuations, and proton temperature. Later, Adhikari et al. (2020a) developed a model that couples equations for the large scale dynamics to the turbulence transport equations to produce a turbulence-driven solar wind. Again, they concluded
a generally good agreement, and additionally found the heating rate of the quasi-2D compo-
nent of the turbulence to be dominant, and to be sufficient to provide the necessary heating
at the coronal base.

Overall, these studies indicate a picture that is consistent with turbulence, driven ulti-
mately by motions at the surface of the Sun, providing the energy necessary to heat the
corona and accelerate the solar wind in a way that matches the *in situ* measurements made
by *PSP*. Future work will involve adding even more realistic turbulence physics into these
models, and testing them under a wider variety of solar wind conditions. One recent study in
this direction is Chen et al. (2021a), which examined the turbulence energy fluxes as a func-
tion of distance to the HCS during Enc. 4. They found that the turbulence properties changed
when *PSP* was within 4° of the HCS, resembling more the standard slow solar wind seen at
1 AU, and suggesting this as the angular width of the streamer belt wind at these distances.
Also, within this streamer belt wind, the turbulence fluxes were significantly lower, being
on average 3 times smaller than required for consistency with the Chandran et al. (2011)
solar wind model. Chen et al. (2021a) concluded, therefore, that additional mechanisms not
in these models are required to explain the solar wind acceleration in the streamer belt wind
near the HCS.

The coming years, with both *PSP* moving even closer to the Sun and the solar cycle coming into its maximum, will provide even better opportunities to further understand the role that turbulence plays in the heating and acceleration of the different types of solar wind, and how this shapes the large-scale structure of our heliosphere.

**8 Large-Scale Structures in the Solar Wind**

During these four years of mission (within the ascending phase of the solar cycle), *PSP* crossed the HCS several times and also observed structures (both remotely and *in situ*) with similar features to the internal magnetic flux ropes (MFRs) associated with the interplanetary coronal mass ejections (ICMEs). This section focuses on *PSP* observations of large-scale structures, *i.e.*, the HCS crossing, ICMEs, and CMEs.

Smaller heliospheric flux ropes are also included in this section because of their similarity to larger ICME flux ropes. The comparison of the internal structure of large- and small-scale flux ropes (LFRs and SFRs, respectively) is revealing. They can both store and transport magnetic energy. Their properties at different heliodistances provide insights into the energy transport in the inner heliosphere. *PSP* brings a unique opportunity for understanding the role of the MFRs in the solar wind formation, evolution, and thus connecting scales in the heliosphere.

**8.1 The Heliospheric Current Sheet**

The HCS separates the two heliospheric magnetic hemispheres: one with a magnetic polar-
ity pointing away from the Sun and another toward the Sun. PSP crossed the HCS multiple
times in each orbit due to its low heliographic latitude orbit. Figure 27 shows a comprehen-
sive set of measurements with periods of HCS crossing identified as gray regions. These
crossings are particularly evident in the magnetic field azimuth angle (φ<sub>B</sub>) and the PAD of
suprathermal electrons. In the Radial-Tangential-Normal (RTN) coordinates used here, the
outward and inward polarities have a near-zero degree and 180° azimuth angle, respectively.
Since the electron heat flux always streams away from the Sun, the streaming direction is
a generally good agreement, and additionally found the heating rate of the quasi-2D compo-
nent of the turbulence to be dominant, and to be sufficient to provide the necessary heating
at the coronal base.

Overall, these studies indicate a picture that is consistent with turbulence, driven ulti-
mately by motions at the surface of the Sun, providing the energy necessary to heat the
corona and accelerate the solar wind in a way that matches the *in situ* measurements made
by *PSP*. Future work will involve adding even more realistic turbulence physics into these
models, and testing them under a wider variety of solar wind conditions. One recent study in
this direction is Chen et al. (2021a), which examined the turbulence energy fluxes as a func-
tion of distance to the HCS during Enc. 4. They found that the turbulence properties changed
when *PSP* was within 4° of the HCS, resembling more the standard slow solar wind seen at
1 AU, and suggesting this as the angular width of the streamer belt wind at these distances.
Also, within this streamer belt wind, the turbulence fluxes were significantly lower, being
on average 3 times smaller than required for consistency with the Chandran et al. (2011)
solar wind model. Chen et al. (2021a) concluded, therefore, that additional mechanisms not
in these models are required to explain the solar wind acceleration in the streamer belt wind
near the HCS.

The coming years, with both PSP moving even closer to the Sun and the solar cycle coming into its maximum, will provide even better opportunities to further understand the role that turbulence plays in the heating and acceleration of the different types of solar wind, and how this shapes the large-scale structure of our heliosphere.

**8 Large-Scale Structures in the Solar Wind**

During these four years of mission (within the ascending phase of the solar cycle), *PSP* crossed the HCS several times and also observed structures (both remotely and *in situ*) with similar features to the internal magnetic flux ropes (MFRs) associated with the interplanetary coronal mass ejections (ICMEs). This section focuses on *PSP* observations of large-scale structures, *i.e.*, the HCS crossing, ICMEs, and CMEs.

Smaller heliospheric flux ropes are also included in this section because of their similarity
to larger ICME flux ropes. The comparison of the internal structure of large- and small-scale
flux ropes (LFRs and SFRs, respectively) is revealing. They can both store and transport
magnetic energy. Their properties at different heliodistances provide insights into the energy
transport in the inner heliosphere. *PSP* brings a unique opportunity for understanding the
role of the MFRs in the solar wind formation, evolution, and thus connecting scales in the
heliosphere.

**8.1 The Heliospheric Current Sheet**

The HCS separates the two heliospheric magnetic hemispheres: one with a magnetic polar-
ity pointing away from the Sun and another toward the Sun. PSP crossed the HCS multiple
times in each orbit due to its low heliographic latitude orbit. Figure 27 shows a comprehen-
sive set of measurements with periods of HCS crossing identified as gray regions. These
crossings are particularly evident in the magnetic field azimuth angle (φ<sub>B</sub>) and the PAD of
suprathermal electrons. In the Radial-Tangential-Normal (RTN) coordinates used here, the
outward and inward polarities have a near-zero degree and 180° azimuth angle, respectively.
Since the electron heat flux always streams away from the Sun, the streaming direction is
<figure><img src="image_29.png" /><figcaption>**Fig. 27** SP solar wind measurements during solar Enc. 5, 10 May 2020 (day of the year [DOY] 130) to 1 Jun. 2020 (DOY 182). The panels from top to bottom are the PAD of 314 eV suprathermal electrons; the normalized PAD of the 314 eV suprathermal electrons; the magnetic field magnitude; the azimuth angle of the magnetic field in the TN plane; the elevation angle of the magnetic field; the solar wind proton number density; the RTN components of the solar wind proton bulk speed; the thermal speed of the solar wind protons; and the S/C radial distance from the Sun. The gray bars mark periods of HCS crossings</figcaption></figure>

parallel (antiparallel) to the magnetic field in the regions of the outward (inward) magnetic polarity, resulting in a magnetic pitch angle of 0° (180°).

Comparing the observed locations of HCS crossings with PFSS model predictions yielded good agreement (Szabo et al. 2020; Laker et al. 2021b). Lowering the source surface radius to $2 R_\odot$ or even below would minimize the timing disagreements, though this would increase the amount of open magnetic flux to unreasonable values. The likely resolution is that the appropriate source surface radius is not a constant value but varies depending on the solar surface structures below. Other sources of disagreement between the model predictions and observations are the emergence of ARs not included in the photospheric magnetic maps used by the PFSS models and the presence of solar wind transients (e.g., ICMEs). Laker et al. (2021b) also found that while the PFSS model predicted a relatively flat HCS, the observed current sheets had a much steeper orientation suggesting significant local corrugation. Szabo et al. (2020) also compared the observed HCS crossing times to global MHD model predictions with similar results.

The internal structure of the HCS near the Sun is very complex (Szabo et al. 2020; Lavraud et al. 2020; Phan et al. 2021). Szabo et al. (2020) has identified structures within the HCS region with magnetic field magnitude depressions, increased solar wind proton bulk speeds, and associated suprathermal electron strahl dropouts. These might evolve into
<figure><img src="image_30.png" /><figcaption>**Fig. 27** SP solar wind measurements during solar Enc. 5, 10 May 2020 (day of the year [DOY] 130) to 1 Jun. 2020 (DOY 182). The panels from top to bottom are the PAD of 314 eV suprathermal electrons; the normalized PAD of the 314 eV suprathermal electrons; the magnetic field magnitude; the azimuth angle of the magnetic field in the TN plane; the elevation angle of the magnetic field; the solar wind proton number density; the RTN components of the solar wind proton bulk speed; the thermal speed of the solar wind protons; and the S/C radial distance from the Sun. The gray bars mark periods of HCS crossings</figcaption></figure>

parallel (antiparallel) to the magnetic field in the regions of the outward (inward) magnetic polarity, resulting in a magnetic pitch angle of 0° (180°).

Comparing the observed locations of HCS crossings with PFSS model predictions yielded good agreement (Szabo et al. 2020; Laker et al. 2021b). Lowering the source surface radius to $2 R_\odot$ or even below would minimize the timing disagreements, though this would increase the amount of open magnetic flux to unreasonable values. The likely resolution is that the appropriate source surface radius is not a constant value but varies depending on the solar surface structures below. Other sources of disagreement between the model predictions and observations are the emergence of ARs not included in the photospheric magnetic maps used by the PFSS models and the presence of solar wind transients (e.g., ICMEs). Laker et al. (2021b) also found that while the PFSS model predicted a relatively flat HCS, the observed current sheets had a much steeper orientation suggesting significant local corrugation. Szabo et al. (2020) also compared the observed HCS crossing times to global MHD model predictions with similar results.

The internal structure of the HCS near the Sun is very complex (Szabo et al. 2020; Lavraud et al. 2020; Phan et al. 2021). Szabo et al. (2020) has identified structures within the HCS region with magnetic field magnitude depressions, increased solar wind proton bulk speeds, and associated suprathermal electron strahl dropouts. These might evolve into
the small density enhancements observed by Lavraud et al. (2020) likely showing magnetic disconnections. In addition, small flux ropes were also identified inside or just outside the HCS, often associated with plasma jets indicating recent magnetic reconnection (Szabo et al. 2020; Lavraud et al. 2020; Phan et al. 2021; Zhao et al. 2021). The near Sun HCS is much thicker than expected; thus, it is surprising that it is the site of frequent magnetic reconnection (Phan et al. 2021). Moreover, 1 AU observations of the HCS reveal significantly different magnetic and plasma signatures implying that the near-Sun HCS is the location of active evolution of the internal structures (Szabo et al. 2020).

The HCS also appears to organize the nearby, low-latitude solar wind. Chen et al. (2021a) observed lower amplitude turbulence, higher magnetic compressibility, steeper magnetic spectrum, lower Alfvénicity, and a 1/f break at much lower frequencies within 4° of the HCS compared to the rest of the solar wind, possibly implying a different solar source of the HCS environment.

## 8.2 Interplanetary Coronal Mass Ejections

The accurate identification and characterization of the physical processes associated with the evolution of the ICMEs require as many measurements of the magnetic field and plasma conditions as possible (see Cane and Richardson 2003; Zurbuchen and Richardson 2006, and references therein).

Our knowledge of the transition from CME to ICME has been limited to the *in situ* data collected at 1 AU and remote-sensing observations from space-based observatories. PSP provides a unique opportunity to link both views through valuable observations that will allow us to distinguish the evidence of the early transition from CME to ICME. Due to its highly elliptical orbit, PSP measures the plasma conditions of the solar wind at different heliospheric distances. Synergies with other space missions and ground observatories allow building a complete picture of the phenomena from the genesis at the Sun to the inner heliosphere.

In general, magnetic structures in the solar wind are MFRs, a subset of which are MCs (Burlaga 1988), and are characterized by enhanced magnetic fields where the field rotates slowly through a large angle. MCs are of great interest as their coherent magnetic field configuration and plasma properties drive space weather and are related to major geomagnetic storms. (Webb et al. 2000). Therefore, understanding their origin, evolution, propagation, and how they can interact with other transients traveling through space and planetary systems is of great interest. ICMEs are structures that travel throughout the heliosphere and transfer energy to the outer edge of the solar system and perhaps beyond.

**Event of 11-12 Nov. 2018: Enc. 1 – PSP at (0.25 AU, -178°)** During the first orbit, PSP collected *in situ* measurements of the thermal solar wind plasma as close as 35.6 $R_⊙$ from the Sun. In this new environment, PSP recorded the signatures of SBO-CMEs: the first on 31 Oct. 2018, at 03:36 UT as it entered the Enc. and the second on 11 Nov. 2018, at 23:50 UT as it exited the Enc.. The signature of the second SBO-CME crossing the S/C was a magnetic field enhancement (i.e., maximum strength 97 nT). The event was seen by STEREO-A but was not visible from L1 or Earth-orbiting S/C as the event was directed away from Earth. The signature and characteristics of this event were the focus of several studies, (see Korreck et al. 2020; Nieves-Chinchilla et al. 2020; Giacalone et al. 2020). SBO-CMEs (Cartwright and Moldwin 2008) are ICMEs that fulfill the following criteria in coronagraph data (1) slow speed ranging from 300 to 500 km s⁻¹; (2) no identifiable surface or low coronal signatures (in this case from Earth point of view); (3) characterized by a gradual swelling of the overlying streamer (blowout type); and (4) follows the tilt of HCS.
the small density enhancements observed by Lavraud et al. (2020) likely showing magnetic disconnections. In addition, small flux ropes were also identified inside or just outside the HCS, often associated with plasma jets indicating recent magnetic reconnection (Szabo et al. 2020; Lavraud et al. 2020; Phan et al. 2021; Zhao et al. 2021). The near Sun HCS is much thicker than expected; thus, it is surprising that it is the site of frequent magnetic reconnection (Phan et al. 2021). Moreover, 1 AU observations of the HCS reveal significantly different magnetic and plasma signatures implying that the near-Sun HCS is the location of active evolution of the internal structures (Szabo et al. 2020).

The HCS also appears to organize the nearby, low-latitude solar wind. Chen et al. (2021a) observed lower amplitude turbulence, higher magnetic compressibility, steeper magnetic spectrum, lower Alfvénicity, and a 1/f break at much lower frequencies within 4° of the HCS compared to the rest of the solar wind, possibly implying a different solar source of the HCS environment.

## 8.2 Interplanetary Coronal Mass Ejections

The accurate identification and characterization of the physical processes associated with the evolution of the ICMEs require as many measurements of the magnetic field and plasma conditions as possible (see Cane and Richardson 2003; Zurbuchen and Richardson 2006, and references therein).

Our knowledge of the transition from CME to ICME has been limited to the *in situ* data collected at 1 AU and remote-sensing observations from space-based observatories. PSP provides a unique opportunity to link both views through valuable observations that will allow us to distinguish the evidence of the early transition from CME to ICME. Due to its highly elliptical orbit, PSP measures the plasma conditions of the solar wind at different heliospheric distances. Synergies with other space missions and ground observatories allow building a complete picture of the phenomena from the genesis at the Sun to the inner heliosphere.

In general, magnetic structures in the solar wind are MFRs, a subset of which are MCs (Burlaga 1988), and are characterized by enhanced magnetic fields where the field rotates slowly through a large angle. MCs are of great interest as their coherent magnetic field configuration and plasma properties drive space weather and are related to major geomagnetic storms. (Webb et al. 2000). Therefore, understanding their origin, evolution, propagation, and how they can interact with other transients traveling through space and planetary systems is of great interest. ICMEs are structures that travel throughout the heliosphere and transfer energy to the outer edge of the solar system and perhaps beyond.

**Event of 11-12 Nov. 2018: Enc. 1 – PSP at (0.25 AU, -178°)** During the first orbit, PSP collected *in situ* measurements of the thermal solar wind plasma as close as 35.6 $R_⊙$ from the Sun. In this new environment, PSP recorded the signatures of SBO-CMEs: the first on 31 Oct. 2018, at 03:36 UT as it entered the Enc. and the second on 11 Nov. 2018, at 23:50 UT as it exited the Enc.. The signature of the second SBO-CME crossing the S/C was a magnetic field enhancement (i.e., maximum strength 97 nT). The event was seen by STEREO-A but was not visible from L1 or Earth-orbiting S/C as the event was directed away from Earth. The signature and characteristics of this event were the focus of several studies, (see Korreck et al. 2020; Nieves-Chinchilla et al. 2020; Giacalone et al. 2020). SBO-CMEs (Cartwright and Moldwin 2008) are ICMEs that fulfill the following criteria in coronagraph data (1) slow speed ranging from 300 to 500 km s⁻¹; (2) no identifiable surface or low coronal signatures (in this case from Earth point of view); (3) characterized by a gradual swelling of the overlying streamer (blowout type); and (4) follows the tilt of HCS.
The source location was determined using remote sensing and in situ observations, the WSA model (Arge and Pizzo 2000), and the Air Force Data Assimilative Photospheric Flux Transport (ADAPT) model (Arge et al. 2004). Hydrodynamical analytical and numerical simulations were also utilized to predict the CME arrival time to PSP. Using a CME propagation model, Korreck et al. (2020) and Nieves-Chinchilla et al. (2020) explored the characteristics of the CME using in situ data recorded closest to the Sun as well as the implications for CME propagation from the coronal source to PSP and space weather. The CME was traveling at an average speed of ~391 km s⁻¹ embedded in an ambient solar wind flow of ~395 km s⁻¹ and a magnetic field of 37 nT. The difference in speed with the ambient solar wind suggests that drag forces drive the SBO-CME.

The internal magnetic structure associated with the SBO displayed signatures of flux-rope but was characterized by changes that deviated from the expected smooth change in the magnetic field direction (flux rope-like configuration), low proton plasma beta, and a drop in the proton temperature. A detailed analysis of the internal magnetic properties suggested high complexity in deviations from an ideal flux rope 3D topology. Reconstructions of the magnetic field configuration revealed a highly distorted structure consistent with the highly elongated “bubble” observed remotely. A double-ring substructure observed in the FOV of COR2 coronagraph on the STEREO-A Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI; Howard et al. 2008) may also indicate a double internal flux rope. Another possible scenario is described as a mixed topology of a closed flux rope combined with the magnetically open structure, justified by the flux dropout observed in the measurements of the electron PAD. In any case, the plethora of structures observed by the EUV imager (SECCHI-EUVI) in the hours preceding the SBO evacuation indicated that the complexity might be related to the formation processes (Nieves-Chinchilla et al. 2020).

Applying a wavelet analysis technique to the *in situ* data from PSP, Zhao et al. (2020b) also identified the related flux rope. They inferred the reduced magnetic helicity, cross helicity, and residual energy. With the method, they also discussed that after crossing the ICME, both the plasma velocity and the magnetic field fluctuate rapidly and positively correlate with each other, indicating that Alfvénic fluctuations are generated in the region downstream of the ICME.

Finally, Giacalone et al. (2020) also discussed the SBO-CME as the driver of a weak shock when the ICME was at $7.4 R_⊙$ accelerating energetic particles. PSP/ISCOIS observed the SEP event (see Fig. 39). Five hours later, PSP/FIELDS and PSP/SWEAP detected the passage of the ICME (see §10 for a detailed discussion).

**Event of 15 Mar. 2019: Enc. 2 – PSP at (0.547 AU, 161°)** An SBO-CME was observed by STEREO-A and SOHO coronagraphs and measured in situ by PSP at 0.547 AU on 15 Mar. 2019 from 12:14 UT to 17:45 UT. The event was studied in detail by Lario et al. (2020). The ICME was preceded by two interplanetary shock waves, registered at 08:56:01 UT and 09:00:07 UT (see Fig. 40 in §10). This study’s authors proposed that the shocks were associated with the interaction between the SBO-CME and a HSS. The analysis of the shocks’ characteristics indicated that despite the weak strength, the successive structures caused the acceleration of energetic particles. This study aimed to demonstrate that although SBO-CMEs are usually slow close to the Sun, subsequent evolution in the interplanetary space might drive shocks that can accelerate particles in the inner heliosphere. The event is discussed in more detail in §10, showing that the time of arrival of energetic particles at PSP (Fig. 41) is consistent with the arrival of the ICME predicted by MHD simulations. With the simulations, Lario et al. (2020) determined when the magnetic connection was established between PSP and the shocks, potentially driven by the ICME.
The source location was determined using remote sensing and *in situ* observations, the WSA model (Arge and Pizzo 2000), and the Air Force Data Assimilative Photospheric Flux Transport (ADAPT) model (Arge et al. 2004). Hydrodynamical analytical and numerical simulations were also utilized to predict the CME arrival time to *PSP*. Using a CME propagation model, Korreck et al. (2020) and Nieves-Chinchilla et al. (2020) explored the characteristics of the CME using *in situ* data recorded closest to the Sun as well as the implications for CME propagation from the coronal source to *PSP* and space weather. The CME was traveling at an average speed of ~391 km s⁻¹ embedded in an ambient solar wind flow of ~395 km s⁻¹ and a magnetic field of 37 nT. The difference in speed with the ambient solar wind suggests that drag forces drive the SBO-CME.

The internal magnetic structure associated with the SBO displayed signatures of flux-rope but was characterized by changes that deviated from the expected smooth change in the magnetic field direction (flux rope-like configuration), low proton plasma beta, and a drop in the proton temperature. A detailed analysis of the internal magnetic properties suggested high complexity in deviations from an ideal flux rope 3D topology. Reconstructions of the magnetic field configuration revealed a highly distorted structure consistent with the highly elongated “bubble” observed remotely. A double-ring substructure observed in the FOV of COR2 coronagraph on the STEREO-A Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI; Howard et al. 2008) may also indicate a double internal flux rope. Another possible scenario is described as a mixed topology of a closed flux rope combined with the magnetically open structure, justified by the flux dropout observed in the measurements of the electron PAD. In any case, the plethora of structures observed by the EUV imager (SECCHI-EUVI) in the hours preceding the SBO evacuation indicated that the complexity might be related to the formation processes (Nieves-Chinchilla et al. 2020).

Applying a wavelet analysis technique to the *in situ* data from PSP, Zhao et al. (2020b) also identified the related flux rope. They inferred the reduced magnetic helicity, cross helicity, and residual energy. With the method, they also discussed that after crossing the ICME, both the plasma velocity and the magnetic field fluctuate rapidly and positively correlate with each other, indicating that Alfvénic fluctuations are generated in the region downstream of the ICME.

Finally, Giacalone et al. (2020) also discussed the SBO-CME as the driver of a weak shock when the ICME was at $7.4 R_⊙$ accelerating energetic particles. PSP/ISCOIS observed the SEP event (see Fig. 39). Five hours later, PSP/FIELDS and PSP/SWEAP detected the passage of the ICME (see §10 for a detailed discussion).

**Event of 15 Mar. 2019: Enc. 2 – PSP at (0.547 AU, 161°)** An SBO-CME was observed by STEREO-A and SOHO coronagraphs and measured *in situ* by PSP at 0.547 AU on 15 Mar. 2019 from 12:14 UT to 17:45 UT. The event was studied in detail by Lario et al. (2020). The ICME was preceded by two interplanetary shock waves, registered at 08:56:01 UT and 09:00:07 UT (see Fig. 40 in §10). This study’s authors proposed that the shocks were associated with the interaction between the SBO-CME and a HSS. The analysis of the shocks’ characteristics indicated that despite the weak strength, the successive structures caused the acceleration of energetic particles. This study aimed to demonstrate that although SBO-CMEs are usually slow close to the Sun, subsequent evolution in the interplanetary space might drive shocks that can accelerate particles in the inner heliosphere. The event is discussed in more detail in §10, showing that the time of arrival of energetic particles at PSP (Fig. 41) is consistent with the arrival of the ICME predicted by MHD simulations. With the simulations, Lario et al. (2020) determined when the magnetic connection was established between PSP and the shocks, potentially driven by the ICME.
**Event of 13 Oct. 2019: Enc. 3 – PSP at (0.81 AU, 75°)** The event observed during Enc. 3 was reported by Winslow et al. (2021). The ICME is associated with the stealth CME evacuation on 10 Oct. 2019, at 00:48 UT. It was characterized by an angular width of 19°, position angle of 82°, no signatures in SDO/AIA and EUVI-A images, and reaching a speed of 282 km s⁻¹ at 20 R⊙. At the time of the eruption, two coronal holes were identified from EUV images and extrapolations of the coronal magnetic field topology computed using the PFSS model (Wang and Sheeley 1992), suggesting that the stealth CME evolved between two HSSs originated at the coronal holes. The first HSS enabled the ICME to travel almost at a constant speed (minimum interaction time ~ 2.5 days), while the second overtook the ICME in the later stages of evolution.

The event was measured when PSP was not taking plasma measurements due to its proximity to aphelion, and there are only reliable magnetic field measurements by PSP/FIELDS instrument. Even with these observational limitations, this event is of particular interest as STEREO-A was located, 0.15 AU in the radial distance, < 1° in latitude and ~7.7° in longitude, away of PSP.

The ICME arrival is characterized by a fast-forward shock observed by PSP on 13 Oct. 2019, at 19:03 UT and by STEREO-A on 14 Oct. 2019, at 07:44 UT. Both S/C observed the same main features in the magnetic field components (exhibiting flux rope-like signatures) except for the HSS that STEREO-A observed as an increasing speed profile and shorter ICME duration. To show the similarity of the main magnetic field features and the effect of the ICME compression due to its interaction with the HSS, the magnetic field and plasma parameters of STEREO-A were plotted (shown in Fig. 28) and overlaid the PSP magnetic field measurements scaled by a factor of 1.235 and shifted to get the same ICME duration as observed by STEREO-A.

**Event of 20 Jan. 2020: Enc. 4 – PSP at (0.32 AU, 80°)** Joyce et al. (2021b) reported a CME event observed by PSP on 18 Jan. 2020, at 05:30 UT. The event was classified as a stealth CME since the eruption signatures were identified on the Sun's surface. Coronal SDO/AIA observations indicated the emission of a set of magnetic substructures or loops followed by the evacuation of the magnetic structure on 18 Jan. 2020, at 14:00 UT. The signatures of a few dispersed brightenings and dimmings observed in EUVI-A 195 Å were identified as the source region (Joyce et al. 2021b).

The ICME arrived at PSP on 20 Jan. 2020, at 19:00 UT, with a clear magnetic obstacle and rotation in the magnetic field direction but no sign of an IP shock wave. The event was also associated with a significant enhancement of SEPs. PSP and STEREO-A were almost aligned (separated by 5° in longitude). The ICME flew by both S/C, allowing for the examination of the evolution of the associated SEPs. Interestingly, this event established a scenario in which weaker structures can also accelerate SEPs. Thus, the presence of SEPs with the absence of the shock was interpreted as PSP crossing the magnetic structure's flank, although no dense feature was observed in coronagraph images propagating in that direction (Giacalone et al. 2002). In §10, the event is discussed in detail, including the discussion of the associated PSP observations of SEPs.

**Event of 25 Jun. 2020: Enc. 5 – PSP at (0.5 AU, 20°)** Palmerio et al. (2021), Kay et al. (2022), and Möstl et al. (2022) studied and modeled the event of 25 Jan. 2020, which occurred during Enc. 5. The lack of clear signatures on the solar surface and low corona led to the interpretation of this event as an SBO-CME and was the primary motivation for these studies.

The models were tested extensively to determine their capabilities to predict the coronal features and their counterparts in space. Palmerio et al. (2021) focused on predictions of
**Event of 13 Oct. 2019: Enc. 3 – PSP at (0.81 AU, 75°)** The event observed during Enc. 3 was reported by Winslow et al. (2021). The ICME is associated with the stealth CME evacuation on 10 Oct. 2019, at 00:48 UT. It was characterized by an angular width of 19°, position angle of 82°, no signatures in SDO/AIA and EUVI-A images, and reaching a speed of 282 km s⁻¹ at 20 R⊙. At the time of the eruption, two coronal holes were identified from EUV images and extrapolations of the coronal magnetic field topology computed using the PFSS model (Wang and Sheeley 1992), suggesting that the stealth CME evolved between two HSSs originated at the coronal holes. The first HSS enabled the ICME to travel almost at a constant speed (minimum interaction time ~ 2.5 days), while the second overtook the ICME in the later stages of evolution.

The event was measured when PSP was not taking plasma measurements due to its proximity to aphelion, and there are only reliable magnetic field measurements by PSP/FIELDS instrument. Even with these observational limitations, this event is of particular interest as STEREO-A was located, 0.15 AU in the radial distance, < 1° in latitude and ~7.7° in longitude, away of PSP.

The ICME arrival is characterized by a fast-forward shock observed by PSP on 13 Oct. 2019, at 19:03 UT and by STEREO-A on 14 Oct. 2019, at 07:44 UT. Both S/C observed the same main features in the magnetic field components (exhibiting flux rope-like signatures) except for the HSS that STEREO-A observed as an increasing speed profile and shorter ICME duration. To show the similarity of the main magnetic field features and the effect of the ICME compression due to its interaction with the HSS, the magnetic field and plasma parameters of STEREO-A were plotted (shown in Fig. 28) and overlaid the PSP magnetic field measurements scaled by a factor of 1.235 and shifted to get the same ICME duration as observed by STEREO-A.

**Event of 20 Jan. 2020: Enc. 4 – PSP at (0.32 AU, 80°)** Joyce et al. (2021b) reported a CME event observed by PSP on 18 Jan. 2020, at 05:30 UT. The event was classified as a stealth CME since the eruption signatures were identified on the Sun's surface. Coronal SDO/AIA observations indicated the emission of a set of magnetic substructures or loops followed by the evacuation of the magnetic structure on 18 Jan. 2020, at 14:00 UT. The signatures of a few dispersed brightenings and dimmings observed in EUVI-A 195 Å were identified as the source region (Joyce et al. 2021b).

The ICME arrived at PSP on 20 Jan. 2020, at 19:00 UT, with a clear magnetic obstacle and rotation in the magnetic field direction but no sign of an IP shock wave. The event was also associated with a significant enhancement of SEPs. PSP and STEREO-A were almost aligned (separated by 5° in longitude). The ICME flew by both S/C, allowing for the examination of the evolution of the associated SEPs. Interestingly, this event established a scenario in which weaker structures can also accelerate SEPs. Thus, the presence of SEPs with the absence of the shock was interpreted as PSP crossing the magnetic structure's flank, although no dense feature was observed in coronagraph images propagating in that direction (Giacalone et al. 2002). In §10, the event is discussed in detail, including the discussion of the associated PSP observations of SEPs.

**Event of 25 Jun. 2020: Enc. 5 – PSP at (0.5 AU, 20°)** Palmerio et al. (2021), Kay et al. (2022), and Möstl et al. (2022) studied and modeled the event of 25 Jan. 2020, which occurred during Enc. 5. The lack of clear signatures on the solar surface and low corona led to the interpretation of this event as an SBO-CME and was the primary motivation for these studies.

The models were tested extensively to determine their capabilities to predict the coronal features and their counterparts in space. Palmerio et al. (2021) focused on predictions of
<figure><img src="image_30.png" /><figcaption>**Fig. 28** Overlay of the *in situ* measurements by STEREO-A (black) and PSP (red). From top to bottom: the magnetic field strength and the radial ($B_R$), tangential ($B_T$) and normal ($B_N$) components, and for STEREO-A only: the proton density (N), temperature (T), and velocity (V). The PSP data are scaled (by a factor of 1.235) and time-shifted to obtain the same ICME duration delimit by the two dashed vertical red lines. The red vertical solid line marks the fast forward shock at PSP, while at STEREO-A with the black vertical solid line. Figure adapted from Winslow et al. (2021)</figcaption></figure>

the location of its source and the magnetic field configuration expected to be measured by PSP. The SDO/AIA and EUVI-A observations were used to determine the source location of the event. The increase in the solar activity around the source region was followed by a small eruption in the northern hemisphere on 21 Jun. 2020, at 02:00 UT (Fig. 29-left). This led to the outbreak of the SBO-CME on 23 Jun. 2020, at 00:54 UT. Using the PFSS model, the authors found that the SBO-CME was triggered by the interaction between the small eruption and the neighboring helmet streamer. The SBO-CME geometry and kinetic aspects were obtained by applying the graduated cylindrical shell (GCS; Thernisien 2011) model to the series of coronagraph images resulting in the estimation of an average speed of 200 km s⁻¹.

The magnetic field configuration from Sun to PSP was obtained by modeling the event using OSPREI suite (Kay et al. 2022). The arrival at PSP was also predicted to be on 25 Jun. 2020, at 15:50 UT (9 minutes before the actual arrival). PSP was located at 0.5 AU and 20° west of the Sun-Earth line.

Möstl et al. (2022, ; Fig. 29-right) studied the same event using multipoint measurements from SolO, BepiColombo (Benkhoff et al. 2021) (0.88 AU, -3.9°), PSP, Wind (1.007 AU, -0.2°), and STEREO-A (0.96 AU, -69.6°). The WSA/THUX (Reiss et al. 2020), HELCATS,
<figure><img src="image_31.png" /><figcaption>**Fig. 28** Overlay of the *in situ* measurements by STEREO-A (black) and PSP (red). From top to bottom: the magnetic field strength and the radial ($B_R$), tangential ($B_T$) and normal ($B_N$) components, and for STEREO-A only: the proton density (N), temperature (T), and velocity (V). The PSP data are scaled (by a factor of 1.235) and time-shifted to obtain the same ICME duration delimit by the two dashed vertical red lines. The red vertical solid line marks the fast forward shock at PSP, while at STEREO-A with the black vertical solid line. Figure adapted from Winslow et al. (2021)</figcaption></figure>

the location of its source and the magnetic field configuration expected to be measured by PSP. The SDO/AIA and EUVI-A observations were used to determine the source location of the event. The increase in the solar activity around the source region was followed by a small eruption in the northern hemisphere on 21 Jun. 2020, at 02:00 UT (Fig. 29-left). This led to the outbreak of the SBO-CME on 23 Jun. 2020, at 00:54 UT. Using the PFSS model, the authors found that the SBO-CME was triggered by the interaction between the small eruption and the neighboring helmet streamer. The SBO-CME geometry and kinetic aspects were obtained by applying the graduated cylindrical shell (GCS; Thernisien 2011) model to the series of coronagraph images resulting in the estimation of an average speed of 200 km s⁻¹.

The magnetic field configuration from Sun to PSP was obtained by modeling the event using OSPREI suite (Kay et al. 2022). The arrival at PSP was also predicted to be on 25 Jun. 2020, at 15:50 UT (9 minutes before the actual arrival). PSP was located at 0.5 AU and 20° west of the Sun-Earth line.

Möstl et al. (2022, ; Fig. 29-right) studied the same event using multipoint measurements from SolO, BepiColombo (Benkhoff et al. 2021) (0.88 AU, -3.9°), PSP, Wind (1.007 AU, -0.2°), and STEREO-A (0.96 AU, -69.6°). The WSA/THUX (Reiss et al. 2020), HELCATS,
<figure><img src="image_31.png" /><figcaption>**Fig. 29** Left: *PSP in situ* magnetic field and plasma measurements of the 21 Jun. 2020 stealth CME. From top to bottom: magnetic field strength and components ($B_R$, $B_T$ and $B_N$), $\theta_B$ and (d) $\phi_B$ magnetic field angles, wind speed ($V_P$), proton number density ($N_P$), and proton temperature ($T_P$). The flux rope interval is shaded in grey. Figure adapted from Palmerio et al. (2021). Right: *in situ* magnetic field data at PSP, BepiColombo and Wind. Solid vertical lines indicate ICME start times, and dashed lines the boundaries of the magnetic obstacle. Figure adapted from Möstl et al. (2022)</figcaption></figure>

and 3DCORE (Weiss et al. 2021) models were used to infer the background solar wind in the solar equatorial plane, the height time plots, and the flux rope, respectively. With the multi-S/C observation, the authors attempted to explain the differences in the *in situ* signatures observed at different locations of a single CME. To accomplish this goal, they modeled the evolution of a circular front shape propagating at a constant speed. The *in situ* arrival ICME speeds at PSP and Wind were 290 km s⁻¹ and 326 km s⁻¹, respectively. The arrival speed at STEREO-A was computed using SSEF30 model described by Davies et al. (2012). The discrepancies between the observed and predicted arrival times ranged from −11 to +18 hrs. The authors attributed this to a strong ICME deformation.

**Event of 29 Nov. 2020: Enc. 6 – PSP at (0.81 AU, 104°)** The CME event of 2020 Nov. 29 has been widely studied and identified as the largest widespread SEP event of solar cycle 25 and the direct first PSP observation of the interaction of two successive ICMEs (Cohen et al. 2021b; Mitchell et al. 2021; Kollhoff et al. 2021; Lario et al. 2021; Mason et al. 2021a; Möstl et al. 2022; Nieves-Chinchilla et al. 2022).

During this event, PSP, SolO, and STEREO-A were located at respective radial distances of 0.81 AU, 0.87 AU, and ~1 AU. As seen from the Earth, they were at longitudinal angular positions of 104° east, 115° west, 65° east, respectively. The remote sensing observations show that at least four successive CMEs were observed during 24-29 Nov. 2020, although only two were directed toward PSP.

During the SEP event, the particles spread over more than 230° in longitude close to 1 AU. Kollhoff et al. (2021) compared the timing when the EUV wave intersects the estimated magnetic foot-points from different S/C with the particle release times from time shift and velocity dispersion analyses. They found that there was no EUV wave related to
<figure><img src="image_32.png" /><figcaption>**Fig. 29** Left: *PSP in situ* magnetic field and plasma measurements of the 21 Jun. 2020 stealth CME. From top to bottom: magnetic field strength and components ($B_R$, $B_T$ and $B_N$), $\theta_B$ and (d) $\phi_B$ magnetic field angles, wind speed ($V_P$), proton number density ($N_P$), and proton temperature ($T_P$). The flux rope interval is shaded in grey. Figure adapted from Palmerio et al. (2021). Right: *in situ* magnetic field data at PSP, BepiColombo and Wind. Solid vertical lines indicate ICME start times, and dashed lines the boundaries of the magnetic obstacle. Figure adapted from Möstl et al. (2022)</figcaption></figure>

and 3DCORE (Weiss et al. 2021) models were used to infer the background solar wind in the solar equatorial plane, the height time plots, and the flux rope, respectively. With the multi-S/C observation, the authors attempted to explain the differences in the *in situ* signatures observed at different locations of a single CME. To accomplish this goal, they modeled the evolution of a circular front shape propagating at a constant speed. The *in situ* arrival ICME speeds at PSP and Wind were 290 km s⁻¹ and 326 km s⁻¹, respectively. The arrival speed at STEREO-A was computed using SSEF30 model described by Davies et al. (2012). The discrepancies between the observed and predicted arrival times ranged from −11 to +18 hrs. The authors attributed this to a strong ICME deformation.

**Event of 29 Nov. 2020: Enc. 6 – PSP at (0.81 AU, 104°)** The CME event of 2020 Nov. 29 has been widely studied and identified as the largest widespread SEP event of solar cycle 25 and the direct first PSP observation of the interaction of two successive ICMEs (Cohen et al. 2021b; Mitchell et al. 2021; Kollhoff et al. 2021; Lario et al. 2021; Mason et al. 2021a; Möstl et al. 2022; Nieves-Chinchilla et al. 2022).

During this event, PSP, SolO, and STEREO-A were located at respective radial distances of 0.81 AU, 0.87 AU, and ~1 AU. As seen from the Earth, they were at longitudinal angular positions of 104° east, 115° west, 65° east, respectively. The remote sensing observations show that at least four successive CMEs were observed during 24-29 Nov. 2020, although only two were directed toward PSP.

During the SEP event, the particles spread over more than 230° in longitude close to 1 AU. Kollhoff et al. (2021) compared the timing when the EUV wave intersects the estimated magnetic foot-points from different S/C with the particle release times from time shift and velocity dispersion analyses. They found that there was no EUV wave related to
the event. The PAD and first-order anisotropies studies at SolO, STEREO-A, and Wind S/C
suggest that diffusive propagation processes were involved.

Nieves-Chinchilla et al. (2022) analyzed multi-S/C observations and included different
models and techniques focusing on creating the heliospheric scenario of the CMEs’ evo-
lution and propagation and the impact on their internal structure at PSP. The observations
of PSP, STEREO-A, and Wind of type II and III radio burst emissions indicate a significant
left-handed polarization, which has never been detected in that frequency range. The authors
identified the period when the interaction/collision between the CMEs took place using the
results of reconstructing the event back at the Sun and simulating the event on the WSA-
ENLIL+Cone and DBM models. They concluded that both ICMEs interacted elastically
while flying by PSP. The impact of such interaction on the internal magnetic structure of
the ICMEs was also considered. Both ICMEs were fully characterized and 3D-reconstructed
with the GCS, elliptical cylindrical (EC), and circular cylindrical (CC) models. The aging
and expansion effects were implemented to evaluate the consequences of the interaction on
the internal structure.

Mitchell et al. (2021) investigated key characteristics of the SEP event, such as the time profile and anisotropy distribution of near-relativistic electrons measured by IS◎IS/EPI-Lo. They observed the brief PAD with a peak between 40° and 90° supporting the idea of a shock-drift acceleration, noting that the electron count rate peaks at the time of the shock driven by the faster of the two ICMEs. They concluded that the ICME shock caused the acceleration of electrons and also discussed that the ICMEs show significant electron anisotropy indicating the ICME’s topology and connectivity to the Sun.

Lario et al. (2021) studied two characteristics of the shock and their impact on the SEP event intensity: (1) the influence of unrelated solar wind structures, and (2) the role of the sheath region behind the shock. The authors found that on arrival at PSP, the SEP event was preceded by an intervening ICME that modified the low energy ion intensity-time profile and energy spectra. The low-energy (≈220 keV) protons accelerated by the shock were excluded from the first ICME, resulting in the observation of inverted energy spectra during its passage.

Cohen et al. (2021b) analyzed the ion spectra during both the decay of the event (where the data are the most complete for H and He) and integrated over the entire event (for O and Fe). They found that the spectra follow a power law multiplied by an exponential with roll-over energies that decrease with the species' increasing rigidities. These signatures are typically found in SEP events where the dominant source is a CME-driven shock, supported by the He/H and Fe/O composition ratios. They also identified signatures in the electron spectrum that may suggest the presence of a population trapped between the ICMEs and pointed out the possibility of having the ICMEs interacting at the time of observation by noting a local ion population with energies up to ~ 1 MeV. The SEP intensities dropped significantly during the passage of the MFR and returned to high values once PSP crossed out of the magnetic structure. Mason et al. (2021a) compared detailed measurements of heavy ion intensities, time dependence, fluences, and spectral slopes with 41 events surveyed by Cohen et al. (2017) from previous solar cycles. They concluded that an interplanetary shock passage could explain the observed signatures. The observed Fe/O ratios dropped sharply above 1 MeV nucleon⁻¹ to values much lower than the averaged SEP survey. They were a few MeV nucleon⁻¹ and ³He/⁴He < 0.03% at ACE and < 1% at SolO. For further details on this SEP event, see the discussed in §10.1.1.

The second ICME hitting PSP was also analyzed by Möstl et al. (2022). The authors com-
bined coronagraph images from SOHO and STEREO and applied the GCS model to obtain
the ICME geometric and kinematic parameters, computing an average speed of 1637 km s⁻¹
the event. The PAD and first-order anisotropies studies at SolO, STEREO-A, and Wind S/C suggest that diffusive propagation processes were involved.

Nieves-Chinchilla et al. (2022) analyzed multi-S/C observations and included different models and techniques focusing on creating the heliospheric scenario of the CMEs' evolution and propagation and the impact on their internal structure at PSP. The observations of PSP, STEREO-A, and Wind of type II and III radio burst emissions indicate a significant left-handed polarization, which has never been detected in that frequency range. The authors identified the period when the interaction/collision between the CMEs took place using the results of reconstructing the event back at the Sun and simulating the event on the WSA-ENLIL+Cone and DBM models. They concluded that both ICMEs interacted elastically while flying by PSP. The impact of such interaction on the internal magnetic structure of the ICMEs was also considered. Both ICMEs were fully characterized and 3D-reconstructed with the GCS, elliptical cylindrical (EC), and circular cylindrical (CC) models. The aging and expansion effects were implemented to evaluate the consequences of the interaction on the internal structure.

Mitchell et al. (2021) investigated key characteristics of the SEP event, such as the time profile and anisotropy distribution of near-relativistic electrons measured by IS◎IS/EPI-Lo. They observed the brief PAD with a peak between 40° and 90° supporting the idea of a shock-drift acceleration, noting that the electron count rate peaks at the time of the shock driven by the faster of the two ICMEs. They concluded that the ICME shock caused the acceleration of electrons and also discussed that the ICMEs show significant electron anisotropy indicating the ICME's topology and connectivity to the Sun.

Lario et al. (2021) studied two characteristics of the shock and their impact on the SEP event intensity: (1) the influence of unrelated solar wind structures, and (2) the role of the sheath region behind the shock. The authors found that on arrival at PSP, the SEP event was preceded by an intervening ICME that modified the low energy ion intensity-time profile and energy spectra. The low-energy ($\lesssim$220 keV) protons accelerated by the shock were excluded from the first ICME, resulting in the observation of inverted energy spectra during its passage.

Cohen et al. (2021b) analyzed the ion spectra during both the decay of the event (where the data are the most complete for H and He) and integrated over the entire event (for O and Fe). They found that the spectra follow a power law multiplied by an exponential with roll-over energies that decrease with the species' increasing rigidities. These signatures are typically found in SEP events where the dominant source is a CME-driven shock, supported by the He/H and Fe/O composition ratios. They also identified signatures in the electron spectrum that may suggest the presence of a population trapped between the ICMEs and pointed out the possibility of having the ICMEs interacting at the time of observation by noting a local ion population with energies up to ~1 MeV. The SEP intensities dropped significantly during the passage of the MFR and returned to high values once PSP crossed out of the magnetic structure. Mason et al. (2021a) compared detailed measurements of heavy ion intensities, time dependence, fluences, and spectral slopes with 41 events surveyed by Cohen et al. (2017) from previous solar cycles. They concluded that an interplanetary shock passage could explain the observed signatures. The observed Fe/O ratios dropped sharply above 1 MeV nucleon⁻¹ to values much lower than the averaged SEP survey. They were a few MeV nucleon⁻¹ and ³He/⁴He < 0.03% at ACE and < 1% at SolO. For further details on this SEP event, see the discussed in §10.1.1.

The second ICME hitting PSP was also analyzed by Möstl et al. (2022). The authors combined coronagraph images from SOHO and STEREO and applied the GCS model to obtain the ICME geometric and kinematic parameters, computing an average speed of 1637 km s⁻¹
at a heliodistance ranging from 6 to 14 $R_⊙$. The ICME arrived at PSP (0.80 AU and –96.8°) on 1 Dec. 2020, at 02:22 UT and at STEREO-A (0.95 AU and –57.6°) on 1 Dec. 2020, at 07:28 UT. They also considered this event an excellent example of the background wind's influence on the possible deformation and evolution of a fast CME and the longitudinal extension of a high-inclination flux rope.

## 8.3 Magnetic Flux Ropes

The *in situ* solar wind measurements show coherent and clear rotations of the magnetic field components at different time scales. These magnetic structures are well known as MFRs. According to their durations and sizes, MFRs are categorized as LFRs (few hours to few days; Janvier et al. 2014) and SFRs (tens of minutes to a few hours; Moldwin et al. 2000).

At 1 AU, it has been found that 30% to 60% of the large-scale MFRs are related to CMEs (Gosling 1990; Richardson and Cane 2010). This subset of MFRs is known as MCs (Burlaga 1988). On the other hand, the SFRs' origin is not well understood. Several studies proposed that SFRs are produced in the near vicinity of the Sun, while others can consider turbulence as a potential SFR source (i.e., Pecora et al. 2019) or else that SFRs are related and originate from SBO-CMEs. It is worth noticing that observations suggest that SBO-CMEs last a few hours, a time scale that falls in the SFR category.

To identify SFRs, Zhao et al. (2020b) analyzed the magnetic field and plasma data from the PSP's first orbit from 22 Oct. to 21 Nov. 2018. They identified 40 SFRs by following the method described by Telloni et al. (2012). They applied a Morlet analysis technique to estimate an SFR duration ranging from 8 to 300 minutes. This statistical analysis suggests that the SFRs are primarily found in the slow solar wind, and their possible source is MHD turbulence. For the third and fourth orbits, they identified a total of 21 and 34 SFRs, respectively (Zhao et al. 2021), including their relation to the streamer belt and HCS crossing.

Alternatively, Chen et al. (2020b) identified 44 SFRs by implementing an automated detection method based on the Grad-Shafranov reconstruction technique (Hu and Sonnerup 2001, 2002) over the *in situ* measurements in a 28-second cadence. They looked for the double-folding pattern in the relation between the transverse pressure and the magnetic vector potential axial component and removed highly Alfvénic structures with a threshold condition over a Walén test. The SFRs were identified during the first two PSP Encs. over the periods 31 Oct. – Dec. 19 2018 (~0.26 – 0.81 AU), and 7 Mar. – 15 May 2019 (~0.66 – 0.78 AU) with durations ranging from 5.6 to 276.3 min. They found that the monthly counts at PSP (27 per month) are notably lower than the average monthly counts at Wind (294 at 1 AU). The authors also noticed that some of the detected SFRs are related to magnetic reconnection processes (two reported by Phan et al. 2020) and HCS (three reported by Szabo et al. 2020). They argue that the SFR occurrence rate (being far less than at 1 AU) and a power-law tendency of the size-scales point towards an SFRs origin from MHD turbulence but note that the number of events analyzed is not sufficient to yield a statistically significant analysis result. 12 SFRs were also identified with the method proposed by Zhao et al. (2020b) with similar duration and two cases with opposite helicity.

## 8.4 Remote Sensing

### 8.4.1 Introduction

PSP/WISPR is a suite of two white light telescopes akin to the heliospheric imagers (HI-1 and HI-2; Eyles et al. 2009) of STEREO/SECCHI (Kaiser et al. 2008). The WISPR telescopes look off to the ram side of the S/C (i.e., in the direction of motion of PSP in its
at a heliodistance ranging from 6 to 14 $R_⊙$. The ICME arrived at PSP (0.80 AU and -96.8°) on 1 Dec. 2020, at 02:22 UT and at STEREO-A (0.95 AU and -57.6°) on 1 Dec. 2020, at 07:28 UT. They also considered this event an excellent example of the background wind's influence on the possible deformation and evolution of a fast CME and the longitudinal extension of a high-inclination flux rope.

## 8.3 Magnetic Flux Ropes

The *in situ* solar wind measurements show coherent and clear rotations of the magnetic field components at different time scales. These magnetic structures are well known as MFRs. According to their durations and sizes, MFRs are categorized as LFRs (few hours to few days; Janvier et al. 2014) and SFRs (tens of minutes to a few hours; Moldwin et al. 2000).

At 1 AU, it has been found that 30% to 60% of the large-scale MFRs are related to CMEs (Gosling 1990; Richardson and Cane 2010). This subset of MFRs is known as MCs (Burlaga 1988). On the other hand, the SFRs' origin is not well understood. Several studies proposed that SFRs are produced in the near vicinity of the Sun, while others can consider turbulence as a potential SFR source (i.e., Pecora et al. 2019) or else that SFRs are related and originate from SBO-CMEs. It is worth noticing that observations suggest that SBO-CMEs last a few hours, a time scale that falls in the SFR category.

To identify SFRs, Zhao et al. (2020b) analyzed the magnetic field and plasma data from the PSP's first orbit from 22 Oct. to 21 Nov. 2018. They identified 40 SFRs by following the method described by Telloni et al. (2012). They applied a Morlet analysis technique to estimate an SFR duration ranging from 8 to 300 minutes. This statistical analysis suggests that the SFRs are primarily found in the slow solar wind, and their possible source is MHD turbulence. For the third and fourth orbits, they identified a total of 21 and 34 SFRs, respectively (Zhao et al. 2021), including their relation to the streamer belt and HCS crossing.

Alternatively, Chen et al. (2020b) identified 44 SFRs by implementing an automated detection method based on the Grad-Shafranov reconstruction technique (Hu and Sonnerup 2001, 2002) over the *in situ* measurements in a 28-second cadence. They looked for the double-folding pattern in the relation between the transverse pressure and the magnetic vector potential axial component and removed highly Alfvénic structures with a threshold condition over a Walén test. The SFRs were identified during the first two PSP Encs. over the periods 31 Oct. – Dec. 19 2018 (~0.26 – 0.81 AU), and 7 Mar. – 15 May 2019 (~0.66 – 0.78 AU) with durations ranging from 5.6 to 276.3 min. They found that the monthly counts at PSP (27 per month) are notably lower than the average monthly counts at Wind (294 at 1 AU). The authors also noticed that some of the detected SFRs are related to magnetic reconnection processes (two reported by Phan et al. 2020) and HCS (three reported by Szabo et al. 2020). They argue that the SFR occurrence rate (being far less than at 1 AU) and a power-law tendency of the size-scales point towards an SFRs origin from MHD turbulence but note that the number of events analyzed is not sufficient to yield a statistically significant analysis result. 12 SFRs were also identified with the method proposed by Zhao et al. (2020b) with similar duration and two cases with opposite helicity.

## 8.4 Remote Sensing

### 8.4.1 Introduction

PSP/WISPR is a suite of two white light telescopes akin to the heliospheric imagers (HI-1 and HI-2; Eyles et al. 2009) of STEREO/SECCHI (Kaiser et al. 2008). The WISPR telescopes look off to the ram side of the S/C (i.e., in the direction of motion of PSP in its
counter-clockwise orbit about the Sun). When PSP is in its nominal attitude (i.e., Sun-pointed and “unrolled”), their combined FOV covers the interplanetary medium on the West side of the Sun, starting at a radial elongation of about 13.5° from the Sun and extending up to about 108°. The FOV of WISPR-i extends up to 53.5°, while the FOV of WISPR-o starts at 50° elongation, both telescopes covering about 40° in latitude. Since the angular size of the Sun increases as PSP gets closer to the Sun, the radial offset from Sun center represents different distances in units of solar radii. For example, on 24 Dec. 2024, at the closest approach of PSP of 0.046 AU (9.86 $R_⊙$), the offset of 13.5° will correspond to $\sim 2.3 R_⊙$.

## 8.4.2 Streamer Imaging with WISPR

Howard et al. (2019) reported on the first imaging of the solar corona taken by WISPR during PSP's first two solar Encs. (0.16 – 0.25 AU). The imaging revealed that both the large and small scale topology of streamers can be resolved by WISPR and that the temporal variability of the streamers can be clearly isolated from spatial effects when PSP is corotating with the Sun Poirier et al. (2020), by exploiting synoptic maps based on sequential WISPR images, revealed the presence of multiple substructures (individual rays) inside streamers and pseudostreamers. This substructure of the streamers was noted in other studies (Thernisien and Howard 2006; Morgan and Cook 2020). Noteworthy in the WISPR synoptic maps was the identification of a bright and narrow set of streamer rays located at the core of the streamer belt (Poirier et al. 2020; Hess et al. 2020). The thickness of this bright region matches the thickness of the heliospheric plasma sheet (HPS) measured in the solar wind (up to 300 Mm) at times of sector boundary crossings (Winterhalter et al. 1994). Thus, WISPR may offer the first clear-cut connection between coronal imaging of streamers and the *in situ* measurements of the rather narrow HPS.

Global PFSS and MHD models of the solar corona during the PSP Encs. generally agree with the large-scale structure inferred from remote sensing observations (e.g., Riley et al. 2019; Poirier et al. 2020). As noted above, they have been used to interpret streamer substructure (Poirier et al. 2020) observed in WISPR observations, as well as during eclipses (Mikić et al. 2018). Equally importantly, they have been used to connect remote solar observations with their *in situ* counterparts (§8.1). Comparisons with white-light, but more importantly from emission images, provides crucial constraints for models that include realistic energy transport processes (van der Holst et al. 2019; Riley et al. 2019). They have already led to the improvement of coronal heating models (Riley et al. 2021), resulting in better matches with *in situ* measurements during multiple PSP Encs.

Images taken from a vantage point situated much closer to the Sun provide more detailed information on the population of transient structures released continually by helmet streamers. The fine-scale structure of streamer blobs is better resolved by WISPR than previous generations of heliospheric imagers. In addition the WISPR images have revealed that small-scale transients, with aspects that are reminiscent of magnetic islands and/or twisted 3D magnetic fields, are emitted at scales smaller than those of streamer blobs (Howard et al. 2019). These very small flux ropes were identified *in situ* as common structures during crossings of the HPS (Sanchez-Diaz et al. 2019) and more recently at PSP (Lavraud et al. 2020). They may also relate to the quasi-periodic structures detected by Viall and Vourlidas (2015) – on-going research is evaluating this hypothesis. Recent MHD simulations have shown that the flux ropes observed in blobs and the magnetic islands between quasi-periodic increases in density could result from a single process known as the tearing-mode instability as the HCS is stretched by the adjacent out-flowing solar wind (Réville et al. 2020b).
counter-clockwise orbit about the Sun). When PSP is in its nominal attitude (i.e., Sun-pointed and “unrolled”), their combined FOV covers the interplanetary medium on the West side of the Sun, starting at a radial elongation of about 13.5° from the Sun and extending up to about 108°. The FOV of WISPR-i extends up to 53.5°, while the FOV of WISPR-o starts at 50° elongation, both telescopes covering about 40° in latitude. Since the angular size of the Sun increases as PSP gets closer to the Sun, the radial offset from Sun center represents different distances in units of solar radii. For example, on 24 Dec. 2024, at the closest approach of PSP of 0.046 AU (9.86 $R_⊙$), the offset of 13.5° will correspond to $\sim 2.3 R_⊙$.

## 8.4.2 Streamer Imaging with WISPR

Howard et al. (2019) reported on the first imaging of the solar corona taken by WISPR during PSP’s first two solar Encs. (0.16 – 0.25 AU). The imaging revealed that both the large and small scale topology of streamers can be resolved by WISPR and that the temporal variability of the streamers can be clearly isolated from spatial effects when PSP is corotating with the Sun Poirier et al. (2020), by exploiting synoptic maps based on sequential WISPR images, revealed the presence of multiple substructures (individual rays) inside streamers and pseudostreamers. This substructure of the streamers was noted in other studies (Thernisien and Howard 2006; Morgan and Cook 2020). Noteworthy in the WISPR synoptic maps was the identification of a bright and narrow set of streamer rays located at the core of the streamer belt (Poirier et al. 2020; Hess et al. 2020). The thickness of this bright region matches the thickness of the heliospheric plasma sheet (HPS) measured in the solar wind (up to 300 Mm) at times of sector boundary crossings (Winterhalter et al. 1994). Thus, WISPR may offer the first clear-cut connection between coronal imaging of streamers and the *in situ* measurements of the rather narrow HPS.

Global PFSS and MHD models of the solar corona during the PSP Encs. generally agree with the large-scale structure inferred from remote sensing observations (e.g., Riley et al. 2019; Poirier et al. 2020). As noted above, they have been used to interpret streamer substructure (Poirier et al. 2020) observed in WISPR observations, as well as during eclipses (Mikić et al. 2018). Equally importantly, they have been used to connect remote solar observations with their *in situ* counterparts (§8.1). Comparisons with white-light, but more importantly from emission images, provides crucial constraints for models that include realistic energy transport processes (van der Holst et al. 2019; Riley et al. 2019). They have already led to the improvement of coronal heating models (Riley et al. 2021), resulting in better matches with *in situ* measurements during multiple PSP Encs.

Images taken from a vantage point situated much closer to the Sun provide more detailed information on the population of transient structures released continually by helmet streamers. The fine-scale structure of streamer blobs is better resolved by WISPR than previous generations of heliospheric imagers. In addition the WISPR images have revealed that small-scale transients, with aspects that are reminiscent of magnetic islands and/or twisted 3D magnetic fields, are emitted at scales smaller than those of streamer blobs (Howard et al. 2019). These very small flux ropes were identified *in situ* as common structures during crossings of the HPS (Sanchez-Diaz et al. 2019) and more recently at PSP (Lavraud et al. 2020). They may also relate to the quasi-periodic structures detected by Viall and Vourlidas (2015) – on-going research is evaluating this hypothesis. Recent MHD simulations have shown that the flux ropes observed in blobs and the magnetic islands between quasi-periodic increases in density could result from a single process known as the tearing-mode instability as the HCS is stretched by the adjacent out-flowing solar wind (Réville et al. 2020b).
<figure><img src="image_32.png" /><figcaption>**Fig. 30** Multi-S/C observations of the first CME imaged by WISPR, on 1 Nov. 2018. (a) SDO/AIA imaging of the CME. Different features are visible in each panel, including the dark, circular cavity (193 Å; 1.6 MK and 20 MK), the bright trailing edge (131 Å; 0.4 MK and 10 MK), a bright blob that is co-spatial with the cavity (171 Å; 0.6 MK) and the prominence at the base of the eruption (304 Å; 0.05 MK). The black line in the 193 Å frame was used to calculate the size of the cavity. The white line in the 171 Å frame is the approximate direction of motion and was used to measure the height and calculate the velocity of the cavity in AIA. (b) The CME as seen by the SOHO/LASCO-C2 and -C3 coronagraphs. (c) The CME as seen by both PSP/WISPR telescopes. The white line denotes the solar equatorial plane. The curvature of the line in WISPR-o is due to the distortion of the detector. Figure adapted from Hess et al. (2020)</figcaption></figure>

### 8.4.3 Coronal Mass Ejection Imaging with WISPR

Within a few hours of being turned on in preparation for the first PSP perihelion passage, the WISPR imager observed its first CME. The WISPR cameras began taking science data at 00:00 UT on 1 Nov. 2018. By 11:00 UT a CME was visible in the inner telescope (Hess et al. 2020). Over the course of the next two days, the CME propagated along the solar equatorial plane throughout both WISPR telescopes, spanning 13.5° – 108.5°, with a speed of about 300 km s⁻¹, consistent with SBO-CMEs (Vourlidas and Webb 2018). The WISPR observations are included in Fig. 30.

The CME was also observed from the Earth perspective by the SDO/AIA EUV imager and the SOHO/LASCO coronagraphs. In AIA, a small prominence was observed beneath a cavity, which slowly rose from the west limb in a non-radial direction. The cavity and prominence are both visible in the left panel Fig. 30. As this structure enters the LASCO-C2 FOV the cavity remains visible, as does a bright claw-like structure at its base, as seen throughout the middle panel of Fig. 30. The non-radial motion continues until the CME reaches the boundary of an overlying helmet streamer, at which point the CME is deflected out through the streamer along the solar equatorial plane.

Because of the alignment of the S/C at the time of the eruption, WISPR was able to see the CME from a similar perspective as LASCO, but from a quarter of the distance. The inner FOV from WISPR was within the LASCO-C3 FOV, meaning that for a brief time WISPR and C3 observations were directly comparable. These direct comparisons demonstrate the improved resolution possible, even in a weaker event, from a closer observational position. This can be seen directly in Fig. 30 in the LASCO frame at 17:16 UT and the WISPR-i frame at 17:15 UT.

The clarity of the observations of the CME cavity in WISPR allowed for tracking of the cavity out to 40 $R_⊙$, as well as detailed modeling of the internal magnetic field of the CME (Rouillard et al. 2020b). Both studies would have been impossible without the details provided by WISPR imaging.
<figure><img src="image_33.png" /><figcaption>**Fig. 30** Multi-S/C observations of the first CME imaged by WISPR, on 1 Nov. 2018. (a) SDO/AIA imaging of the CME. Different features are visible in each panel, including the dark, circular cavity (193 Å; 1.6 MK and 20 MK), the bright trailing edge (131 Å; 0.4 MK and 10 MK), a bright blob that is co-spatial with the cavity (171 Å; 0.6 MK) and the prominence at the base of the eruption (304 Å; 0.05 MK). The black line in the 193 Å frame was used to calculate the size of the cavity. The white line in the 171 Å frame is the approximate direction of motion and was used to measure the height and calculate the velocity of the cavity in AIA. (b) The CME as seen by the SOHO/LASCO-C2 and -C3 coronagraphs. (c) The CME as seen by both PSP/WISPR telescopes. The white line denotes the solar equatorial plane. The curvature of the line in WISPR-o is due to the distortion of the detector. Figure adapted from Hess et al. (2020)</figcaption></figure>

### 8.4.3 Coronal Mass Ejection Imaging with WISPR

Within a few hours of being turned on in preparation for the first PSP perihelion passage, the WISPR imager observed its first CME. The WISPR cameras began taking science data at 00:00 UT on 1 Nov. 2018. By 11:00 UT a CME was visible in the inner telescope (Hess et al. 2020). Over the course of the next two days, the CME propagated along the solar equatorial plane throughout both WISPR telescopes, spanning 13.5° – 108.5°, with a speed of about 300 km s⁻¹, consistent with SBO-CMEs (Vourlidas and Webb 2018). The WISPR observations are included in Fig. 30.

The CME was also observed from the Earth perspective by the SDO/AIA EUV imager and the SOHO/LASCO coronagraphs. In AIA, a small prominence was observed beneath a cavity, which slowly rose from the west limb in a non-radial direction. The cavity and prominence are both visible in the left panel Fig. 30. As this structure enters the LASCO-C2 FOV the cavity remains visible, as does a bright claw-like structure at its base, as seen throughout the middle panel of Fig. 30. The non-radial motion continues until the CME reaches the boundary of an overlying helmet streamer, at which point the CME is deflected out through the streamer along the solar equatorial plane.

Because of the alignment of the S/C at the time of the eruption, WISPR was able to see the CME from a similar perspective as LASCO, but from a quarter of the distance. The inner FOV from WISPR was within the LASCO-C3 FOV, meaning that for a brief time WISPR and C3 observations were directly comparable. These direct comparisons demonstrate the improved resolution possible, even in a weaker event, from a closer observational position. This can be seen directly in Fig. 30 in the LASCO frame at 17:16 UT and the WISPR-i frame at 17:15 UT.

The clarity of the observations of the CME cavity in WISPR allowed for tracking of the cavity out to 40 $R_⊙$, as well as detailed modeling of the internal magnetic field of the CME (Rouillard et al. 2020b). Both studies would have been impossible without the details provided by WISPR imaging.
<figure><img src="image_33.png" /><figcaption>**Fig. 31** (a) A LASCO/C3 image from 5 Nov. 2018 showing two small streamer blobs marked by the two arrows. The northern blob (red arrow) is observed by PSP/WISPR during PSP's first perihelion passage. (b) The upper panels are a sequence of four images from WISPR's inner detector of the northern streamer blob eruption from (a), with dotted lines outlining the transient. Synthetic images are shown below the real images, based on the 3D reconstruction of the event. (c) Reconstructed 3D flux rope structure of the streamer blob. The flux rope is shown at two times, t1 and t2, corresponding to 03:48 UT and 09:40 UT, respectively. The red circles indicate the location of PSP at these two times, and the size of the Sun is to scale. Figure adapted from Wood et al. (2020)</figcaption></figure>

Another transient observed by WISPR during the first *PSP* perihelion passage was a small eruption seen by the WISPR-i detector on 5 Nov. 2018, only a day before *PSP*'s first close perihelion passage (Wood et al. 2020). As shown in Fig. 31(a), the LASCO/C3 coronagraph on board *SOHO* observed two small jet-like eruptions on that day, with the northern of the two (red arrow) corresponding to the one observed by WISPR. The appearance of the event from 1 AU is very consistent with the class of small transients called “streamer blobs” (Sheeley et al. 1997; Wang et al. 1998), although it is also listed in catalogs of CMEs compiled from *SOHO/LASCO* data, and so could also be described as a small CME.

At the time of the CME, *PSP* was located just off the right side of the LASCO/C3 image in Fig. 31a, lying almost perfectly in the C3 image plane. The transient's appearance in WISPR images is very different than that provided by the LASCO/C3 perspective, being so much closer to both the Sun and the event itself. This is the first close-up image of a streamer blob. In the WISPR images in Fig. 31b, the transient is not jet-like at all. Instead, it looks very much like a flux rope, with two legs stretching back toward the Sun, although one of the legs of the flux rope mostly lies above the WISPR FOV. This leg basically passes over *PSP* as the transient moves outward.

A 3D reconstruction of the flux rope morphology of the transient is shown in Fig. 31c, based not only on the LASCO/C3 and PSP/WISPR data, but also on images from the COR2 coronagraph on STEREO-A, making this the first CME reconstruction performed based on images from three different perspectives that include one very close to the Sun. Although typical of streamer blobs in appearance, a kinematic analysis of the 5 Nov. 2018 event reveals that it has a more impulsive acceleration than previously studied blobs.

### 8.4.4 Analysis of WISPR Coronal Mass Ejections

The rapid, elliptical orbit of *PSP* presents new challenges for the analysis of the white light images from WISPR due to the changing distance from the Sun. While the FOV of WISPR's two telescopes are fixed in angular size, the physical size of the coronal region imaged changes dramatically, as discussed in Liewer et al. (2019). In addition, because of *PSP*'s rapid motion in solar longitude, the projected latitude of a feature changes, as seen by WISPR, even if the feature has a constant heliocentric latitude. Because of these effects, techniques used in the past for studying the kinematics of solar ejecta may no longer be sufficient. The motion observed in the images is now a combination of the motion of the ejecta
<figure><img src="image_34.png" /><figcaption>**Fig. 31** (a) A LASCO/C3 image from 5 Nov. 2018 showing two small streamer blobs marked by the two arrows. The northern blob (red arrow) is observed by PSP/WISPR during PSP's first perihelion passage. (b) The upper panels are a sequence of four images from WISPR's inner detector of the northern streamer blob eruption from (a), with dotted lines outlining the transient. Synthetic images are shown below the real images, based on the 3D reconstruction of the event. (c) Reconstructed 3D flux rope structure of the streamer blob. The flux rope is shown at two times, t1 and t2, corresponding to 03:48 UT and 09:40 UT, respectively. The red circles indicate the location of PSP at these two times, and the size of the Sun is to scale. Figure adapted from Wood et al. (2020)</figcaption></figure>

Another transient observed by WISPR during the first *PSP* perihelion passage was a small eruption seen by the WISPR-i detector on 5 Nov. 2018, only a day before *PSP*'s first close perihelion passage (Wood et al. 2020). As shown in Fig. 31(a), the LASCO/C3 coronagraph on board *SOHO* observed two small jet-like eruptions on that day, with the northern of the two (red arrow) corresponding to the one observed by WISPR. The appearance of the event from 1 AU is very consistent with the class of small transients called “streamer blobs” (Sheeley et al. 1997; Wang et al. 1998), although it is also listed in catalogs of CMEs compiled from *SOHO/LASCO* data, and so could also be described as a small CME.

At the time of the CME, *PSP* was located just off the right side of the LASCO/C3 image in Fig. 31a, lying almost perfectly in the C3 image plane. The transient's appearance in WISPR images is very different than that provided by the LASCO/C3 perspective, being so much closer to both the Sun and the event itself. This is the first close-up image of a streamer blob. In the WISPR images in Fig. 31b, the transient is not jet-like at all. Instead, it looks very much like a flux rope, with two legs stretching back toward the Sun, although one of the legs of the flux rope mostly lies above the WISPR FOV. This leg basically passes over *PSP* as the transient moves outward.

A 3D reconstruction of the flux rope morphology of the transient is shown in Fig. 31c, based not only on the LASCO/C3 and PSP/WISPR data, but also on images from the COR2 coronagraph on STEREO-A, making this the first CME reconstruction performed based on images from three different perspectives that include one very close to the Sun. Although typical of streamer blobs in appearance, a kinematic analysis of the 5 Nov. 2018 event reveals that it has a more impulsive acceleration than previously studied blobs.

### 8.4.4 Analysis of WISPR Coronal Mass Ejections

The rapid, elliptical orbit of *PSP* presents new challenges for the analysis of the white light images from WISPR due to the changing distance from the Sun. While the FOV of WISPR's two telescopes are fixed in angular size, the physical size of the coronal region imaged changes dramatically, as discussed in Liewer et al. (2019). In addition, because of *PSP*'s rapid motion in solar longitude, the projected latitude of a feature changes, as seen by WISPR, even if the feature has a constant heliocentric latitude. Because of these effects, techniques used in the past for studying the kinematics of solar ejecta may no longer be sufficient. The motion observed in the images is now a combination of the motion of the ejecta
and of the S/C. On the other hand, the rapid motion gives multiple view points of coro-
nal features and this can be exploited using triangulation. Prior to launch, synthetic white
light WISPR images, created using the sophisticated ray-tracing software (Thernisien et al.
2009), were used to develop new techniques for analyzing observed motions of ejecta. Nis-
ticò et al. (2020) performed extensive studies of the evolution of the brightness due to the
motion of both the S/C and the feature. They concluded that the total brightness evolution
could be exploited to obtain a more precise triangulation of the observed features than might
be possible otherwise.

Tracking and Fitting Technique for Trajectory Determination

Liewer et al. (2020) developed a technique for determining the trajectories of CMEs and other ejecta that takes into account the rapid motion of PSP. The technique assumes that the ejecta, treated as a point, moves radially at a constant velocity. This technique builds on techniques developed for the analysis of J-maps (Sheeley et al. 1999) created from LASCO and SECCHI white light images. For ejecta moving radially at a constant velocity in a heliocentric frame, there are four trajectory parameters: longitude, latitude, velocity and radius (distance from the Sun) at some time t₀. Viewed from the S/C, the ejecta is seen to change position in a time sequence of images. The position in the image can be defined by two angles that specify the telescope’s LOS at that pixel location. We use a projective cartesian observer-centric frame of reference that is defined by the Sun-PSP vector and the PSP orbit plane. One angle (γ) measures the angle from the Sun parallel to the PSP orbit plane and the second angle (β) measures the angle out of the orbit plane. We call this coordinate system the PSP orbit frame. Using basic trigonometry, two equations were derived relating the coordinates in the heliocentric frame to those measured in the S/C frame (γ, β) as a function of time. The geometry relating the ejecta’s coordinates in the two frames is shown in Fig. 1 of Liewer et al. (2020) for the case with the inclination of PSP’s orbit plane w.r.t. the solar equatorial plane neglected. The coordinates of the S/C are [r₁, φ₁, 0], and the coordinates of the ejecta are [r₂, φ₂, δ₂]. The two equations are

$$
\frac{\tan \beta(t)}{\sin \gamma(t)} = \frac{\tan \delta_2}{\sin[\phi_2 - \phi_1(t)]}, \quad (4)
$$

$$
\cot \gamma(t) = \frac{r_1(t) - r_2(t) \cos \delta_2 \cos[\phi_2 - \phi_1(t)]}{r_2(t) \cos \delta_2 \sin[\phi_2 - \phi_1(t)]}. \quad (5)
$$

By tracking the point ejecta in a time sequence of WISPR images, we generate a set of
angular coordinates [γ(tᵢ), β(tᵢ)] for the ejecta in the PSP orbit frame. In principle, ejecta
coordinates in the S/C frame are only needed at two times to solve the above two equations
for the four unknown trajectory parameters. However, we obtain more accurate results by
tracking the ejecta in considerably more than two images. The ejecta trajectory parameters
in the heliocentric frame are determined by fitting the above equations to the tracking data
points [γ(tᵢ), β(tᵢ)]. Our fitting technique is described in Liewer et al. (2020), which also
gives the equations with the corrections for the inclination of PSP’s orbit w.r.t. the solar
equatorial plane.

The tracking and fitting technique was applied to a small CME seen by WISPR in the
second solar Enc. on 1-2 Apr. 2019 (Liewer et al. 2020). Figure 32 shows two of the WISPR
images used in the tracking; the feature tracked, an eye-like dark oval, is shown as the
red X. The direction of the trajectory found for this CME is indicated with an arrow in
Fig. 33, which also shows the location and fields of view of STEREO-A and PSP. The tra-
jectory solution in heliocentric inertial (HCI) coordinates was longitude = 67 ± 1°, latitude
and of the S/C. On the other hand, the rapid motion gives multiple view points of coronal features and this can be exploited using triangulation. Prior to launch, synthetic white light WISPR images, created using the sophisticated ray-tracing software (Thernisien et al. 2009), were used to develop new techniques for analyzing observed motions of ejecta. Nisticò et al. (2020) performed extensive studies of the evolution of the brightness due to the motion of both the S/C and the feature. They concluded that the total brightness evolution could be exploited to obtain a more precise triangulation of the observed features than might be possible otherwise.

## Tracking and Fitting Technique for Trajectory Determination

Liewer et al. (2020) developed a technique for determining the trajectories of CMEs and other ejecta that takes into account the rapid motion of PSP. The technique assumes that the ejecta, treated as a point, moves radially at a constant velocity. This technique builds on techniques developed for the analysis of J-maps (Sheeley et al. 1999) created from LASCO and SECCHI white light images. For ejecta moving radially at a constant velocity in a heliocentric frame, there are four trajectory parameters: longitude, latitude, velocity and radius (distance from the Sun) at some time $t_0$. Viewed from the S/C, the ejecta is seen to change position in a time sequence of images. The position in the image can be defined by two angles that specify the telescope’s LOS at that pixel location. We use a projective cartesian observer-centric frame of reference that is defined by the Sun-PSP vector and the PSP orbit plane. One angle ($\gamma$) measures the angle from the Sun parallel to the PSP orbit plane and the second angle ($\beta$) measures the angle out of the orbit plane. We call this coordinate system the PSP orbit frame. Using basic trigonometry, two equations were derived relating the coordinates in the heliocentric frame to those measured in the S/C frame ($\gamma, \beta$) as a function of time. The geometry relating the ejecta’s coordinates in the two frames is shown in Fig. 1 of Liewer et al. (2020) for the case with the inclination of PSP’s orbit plane w.r.t. the solar equatorial plane neglected. The coordinates of the S/C are [$r_1, \phi_1, 0$], and the coordinates of the ejecta are [$r_2, \phi_2, \delta_2$]. The two equations are

$$ \frac{\tan \beta(t)}{\sin \gamma(t)} = \frac{\tan \delta_2}{\sin[\phi_2 - \phi_1(t)]} \quad (4) $$

$$ \cot \gamma(t) = \frac{r_1(t) - r_2(t) \cos \delta_2 \cos[\phi_2 - \phi_1(t)]}{r_2(t) \cos \delta_2 \sin[\phi_2 - \phi_1(t)]} \quad (5) $$

By tracking the point ejecta in a time sequence of WISPR images, we generate a set of angular coordinates [$\gamma(t_i), \beta(t_i)$] for the ejecta in the PSP orbit frame. In principle, ejecta coordinates in the S/C frame are only needed at two times to solve the above two equations for the four unknown trajectory parameters. However, we obtain more accurate results by tracking the ejecta in considerably more than two images. The ejecta trajectory parameters in the heliocentric frame are determined by fitting the above equations to the tracking data points [$\gamma(t_i), \beta(t_i)$]. Our fitting technique is described in Liewer et al. (2020), which also gives the equations with the corrections for the inclination of PSP’s orbit w.r.t. the solar equatorial plane.

The tracking and fitting technique was applied to a small CME seen by WISPR in the second solar Enc. on 1-2 Apr. 2019 (Liewer et al. 2020). Figure 32 shows two of the WISPR images used in the tracking; the feature tracked, an eye-like dark oval, is shown as the red X. The direction of the trajectory found for this CME is indicated with an arrow in Fig. 33, which also shows the location and fields of view of STEREO-A and PSP. The trajectory solution in heliocentric inertial (HCI) coordinates was longitude = 67 $\pm$ 1°, latitude
<figure><img src="image_34.png" /><figcaption>**Fig. 32** WISPR-i running-difference images at two times for the CME of 2 Apr. 2019 showing the tracked feature, the lower dark “eye” (marked with red X’s). The image covers approximately 13.5° – 53.0° elongation from the Sun center. The streaks seen in the images are due to reflections off debris created by dust impacts on the PSP S/C</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>**Fig. 33** Trajectory solution for the 2 Apr. 2019 flux rope (magenta arrow) shown in relation to PSP, STEREO-A, and Earth at 18:09 UT. The fine solid lines indicate the fields-of-view of the telescopes on PSP and STEREO-A. The CME direction was found to be HCI longitude = 67° ± 1°. Note that the arrow only indicates the direction of the CME, and it is not meant to indicate the distance from the Sun. The HCI longitudes of the Earth and STEREO-A are 117° and 19°, respectively. The blue dashed ellipse is PSP’s orbit. The plot is in the Heliocentric Earth Ecliptic (HEE) reference frame and distances are in AU</figcaption></figure>

= 6.0 ± 0.3°, V = 333 ± 1 km s⁻¹, and $r_2(t_0) = 13.38 ± 0.01$ R⊙, where $t_0$= 12:09 UT on 2 Apr. 2019. There were simultaneous observations of the CME from STEREO-A and PSP, which enabled us to use the second viewpoint observation of STEREO-A/HI-1 to verify the technique and the results. This was done by generating a set of 3D trajectory points using the fitting solution above that included the time of the STEREO-A/HI-1 observations. These trajectory points were then projected onto an image from HI-1A using the World Coordinate System (WCS) information in the Flexible Image Transport System (FITS) image header. This is illustrated in Fig. 34, which shows the trajectory points generated from the solutions
<figure><img src="image_35.png" /><figcaption>**Fig. 32** WISPR-i running-difference images at two times for the CME of 2 Apr. 2019 showing the tracked feature, the lower dark “eye” (marked with red X’s). The image covers approximately 13.5° – 53.0° elongation from the Sun center. The streaks seen in the images are due to reflections off debris created by dust impacts on the PSP S/C</figcaption></figure>

<figure><img src="image_36.png" /><figcaption>**Fig. 33** Trajectory solution for the 2 Apr. 2019 flux rope (magenta arrow) shown in relation to PSP, STEREO-A, and Earth at 18:09 UT. The fine solid lines indicate the fields-of-view of the telescopes on PSP and STEREO-A. The CME direction was found to be HCI longitude = 67° ± 1°. Note that the arrow only indicates the direction of the CME, and it is not meant to indicate the distance from the Sun. The HCI longitudes of the Earth and STEREO-A are 117° and 19°, respectively. The blue dashed ellipse is PSP’s orbit. The plot is in the Heliocentric Earth Ecliptic (HEE) reference frame and distances are in AU</figcaption></figure>

= 6.0 ± 0.3°, V = 333 ± 1 km s⁻¹, and $r_2(t_0) = 13.38 ± 0.01$ R⊙, where $t_0$= 12:09 UT on 2 Apr. 2019. There were simultaneous observations of the CME from STEREO-A and PSP, which enabled us to use the second viewpoint observation of STEREO-A/HI-1 to verify the technique and the results. This was done by generating a set of 3D trajectory points using the fitting solution above that included the time of the STEREO-A/HI-1 observations. These trajectory points were then projected onto an image from HI-1A using the World Coordinate System (WCS) information in the Flexible Image Transport System (FITS) image header. This is illustrated in Fig. 34, which shows the trajectory points generated from the solutions
<figure><img src="image_36.png" /><figcaption>**Fig. 34** Trajectory of the flux rope of 2 Apr. 2019, found from the WISPR data using the tracking and fitting technique, projected to images from STEREO-A/HI-1 at 18:09 UT. The trajectory from tracking and fitting (+ signs) is shown from 12:09 to 18:09 UT in hourly increments, as seen from STEREO-A. The location of the prediction for the last time (18:09 UT) is in good agreement with the location of the tracked feature seen in the HI-1A image, thus verifying the trajectory. The grid lines are the coordinate lines of the WCS frame specified in the HI-1A FITS header. The size and location of the Sun (yellow globe) is shown to scale</figcaption></figure>

from 12:09 to 18:09 UT in hourly increments projected onto the HI-1A image at 18:09 UT on 2 Apr. 2019. Note that the last point, corresponding to the time of the HI-1A image, falls quite near a similar feature on the CME as was tracked in the WISPR images (Fig. 32). Thus, the trajectory determined from the WISPR data agrees with the STEREO-A observations from a second view point. This technique was also applied to the first CME seen by WISPR on 2 Nov. 2018. Details of the tracking and the results are in Liewer et al. (2020), and independent analyses of the CME kinematics and trajectory for the 2 Apr. 2019 event were carried out by Braga and Vourlidas (2021) and Wood et al. (2021), with similar results.

## WISPR and STEREO Observations of the Evolution of a Streamer Blowout CME

WISPR obtained detailed images of the flux rope structure of a CME on 26-27 Jan. 2020. The tracking and fitting procedure was also used here to determine the trajectory. The direction of the trajectory is shown in Fig. 35, along with locations and FOVs of STEREO A and WISPR. The trajectory solution parameters are HCl longitude and latitude = (65°±2°, 2°± 2°), $v = 248 \pm 16$ km s⁻¹ and $r_2(t_0)/R_⊙ = 30.3 \pm 0.3$ at $t_0 = 20:04$ UT on 26 Jan. 2020. The CME was also observed by STEREO-A/COR2 starting on 25 Jan. 2020. The brightening of the streamer before the eruption, the lack of a bright leading edge, and the outflow following the ejecta led to its identification as a SBO-CME (Liewer et al. 2021). Data from STEREO-A/EUVI suggested that this CME originated as a rising flux rope on 23 Jan. 2020, which was constrained in the corona until its eruption of 25 Jan. 2020. The detail of the observations and the supporting data from AIA and the Helioseismic and Magnetic Imager (HMI; Scherrer et al. 2012) on SDO can be found in Liewer et al. (2021). The direction determined from the tracking and fitting was consistent with this interpretation; the direction was approximately 30° west of a new AR, which was also a possible source.

To verify the CME's trajectory determined by tracking and fitting, we again made use of simultaneous observations from STEREO-A, but in this case we used triangulation to determine the 3D location of the CME at the time of a simultaneous image pair. This was only possible because details of the structure of the CME were evident from both viewpoints so that the same feature could be located in both images. Figure 36 shows the simultaneous WISPR-o and HI-1A images of the CME at time 26 Jan. 2020, at 20:49 UT when the S/C
<figure><img src="image_37.png" /><figcaption>**Fig. 34** Trajectory of the flux rope of 2 Apr. 2019, found from the WISPR data using the tracking and fitting technique, projected to images from STEREO-A/HI-1 at 18:09 UT. The trajectory from tracking and fitting (+ signs) is shown from 12:09 to 18:09 UT in hourly increments, as seen from STEREO-A. The location of the prediction for the last time (18:09 UT) is in good agreement with the location of the tracked feature seen in the HI-1A image, thus verifying the trajectory. The grid lines are the coordinate lines of the WCS frame specified in the HI-1A FITS header. The size and location of the Sun (yellow globe) is shown to scale</figcaption></figure>

from 12:09 to 18:09 UT in hourly increments projected onto the HI-1A image at 18:09 UT on 2 Apr. 2019. Note that the last point, corresponding to the time of the HI-1A image, falls quite near a similar feature on the CME as was tracked in the WISPR images (Fig. 32). Thus, the trajectory determined from the WISPR data agrees with the STEREO-A observations from a second view point. This technique was also applied to the first CME seen by WISPR on 2 Nov. 2018. Details of the tracking and the results are in Liewer et al. (2020), and independent analyses of the CME kinematics and trajectory for the 2 Apr. 2019 event were carried out by Braga and Vourlidas (2021) and Wood et al. (2021), with similar results.

## WISPR and STEREO Observations of the Evolution of a Streamer Blowout CME

WISPR obtained detailed images of the flux rope structure of a CME on 26-27 Jan. 2020. The tracking and fitting procedure was also used here to determine the trajectory. The direction of the trajectory is shown in Fig. 35, along with locations and FOVs of STEREO A and WISPR. The trajectory solution parameters are HCl longitude and latitude = (65°±2°, 2°± 2°), $v = 248 \pm 16$ km s⁻¹ and $r_2(t_0)/R_⊙ = 30.3 \pm 0.3$ at $t_0 = 20:04$ UT on 26 Jan. 2020. The CME was also observed by STEREO-A/COR2 starting on 25 Jan. 2020. The brightening of the streamer before the eruption, the lack of a bright leading edge, and the outflow following the ejecta led to its identification as a SBO-CME (Liewer et al. 2021). Data from STEREO-A/EUVI suggested that this CME originated as a rising flux rope on 23 Jan. 2020, which was constrained in the corona until its eruption of 25 Jan. 2020. The detail of the observations and the supporting data from AIA and the Helioseismic and Magnetic Imager (HMI; Scherrer et al. 2012) on SDO can be found in Liewer et al. (2021). The direction determined from the tracking and fitting was consistent with this interpretation; the direction was approximately 30° west of a new AR, which was also a possible source.

To verify the CME's trajectory determined by tracking and fitting, we again made use of simultaneous observations from STEREO-A, but in this case we used triangulation to determine the 3D location of the CME at the time of a simultaneous image pair. This was only possible because details of the structure of the CME were evident from both viewpoints so that the same feature could be located in both images. Figure 36 shows the simultaneous WISPR-o and HI-1A images of the CME at time 26 Jan. 2020, at 20:49 UT when the S/C
<figure><img src="image_37.png" /><figcaption>**Fig. 35** Trajectory of the 26-27 Jan. 2020 CME (magenta arrow) in relation to PSP, STEREO-A, and Earth, projected in the HEE reference frame on 26 Jan. 2020, at 20:49 UT. The fields-of-view of the WISPR-i and WISPR-o telescopes on PSP and COR2 and HI-1 on STEREO-A are indicated by solid lines. The plot is in the HEE coordinate frame and distances are in AU</figcaption></figure>

<figure><img src="image_38.png" /><figcaption>**Fig. 36** Image pair used to determine the location of the 26-27 Jan. 2020 CME by triangulation. Left: WISPR-o image of 26 Jan. 2020, at 20:49 UT with the selected feature circled in red. Right: Simultaneous HI-1 image showing the location of the same feature identified in the WISPR-o image. The location of the CME found by triangulation of this feature was in excellent agreement with the trajectory found from tracking and fitting (see core text). The Sun (yellow globe) in the panel on the right is shown to scale. The HI-1 image is projected in the Helioprojective Cartesian (HPC) system (red and blue grid lines) with the Sun (yellow globe) drawn to scale</figcaption></figure>

were separated by 46°. The red X in each image marks the what we identify as the same feature in both images (a dark spot behind the bright V). Using a triangulation technique on this image pair Liewer et al. (2021) gave the result distance from the Sun $r/R_⊙ = 31 \pm 2$, HCl longitude and latitude = (66° ± 3°, -2° ± 2°). These angles are in excellent agreement with the longitude found by tracking and fitting given above. The distance from the Sun is also in excellent agreement with the predicted distance at this time of $r_2/R_⊙ = 31.2 \pm 0.3$, validating our trajectory solution. Thus the trajectory was confirmed, which further supported our interpretation of the evolution of this slowly evolving SBO-CME.
<figure><img src="image_38.png" /><figcaption>**Fig. 35** Trajectory of the 26-27 Jan. 2020 CME (magenta arrow) in relation to PSP, STEREO-A, and Earth, projected in the HEE reference frame on 26 Jan. 2020, at 20:49 UT. The fields-of-view of the WISPR-i and WISPR-o telescopes on PSP and COR2 and HI-1 on STEREO-A are indicated by solid lines. The plot is in the HEE coordinate frame and distances are in AU</figcaption></figure>

<figure><img src="image_39.png" /><figcaption>**Fig. 36** Image pair used to determine the location of the 26-27 Jan. 2020 CME by triangulation. Left: WISPR-o image of 26 Jan. 2020, at 20:49 UT with the selected feature circled in red. Right: Simultaneous HI-1 image showing the location of the same feature identified in the WISPR-o image. The location of the CME found by triangulation of this feature was in excellent agreement with the trajectory found from tracking and fitting (see core text). The Sun (yellow globe) in the panel on the right is shown to scale. The HI-1 image is projected in the Helioprojective Cartesian (HPC) system (red and blue grid lines) with the Sun (yellow globe) drawn to scale</figcaption></figure>

were separated by 46°. The red X in each image marks the what we identify as the same feature in both images (a dark spot behind the bright V). Using a triangulation technique on this image pair Liewer et al. (2021) gave the result distance from the Sun $r/R_⊙ = 31 \pm 2$, HCl longitude and latitude = (66° ± 3°, -2° ± 2°). These angles are in excellent agreement with the longitude found by tracking and fitting given above. The distance from the Sun is also in excellent agreement with the predicted distance at this time of $r_2/R_⊙ = 31.2 \pm 0.3$, validating our trajectory solution. Thus the trajectory was confirmed, which further supported our interpretation of the evolution of this slowly evolving SBO-CME.
# 9 Solar Radio Emission

At low frequencies, below ~10–20 MHz, radio emission cannot be observed well from ground-based observatories due to the terrestrial ionosphere. Solar radio emission at these frequencies consists of radio bursts, which are signatures of the acceleration and propagation of non-thermal electrons. Type II and type III radio bursts are commonly observed, with the former resulting from electrons accelerated at shock fronts associated with CMEs, and the latter from electron beams accelerated by solar flares (see Fig. 38C).

Solar radio bursts offer information on the kinematics of the propagating source, and are a remote probe of the properties of the local plasma through which the source is propagating. Radio observations on PSP are made by the FIELDS RFS, which measures electric fields from 10 kHz to 19.2 MHz (Pulupa et al. 2017). At frequencies below and near $f_{pe}$, the RFS measurements are dominated by the quasi-thermal noise (QTN).

PSP launched at solar minimum, when the occurrence rate of solar radio bursts is relatively low. Several PSP Encs. (Enc. 1, Enc. 3, Enc. 4) near the start of the mission were very quiet in radio, containing only a few small type III bursts. The second PSP Enc. (Enc. 2), in Apr. 2019, was a notable exception, featuring multiple strong type III radio bursts and a type III storm (Pulupa et al. 2020). As solar activity began rising in late 2020 and 2021, with Encs. 5 and beyond, the occurrence of radio bursts is also increasing. Taking advantage of the quiet Encs. near the start of the mission, Chhabra (2021) applied a correlation technique developed by Viall and Klimchuk (2013) for imaging data to RFS light curves, searching for evidence of heating of the coronal by small-scale nanoflares which are too faint to appear to the eye in RFS spectrograms.

During PSP Encs., the cadence of RFS spectra range is typically 3.5 or 7 seconds, which is higher than the typical cadence of radio spectra available from previous S/C such as STEREO and Wind. The relatively high cadence of RFS data is particularly useful in the study of type III radio bursts above 1 MHz (in the RFS High Frequency Receiver [HFR] range), which typically last ≲ 1 minute at these frequencies. Using the HFR data enabled Pulupa et al. (2020) to measure circular polarization near the start of several type III bursts in Enc. 2. Krupar et al. (2020) characterized the decay times of type III radio bursts up to 10 MHz, observing increased decay times above 1 MHz compared to extrapolation using previous measurements from STEREO. Modeling suggests that these decay times may correspond to increased density fluctuations near the Alfvén point.

Recent studies have used RFS data to investigate basic properties of type III bursts, in comparison to previous observations and theories. Chen et al. (2021b) examined a single Enc. 2 type IIIb burst featuring fine structure (striae) in detail. Chen et al. (2021b) found consistency between RFS observations and results of a model with emission generated via the electron cyclotron maser instability (Wu et al. 2004), over the several-MHz frequency range corresponding to solar distances where $f_{ce} > f_{pe}$.

Ma et al. (2021a) performed a statistical survey of the lower cutoff frequency of type III bursts using the first five PSP Encs., finding a higher average cutoff frequency than previous observations from *Ulysses* and *Wind*. Ma et al. (2021a) proposed several explanations for this discrepancy, including solar cycle and event selection effects.

The launch of *SoIo* in Feb. 2020 marked the first time four S/C (*Wind*, STEREO-A, PSP, and *SoIo*) with radio instrumentation were operational in the inner heliosphere. Musset et al. (2021) combined observations from these four S/C along with a model of the burst emission to determine the directivity of individual type III radio bursts, a measurement previously only possible using statistical analysis of large numbers of bursts.
## 9 Solar Radio Emission

At low frequencies, below ~10–20 MHz, radio emission cannot be observed well from ground-based observatories due to the terrestrial ionosphere. Solar radio emission at these frequencies consists of radio bursts, which are signatures of the acceleration and propagation of non-thermal electrons. Type II and type III radio bursts are commonly observed, with the former resulting from electrons accelerated at shock fronts associated with CMEs, and the latter from electron beams accelerated by solar flares (see Fig. 38C).

Solar radio bursts offer information on the kinematics of the propagating source, and are a remote probe of the properties of the local plasma through which the source is propagating. Radio observations on PSP are made by the FIELDS RFS, which measures electric fields from 10 kHz to 19.2 MHz (Pulupa et al. 2017). At frequencies below and near $f_{pe}$, the RFS measurements are dominated by the quasi-thermal noise (QTN).

PSP launched at solar minimum, when the occurrence rate of solar radio bursts is relatively low. Several PSP Encs. (Enc. 1, Enc. 3, Enc. 4) near the start of the mission were very quiet in radio, containing only a few small type III bursts. The second PSP Enc. (Enc. 2), in Apr. 2019, was a notable exception, featuring multiple strong type III radio bursts and a type III storm (Pulupa et al. 2020). As solar activity began rising in late 2020 and 2021, with Encs. 5 and beyond, the occurrence of radio bursts is also increasing. Taking advantage of the quiet Encs. near the start of the mission, Chhabra (2021) applied a correlation technique developed by Viall and Klimchuk (2013) for imaging data to RFS light curves, searching for evidence of heating of the coronal by small-scale nanoflares which are too faint to appear to the eye in RFS spectrograms.

During PSP Encs., the cadence of RFS spectra range is typically 3.5 or 7 seconds, which is higher than the typical cadence of radio spectra available from previous S/C such as STEREO and Wind. The relatively high cadence of RFS data is particularly useful in the study of type III radio bursts above 1 MHz (in the RFS High Frequency Receiver [HFR] range), which typically last ≲ 1 minute at these frequencies. Using the HFR data enabled Pulupa et al. (2020) to measure circular polarization near the start of several type III bursts in Enc. 2. Krupar et al. (2020) characterized the decay times of type III radio bursts up to 10 MHz, observing increased decay times above 1 MHz compared to extrapolation using previous measurements from STEREO. Modeling suggests that these decay times may correspond to increased density fluctuations near the Alfvén point.

Recent studies have used RFS data to investigate basic properties of type III bursts, in comparison to previous observations and theories. Chen et al. (2021b) examined a single Enc. 2 type IIIb burst featuring fine structure (striae) in detail. Chen et al. (2021b) found consistency between RFS observations and results of a model with emission generated via the electron cyclotron maser instability (Wu et al. 2004), over the several-MHz frequency range corresponding to solar distances where $f_{ce} > f_{pe}$.

Ma et al. (2021a) performed a statistical survey of the lower cutoff frequency of type III bursts using the first five PSP Encs., finding a higher average cutoff frequency than previous observations from *Ulysses* and *Wind*. Ma et al. (2021a) proposed several explanations for this discrepancy, including solar cycle and event selection effects.

The launch of *SoIo* in Feb. 2020 marked the first time four S/C (*Wind*, STEREO-A, PSP, and *SoIo*) with radio instrumentation were operational in the inner heliosphere. Musset et al. (2021) combined observations from these four S/C along with a model of the burst emission to determine the directivity of individual type III radio bursts, a measurement previously only possible using statistical analysis of large numbers of bursts.
<figure><img src="image_39.png" /><figcaption>**Fig. 37** IS◎IS/EPI-Lo time-of-flight measurements for the two SEP events on 2 Apr. 2019 (DOY 92) and 4 Apr. 2019 (DOY 94) are shown in green, blue, and red for the stated energies. IS◎IS/EPI-Hi/LET1 observations are shown in black. Figure adapted from Leske et al. (2020)</figcaption></figure>

# 10 Energetic Particles

The first three years of the *PSP* mission have provided key insights into the acceleration and transport of energetic particles in the inner heliosphere and has enabled a comprehensive understanding into the variability of solar radio emission. *PSP* observed a multitude of solar radio emissions, SEP events, CMEs, CIRs and SIRs, inner heliospheric anomalous cosmic rays (ACRs), and energetic electron events; all of which are critical to explore the fundamental physics of particle acceleration and transport in the near-Sun environment and throughout the universe.

## 10.1 Solar Energetic Particles

On 2 and 4 Apr. 2019, *PSP* observed two small SEP events (Leske et al. 2020; Kouloumvakos et al. 2020; Zhao et al. 2020a) while at ~0.17 AU (Fig. 37). The event on 4 Apr. 2019 was associated with both a type III radio emission seen by *PSP* as well as surges in the EUV observed by *STEREO*-A, all of which determined the source was an AR ~80° east of the *PSP* footpoint (Leske et al. 2020). To better understand the origin of these SEP events, Kouloumvakos et al. (2020) conducted a series of simulations constrained by remote sensing observations from *SDO*AIA, *STEREO*-A/EUVI and COR2, *SOHO/LASCO*, and *PSP/WISPR* to determine the magnetic connectivity of *PSP*, model the 3D structure and evolution of the EUV waves, investigate possible shock formation, and connect these simulations to the SEP observations. This robust simulation work suggests that the SEP events were from multiple ejections from AR 12738. The 2 Apr. 2019 event likely originated from two ejections that formed a shock in the lower corona (Kouloumvakos et al. 2020). Meanwhile, the 4 Apr. 2019 event was likely the result of a slow SBO, which reconfigured the global magnetic topology to be conducive for transport of solar particles away from the AR toward *PSP*. Interestingly, however, Leske et al. (2020) did not observe ³He for this event, as would be expected from flare-related SEPs. Zhao et al. (2020a) explained gradual rise of the 4 Apr. 2019 low-energy H⁺ event compared to the more energetic enhancement on 2 Apr. 2019 as being indicative of different diffusion conditions.

The same AR (AR 12738) was later responsible for a ³He-rich SEP event on 20-21 Apr. 2019 observed by *PSP* at ~0.46 AU that was also measured by *SOHO* at ~1 AU (shown in Fig. 38) (Wiedenbeck et al. 2020). This SEP event was observed along with type III
<figure><img src="image_40.png" /><figcaption>**Fig. 37** IS◎IS/EPI-Lo time-of-flight measurements for the two SEP events on 2 Apr. 2019 (DOY 92) and 4 Apr. 2019 (DOY 94) are shown in green, blue, and red for the stated energies. IS◎IS/EPI-Hi/LET1 observations are shown in black. Figure adapted from Leske et al. (2020)</figcaption></figure>

# 10 Energetic Particles

The first three years of the PSP mission have provided key insights into the acceleration and transport of energetic particles in the inner heliosphere and has enabled a comprehensive understanding into the variability of solar radio emission. PSP observed a multitude of solar radio emissions, SEP events, CMEs, CIRs and SIRs, inner heliospheric anomalous cosmic rays (ACRs), and energetic electron events; all of which are critical to explore the fundamental physics of particle acceleration and transport in the near-Sun environment and throughout the universe.

## 10.1 Solar Energetic Particles

On 2 and 4 Apr. 2019, PSP observed two small SEP events (Leske et al. 2020; Kouloumvakos et al. 2020; Zhao et al. 2020a) while at ~0.17 AU (Fig. 37). The event on 4 Apr. 2019 was associated with both a type III radio emission seen by PSP as well as surges in the EUV observed by STEREO-A, all of which determined the source was an AR ~80° east of the PSP footpoint (Leske et al. 2020). To better understand the origin of these SEP events, Kouloumvakos et al. (2020) conducted a series of simulations constrained by remote sensing observations from SDO/AIA, STEREO-A/EUVI and COR2, SOHO/LASCO, and PSP/WISPR to determine the magnetic connectivity of PSP, model the 3D structure and evolution of the EUV waves, investigate possible shock formation, and connect these simulations to the SEP observations. This robust simulation work suggests that the SEP events were from multiple ejections from AR 12738. The 2 Apr. 2019 event likely originated from two ejections that formed a shock in the lower corona (Kouloumvakos et al. 2020). Meanwhile, the 4 Apr. 2019 event was likely the result of a slow SBO, which reconfigured the global magnetic topology to be conducive for transport of solar particles away from the AR toward PSP. Interestingly, however, Leske et al. (2020) did not observe ³He for this event, as would be expected from flare-related SEPs. Zhao et al. (2020a) explained gradual rise of the 4 Apr. 2019 low-energy H⁺ event compared to the more energetic enhancement on 2 Apr. 2019 as being indicative of different diffusion conditions.

The same AR (AR 12738) was later responsible for a ³He-rich SEP event on 20-21 Apr. 2019 observed by PSP at ~0.46 AU that was also measured by SOHO at ~1 AU (shown in Fig. 38) (Wiedenbeck et al. 2020). This SEP event was observed along with type III
<figure><img src="image_40.png" /><figcaption>**Fig. 38** Remote and *in situ* observations for the 20-21 Apr. 2019 $^3$He-rich SEP event. (a) Jet onset times and CME release times as reported by Schwadron et al. (2020), (b) 5-min 0.05-0.4 nm (blue) and 0.1-0.8 nm (red) X-ray flux from the Geostationary Operational Environmental Satellite (GOES), (c) radio emissions from Wind/WAVES (Bougeret et al. 1995), (d) electron fluxes for 53 (black), 79 (red), and 133 (blue) keV from the ACE Electron Proton Alpha Monitor (EPAM; Gold et al. 1998), (e) velocity dispersion with red line indicating the dispersion slope from the ACE Ultra Low Energy Isotope Spectrometer (ULEIS; Mason et al. 1998), (f) ACE/ULEIS 1 MeV He flux, (g) ACE/ULEIS He mass vs. time, and (h) PSP/IS$\odot$IS/EPI-Hi mass vs. time. Grey boxes in panel (h) indicate times without IS$\odot$IS observations. Figure adapted from Wiedenbeck et al. (2020)</figcaption></figure>

radio bursts and helical jets. The $^3$He/$^4$He ratios at PSP and SOHO were ~250 times the nominal solar wind ratio; such large enhancements are often seen in impulsive SEP events. This event demonstrated the utility of IS$\odot$IS/EPI-Hi to contribute to our understanding of the radial evolution of $^3$He-rich SEP events, which can help constrain studies of potential limits on the amount of $^3$He that can be accelerated by an AR (e.g., Ho et al. 2005).

Cohen et al. (2021a) later investigated the helium content of six SEP events from May to Jun. 2020 during the fifth orbit of PSP. These events demonstrated that SEP events, even from the same AR, can have significantly different $^3$He/$^4$He and He/H ratios. Additionally, EUV and coronagraph observations of these events suggest that the SEPs were accelerated very low in the corona. Using velocity-dispersion analysis, Chhiber et al. (2021b) concluded that the path length of these SEP events to the source was ~0.625 AU, greatly exceeding that of a simple Parker spiral. To explain the large path length of these particles, Chhiber et al. (2021b) developed an approach to estimate how the random-walk of magnetic field lines could affect particle path length, which well explained the computed path length from the velocity-dispersion analysis.

During the first orbit of PSP, shortly after the first perihelion pass, a CME was observed locally at PSP, which was preceded by a significant enhancement in SEPs with energies below a few hundred keV/nuc (Giacalone et al. 2020; Mitchell et al. 2020a). The CME was
<figure><img src="image_41.png" /><figcaption>**Fig. 38** Remote and *in situ* observations for the 20-21 Apr. 2019 $^3$He-rich SEP event. (a) Jet onset times and CME release times as reported by Schwadron et al. (2020), (b) 5-min 0.05-0.4 nm (blue) and 0.1-0.8 nm (red) X-ray flux from the Geostationary Operational Environmental Satellite (GOES), (c) radio emissions from Wind/WAVES (Bougeret et al. 1995), (d) electron fluxes for 53 (black), 79 (red), and 133 (blue) keV from the ACE Electron Proton Alpha Monitor (EPAM; Gold et al. 1998), (e) velocity dispersion with red line indicating the dispersion slope from the ACE Ultra Low Energy Isotope Spectrometer (ULEIS; Mason et al. 1998), (f) ACE/ULEIS 1 MeV He flux, (g) ACE/ULEIS He mass vs. time, and (h) PSP/IS$\odot$IS/EPI-Hi mass vs. time. Grey boxes in panel (h) indicate times without IS$\odot$IS observations. Figure adapted from Wiedenbeck et al. (2020)</figcaption></figure>

radio bursts and helical jets. The $^3$He/$^4$He ratios at PSP and SOHO were ~250 times the nominal solar wind ratio; such large enhancements are often seen in impulsive SEP events. This event demonstrated the utility of IS$\odot$IS/EPI-Hi to contribute to our understanding of the radial evolution of $^3$He-rich SEP events, which can help constrain studies of potential limits on the amount of $^3$He that can be accelerated by an AR (e.g., Ho et al. 2005).

Cohen et al. (2021a) later investigated the helium content of six SEP events from May to Jun. 2020 during the fifth orbit of PSP. These events demonstrated that SEP events, even from the same AR, can have significantly different $^3$He/$^4$He and He/H ratios. Additionally, EUV and coronagraph observations of these events suggest that the SEPs were accelerated very low in the corona. Using velocity-dispersion analysis, Chhiber et al. (2021b) concluded that the path length of these SEP events to the source was ~0.625 AU, greatly exceeding that of a simple Parker spiral. To explain the large path length of these particles, Chhiber et al. (2021b) developed an approach to estimate how the random-walk of magnetic field lines could affect particle path length, which well explained the computed path length from the velocity-dispersion analysis.

During the first orbit of PSP, shortly after the first perihelion pass, a CME was observed locally at PSP, which was preceded by a significant enhancement in SEPs with energies below a few hundred keV/nuc (Giacalone et al. 2020; Mitchell et al. 2020a). The CME was
<figure><img src="image_41.png" /><figcaption>**Fig. 39** Multi-instrument data for a CME and SEP event observed by PSP and STEREO-A. (a) shows the heliocentric distance of the CME, (b-c) show solar wind density and solar wind speed from the SWEAP instrument, (d) shows the vector and magnitude of the magnetic field from the FIELDS instrument, and (e-f) show data from the IS⊙IS/EPI-Lo instrument. Figure adapted from Giacalone et al. (2020) where further details are provided</figcaption></figure>

observed to cross PSP on 12 Nov. 2018 (DOY 316), and at this time, PSP was approximately 0.23 AU from the Sun. The CME was observed remotely by STEREO-A which was in a position to observe the CME erupting from the east limb of the Sun (w.r.t. STEREO-A) and moving directly towards PSP. PSP was on the opposite side of the Sun relative to Earth. Through analysis of STEREO-A coronagraph images, the speed of the CME was determined to be 360 km s⁻¹, which is slower than typical SEP-producing CMEs seen by S/C near 1 AU. Moreover, in the few days that preceded the CME, there were very few energetic particles observed, representing a very quiet period. Thus, this represented a unique observation of energetic particles associated with a slow CME near the Sun.

Figure 39 shows a multi-instrument, multi-panel plot of data collected during this slow-CME/SEP event. Figure 39a shows the position of the CME as a function of time based on STEREO-A observations as well as PSP (the cyan point), while the lower Fig. 39e-f shows 30-300 keV energetic particles from the IS⊙IS/EPI-Lo instrument. The CME was observed to erupt and move away from the Sun well before the start of the SEP event, but the SEP intensities rose from the background, peaked, and then decayed before the CME
<figure><img src="image_42.png" /><figcaption>**Fig. 39** Multi-instrument data for a CME and SEP event observed by PSP and STEREO-A. (a) shows the heliocentric distance of the CME, (b-c) show solar wind density and solar wind speed from the SWEAP instrument, (d) shows the vector and magnitude of the magnetic field from the FIELDS instrument, and (e-f) show data from the IS⊙IS/EPI-Lo instrument. Figure adapted from Giacalone et al. (2020) where further details are provided</figcaption></figure>

observed to cross PSP on 12 Nov. 2018 (DOY 316), and at this time, PSP was approximately 0.23 AU from the Sun. The CME was observed remotely by STEREO-A which was in a position to observe the CME erupting from the east limb of the Sun (w.r.t. STEREO-A) and moving directly towards PSP. PSP was on the opposite side of the Sun relative to Earth. Through analysis of STEREO-A coronagraph images, the speed of the CME was determined to be 360 km s⁻¹, which is slower than typical SEP-producing CMEs seen by S/C near 1 AU. Moreover, in the few days that preceded the CME, there were very few energetic particles observed, representing a very quiet period. Thus, this represented a unique observation of energetic particles associated with a slow CME near the Sun.

Figure 39 shows a multi-instrument, multi-panel plot of data collected during this slow-CME/SEP event. Figure 39a shows the position of the CME as a function of time based on STEREO-A observations as well as PSP (the cyan point), while the lower Fig. 39e-f shows 30-300 keV energetic particles from the IS⊙IS/EPI-Lo instrument. The CME was observed to erupt and move away from the Sun well before the start of the SEP event, but the SEP intensities rose from the background, peaked, and then decayed before the CME
crossed *PSP*. There was no shock observed locally at *PSP*, and there is no clear evidence of local acceleration of SEPs at the CME crossing. It was suggested by Giacalone et al. (2020) that the CME briefly drove either a weak shock or a plasma compression when it was closer to the Sun, and was capable of accelerating particles which then propagated ahead of the CME and observed by *PSP*. In fact, modeling of the CME and local plasma parameters, also presented in this paper, suggested there may have been a weak shock over parts of the (modeled) CME-shock surface, but is not clear whether *PSP* was magnetically connected to these locations. The energetic particle event was characterized by a clear velocity dispersion in which higher-energy particles arrived well before the lower energy particles. Moreover, the time-intensity profiles at specific energies, seen in Fig. 39e, show a relatively smooth rise from the background to the peak, and a gradual decay. The particles were observed to be initially anisotropic, moving radially away from the Sun, but at the peak of the event were observed to be more isotropic. Giacalone et al. (2020) interpreted this in terms of the diffusive transport of particles accelerated by the CME starting about the time it was $7.5 R_⊙$ and continuing with time but with a decreasing intensity. They used a diffusive-transport model and fit the observed time-intensity profiles, which gave values for the scattering mean-free path, parallel to the magnetic field, of 30 keV to 100 keV protons to be from 0.04 – 0.09 AU at the location of *PSP*.

Another important feature of this event was the generally steep energy spectrum of the low-energy ions. This suggested a very weak event. In the comparison between the model used by Giacalone et al. (2020) and the observations, it was found that a source spectrum, assumed to be at the CME when it was close to the Sun, had an approximately $E^{-5.5}$ power-law dependence.

At the time, this was the closest to the Sun that a CME-related SEP event had been observed *in situ*. Giacalone et al. (2020) used their diffusive transport model to estimate the total fluence that this event would have had at 1 AU, in order to compare with previous observations of CME-related SEP events. It was determined that this event would have been so weak to not even appear on a figure showing a wide range of values of the SEP fluence as a function of CME speed produced by Kahler (2001).

Mitchell et al. (2020a) presented a separate analysis of this same CME-SEP event suggested an alternative acceleration mechanism. They noted that since *PSP* did not observe a shock locally, and modeling of the CME suggested it may not have ever driven a shock, the acceleration mechanism was not likely diffusive shock acceleration. Instead, they suggested it may be similar to that associated with aurora in planetary magnetospheres (e.g., Mitchell et al. 2009, and references therein). This study focused on two important observed aspects: the velocity dispersion profile and the composition of the SEP event. In the proposed mechanism, which was referred to as “the pressure cooker” (e.g., Gorney et al. 1985), energetic particles are confined below the CME in the solar corona in a region bound by an electric potential above and strong magnetic fields below. The electric field is the result of strong field-aligned electric currents associated with distorted magnetic fields and plasma flow, perhaps associated with magnetic reconnection, between the CME and corona during its eruption. Particles are confined in this region until their energy is sufficient to overcome the electric potential. There are two key results from this process. One is that the highest energy particles will overcome this barrier earlier, and, hence, will arrive at *PSP* earlier than low energy particles, which are presumably released much later when the CME has erupted from the Sun. The other is that the mechanism produces a maximum energy that depends on the charge of the species. Although the event was quite weak, there were sufficient counts of He, O, and Fe, that when combined with assumptions about the composition of these species in the corona, agreed with the observed high-energy cut-off as a function of particle species.
crossed *PSP*. There was no shock observed locally at *PSP*, and there is no clear evidence of local acceleration of SEPs at the CME crossing. It was suggested by Giacalone et al. (2020) that the CME briefly drove either a weak shock or a plasma compression when it was closer to the Sun, and was capable of accelerating particles which then propagated ahead of the CME and observed by *PSP*. In fact, modeling of the CME and local plasma parameters, also presented in this paper, suggested there may have been a weak shock over parts of the (modeled) CME-shock surface, but is not clear whether *PSP* was magnetically connected to these locations. The energetic particle event was characterized by a clear velocity dispersion in which higher-energy particles arrived well before the lower energy particles. Moreover, the time-intensity profiles at specific energies, seen in Fig. 39e, show a relatively smooth rise from the background to the peak, and a gradual decay. The particles were observed to be initially anisotropic, moving radially away from the Sun, but at the peak of the event were observed to be more isotropic. Giacalone et al. (2020) interpreted this in terms of the diffusive transport of particles accelerated by the CME starting about the time it was $7.5 R_⊙$ and continuing with time but with a decreasing intensity. They used a diffusive-transport model and fit the observed time-intensity profiles, which gave values for the scattering mean-free path, parallel to the magnetic field, of 30 keV to 100 keV protons to be from 0.04 – 0.09 AU at the location of *PSP*.

Another important feature of this event was the generally steep energy spectrum of the low-energy ions. This suggested a very weak event. In the comparison between the model used by Giacalone et al. (2020) and the observations, it was found that a source spectrum, assumed to be at the CME when it was close to the Sun, had an approximately $E^{-5.5}$ power-law dependence.

At the time, this was the closest to the Sun that a CME-related SEP event had been observed *in situ*. Giacalone et al. (2020) used their diffusive transport model to estimate the total fluence that this event would have had at 1 AU, in order to compare with previous observations of CME-related SEP events. It was determined that this event would have been so weak to not even appear on a figure showing a wide range of values of the SEP fluence as a function of CME speed produced by Kahler (2001).

Mitchell et al. (2020a) presented a separate analysis of this same CME-SEP event suggested an alternative acceleration mechanism. They noted that since *PSP* did not observe a shock locally, and modeling of the CME suggested it may not have ever driven a shock, the acceleration mechanism was not likely diffusive shock acceleration. Instead, they suggested it may be similar to that associated with aurora in planetary magnetospheres (e.g., Mitchell et al. 2009, and references therein). This study focused on two important observed aspects: the velocity dispersion profile and the composition of the SEP event. In the proposed mechanism, which was referred to as “the pressure cooker” (e.g., Gorney et al. 1985), energetic particles are confined below the CME in the solar corona in a region bound by an electric potential above and strong magnetic fields below. The electric field is the result of strong field-aligned electric currents associated with distorted magnetic fields and plasma flow, perhaps associated with magnetic reconnection, between the CME and corona during its eruption. Particles are confined in this region until their energy is sufficient to overcome the electric potential. There are two key results from this process. One is that the highest energy particles will overcome this barrier earlier, and, hence, will arrive at *PSP* earlier than low energy particles, which are presumably released much later when the CME has erupted from the Sun. The other is that the mechanism produces a maximum energy that depends on the charge of the species. Although the event was quite weak, there were sufficient counts of He, O, and Fe, that when combined with assumptions about the composition of these species in the corona, agreed with the observed high-energy cut-off as a function of particle species.
PSP was in a fortunate location, during a fortuitously quiet period, and provided a unique opportunity to study energetic particles accelerated by a very slow and weak CME closer to the Sun that had been see in situ previously. On the one hand, the observations suggests that very weak shocks, or even non-shock plasma compressions driven by a slow CME, are capable of accelerating particles. On the other hand, the pressure cooker method provides an interesting parallel with processes that occur in planetary ionospheres and magnetosphere. Moreover, the observation of the SEP event provided the opportunity to determine the parallel mean-free path of the particles, at 0.23 AU, as the particles were transported from their source to PSP.

In Mar. 2019, PSP encountered a SBO-CME with unique properties which was analyzed by Lario et al. (2020). SBO-CMEs are generally well-structured, slow CMEs that emerge from the streamer belt in extended PILs outside of ARs. Figure 40 shows an overview of the plasma, magnetic field, electron and energetic particle conditions associated with the CME. Despite the relatively low speed of the SBO-CME close to the Sun determined by remote observation from SOHO and STEREO-A (~311 km s⁻¹), the transit time to PSP indicated a faster speed and two shocks were observed at PSP prior to the arrival of the CME. The low initial speed of the SBO-CME makes it unlikely that it would have driven a shock in the corona and Lario et al. (2020) proposed that the formation of the shocks farther from the Sun were likely caused by compression effects of a HSS that followed the CME and that the formation of the two-shock structure may have been caused by distortions in the CME resulting from the HSS. This demonstrates the importance of the surrounding plasma conditions on the viability of energetic particle acceleration in CME events, though the associated energetic particle event in this case was limited to low energies <100 keV/nuc.

In order to determine the point at which PSP would have been magnetically connected to the CME, Lario et al. (2020) ran two ENLIL simulations, one with just ambient solar wind conditions and another including the CME. By evaluating the solar wind speed along the magnetic field line connecting PSP to the Sun, they find the point at which the solar wind speed in the CME simulation exceeds that of the ambient simulation, which establishes the point at which PSP is connected to the CME, termed the “Connecting with the OBserving” point or “cobpoint” (Heras et al. 1995). Figure 41 shows the coordinates of the cobpoint determined by this analysis alongside energetic particle anisotropy measurements. The energetic particle event is shown to be highly anisotropic, with enhanced particle intensities seen in the sunward-facing sensors of the instrument, and that the onset of energetic particles coincided with the establishment of the cobpoint, connecting the CME to PSP. Also notable is that the increase in the speed jump at the cobpoint coincides with an increase in the measured energetic particle intensities prior to shock arrival. This analysis demonstrates the importance of energetic particle measurements made by IS○IS in constraining modelling of large scale magnetic field structures such as CMEs.

Joyce et al. (2021b) analyzed a CME that was measured by PSP on 20 Jan. 2020, when the S/C was 0.32 AU from the Sun. The eruption of the CME was well imaged by both STEREO-A and SOHO and was observed to have a speed of ~380 km s⁻¹, consistent with the transit time to PSP, and possessed characteristics indicative of a stealth-type CME. Figure 42 shows a unique evolution of the energetic particle anisotropy during this event, with changes in the anisotropy seeming to coincide with changes of the normal component of the magnetic field (B_N). Of particular interest is a period where B_N is close to zero and the dominant anisotropy is of energetic particles propagating toward the Sun (highlighted in yellow in Fig. 42, as well as two periods when B_N spikes northward and there is a near complete dropout in energetic particle flux (highlighted in orange). The period dominated by particles propagating toward the Sun has the highest fluxes extending to the highest energies in the
PSP was in a fortunate location, during a fortuitously quiet period, and provided a unique opportunity to study energetic particles accelerated by a very slow and weak CME closer to the Sun that had been see in situ previously. On the one hand, the observations suggests that very weak shocks, or even non-shock plasma compressions driven by a slow CME, are capable of accelerating particles. On the other hand, the pressure cooker method provides an interesting parallel with processes that occur in planetary ionospheres and magnetosphere. Moreover, the observation of the SEP event provided the opportunity to determine the parallel mean-free path of the particles, at 0.23 AU, as the particles were transported from their source to PSP.

In Mar. 2019, PSP encountered a SBO-CME with unique properties which was analyzed by Lario et al. (2020). SBO-CMEs are generally well-structured, slow CMEs that emerge from the streamer belt in extended PILs outside of ARs. Figure 40 shows an overview of the plasma, magnetic field, electron and energetic particle conditions associated with the CME. Despite the relatively low speed of the SBO-CME close to the Sun determined by remote observation from SOHO and STEREO-A (~311 km s⁻¹), the transit time to PSP indicated a faster speed and two shocks were observed at PSP prior to the arrival of the CME. The low initial speed of the SBO-CME makes it unlikely that it would have driven a shock in the corona and Lario et al. (2020) proposed that the formation of the shocks farther from the Sun were likely caused by compression effects of a HSS that followed the CME and that the formation of the two-shock structure may have been caused by distortions in the CME resulting from the HSS. This demonstrates the importance of the surrounding plasma conditions on the viability of energetic particle acceleration in CME events, though the associated energetic particle event in this case was limited to low energies <100 keV/nuc.

In order to determine the point at which PSP would have been magnetically connected to the CME, Lario et al. (2020) ran two ENLIL simulations, one with just ambient solar wind conditions and another including the CME. By evaluating the solar wind speed along the magnetic field line connecting PSP to the Sun, they find the point at which the solar wind speed in the CME simulation exceeds that of the ambient simulation, which establishes the point at which PSP is connected to the CME, termed the “Connecting with the OBserving” point or “cobpoint” (Heras et al. 1995). Figure 41 shows the coordinates of the cobpoint determined by this analysis alongside energetic particle anisotropy measurements. The energetic particle event is shown to be highly anisotropic, with enhanced particle intensities seen in the sunward-facing sensors of the instrument, and that the onset of energetic particles coincided with the establishment of the cobpoint, connecting the CME to PSP. Also notable is that the increase in the speed jump at the cobpoint coincides with an increase in the measured energetic particle intensities prior to shock arrival. This analysis demonstrates the importance of energetic particle measurements made by IS○IS in constraining modelling of large scale magnetic field structures such as CMEs.

Joyce et al. (2021b) analyzed a CME that was measured by PSP on 20 Jan. 2020, when the S/C was 0.32 AU from the Sun. The eruption of the CME was well imaged by both STEREO-A and SOHO and was observed to have a speed of ~380 km s⁻¹, consistent with the transit time to PSP, and possessed characteristics indicative of a stealth-type CME. Figure 42 shows a unique evolution of the energetic particle anisotropy during this event, with changes in the anisotropy seeming to coincide with changes of the normal component of the magnetic field (B_N). Of particular interest is a period where B_N is close to zero and the dominant anisotropy is of energetic particles propagating toward the Sun (highlighted in yellow in Fig. 42, as well as two periods when B_N spikes northward and there is a near complete dropout in energetic particle flux (highlighted in orange). The period dominated by particles propagating toward the Sun has the highest fluxes extending to the highest energies in the
<figure><img src="image_42.png" /><figcaption>**Fig. 40** Overview of plasma (measured by SWEAP), magnetic field (measured by FIELDS), electron (SWEAP) and energetic particle (measured by IS◎IS) conditions during the Mar. 2019 SBO-CME event observed by PSP. From top to bottom: (a) radial velocity, (b)tangential velocity component, and (c) normal velocity components of the solar wind proton velocity in RTN coordinates, (d) solar wind proton density, (e) solar wind proton temperature, magnetic field (f) magnitude, (g) elevation and (h) azimuth angles in RTN coordinates, (i) proton plasma beta, (j) the sum of the magnetic and thermal pressures, (k) ram pressure, (l) 314 eV electron PADs, (m) normalized 314 eV electron PADs, and (n) ~30–500 keV TOF-only ion intensities measured by IS◎IS/EPI-Lo. The vertical solid line indicates the passing of the two shocks associated with the CME which are too close in time to be separately resolved here, the vertical dashed lines indicate the boundaries of the CME, and the blue arrow indicates the eruption time of the SBO-CME at the Sun. Figure adapted from Lario et al. (2020)</figcaption></figure>

event and Joyce et al. (2021b) argued that this may be evidence that PSP, located on the western flank of the CME throughout the event, may have briefly been connected to a region
<figure><img src="image_43.png" /><figcaption>**Fig. 40** Overview of plasma (measured by SWEAP), magnetic field (measured by FIELDS), electron (SWEAP) and energetic particle (measured by IS◎IS) conditions during the Mar. 2019 SBO-CME event observed by PSP. From top to bottom: (a) radial velocity, (b)tangential velocity component, and (c) normal velocity components of the solar wind proton velocity in RTN coordinates, (d) solar wind proton density, (e) solar wind proton temperature, magnetic field (f) magnitude, (g) elevation and (h) azimuth angles in RTN coordinates, (i) proton plasma beta, (j) the sum of the magnetic and thermal pressures, (k) ram pressure, (l) 314 eV electron PADs, (m) normalized 314 eV electron PADs, and (n) ~30–500 keV TOF-only ion intensities measured by IS◎IS/EPI-Lo. The vertical solid line indicates the passing of the two shocks associated with the CME which are too close in time to be separately resolved here, the vertical dashed lines indicate the boundaries of the CME, and the blue arrow indicates the eruption time of the SBO-CME at the Sun. Figure adapted from Lario et al. (2020)</figcaption></figure>

event and Joyce et al. (2021b) argued that this may be evidence that PSP, located on the western flank of the CME throughout the event, may have briefly been connected to a region
<figure><img src="image_43.png" /><figcaption>**Fig. 41** Cobpoint characteristics as determined from ENLIL simulations and TOF-only energetic ion data measured by IS○IS/EPI-Lo. From top to bottom: (a) heliocentric radial distance of PSP cobpoint, (b) Heliocentric Earth Equatorial (HEEQ) longitude of PSP cobpoint, (c) speed jump ratio measured at PSP cobpoint by comparing the ENLIL background simulation to the simulation including the CME, (d) speed of the cobpoint, (e) ion intensities ~20–500, (f) ion intensities measured in the Sun-facing wedges of EPI-Lo, (g) ion intensities measured in the EPI-Lo wedges facing away from the Sun, (h) ion intensities measured in the transverse wedges of EPI-Lo. The verticle solid line indicate the passage of the two shocks, the vertical dotted lines show the boundaries of the CME, the vertical purple dashed lie indicates the time when PSP became connected to the compression region in front of the CME, and the purple vertical arrows indicate the time that the SBO-CME was accelerated at the Sun. Figure adapted from Lario et al. (2020)</figcaption></figure>
<figure><img src="image_44.png" /><figcaption>**Fig. 41** Cobpoint characteristics as determined from ENLIL simulations and TOF-only energetic ion data measured by IS○IS/EPI-Lo. From top to bottom: (a) heliocentric radial distance of PSP cobpoint, (b) Heliocentric Earth Equatorial (HEEQ) longitude of PSP cobpoint, (c) speed jump ratio measured at PSP cobpoint by comparing the ENLIL background simulation to the simulation including the CME, (d) speed of the cobpoint, (e) ion intensities ~20–500, (f) ion intensities measured in the Sun-facing wedges of EPI-Lo, (g) ion intensities measured in the EPI-Lo wedges facing away from the Sun, (h) ion intensities measured in the transverse wedges of EPI-Lo. The verticle solid line indicate the passage of the two shocks, the vertical dotted lines show the boundaries of the CME, the vertical purple dashed lie indicates the time when PSP became connected to the compression region in front of the CME, and the purple vertical arrows indicate the time that the SBO-CME was accelerated at the Sun. Figure adapted from Lario et al. (2020)</figcaption></figure>
<figure><img src="image_44.png" /><figcaption>**Fig. 42** Overview of energetic particle anisotropy and magnetic field conditions during the Jan. 2020 CME. Energetic particle measurements are from the TOF-only channel of IS⊙IS/EPI-Lo and magnetic field data are from FIELDS. From top to bottom: omnidirectional ion spectrogram, ion spectrum from Sun (0 – 60° from nominal Parker spiral direction), ion spectrogram toward the Sun (120 – 180°), ion spectrogram in the transverse direction (60 – 120°), and the magnetic field vector in RTN coordinates, with the magnetic field magnitude in black. The period highlighted in yellow shows a strong influx of particles propagating toward the Sun, while periods of energetic particle dropouts are highlighted in orange. Figure adapted from Joyce et al. (2021b)</figcaption></figure>

of stronger energetic particle acceleration, likely closer to the nose of the CME where the compression is likely strongest.

STEREO-A was well-aligned radially with PSP during this time period and observed the same CME also from the western flank. Figure 43 shows the comparison between energetic particle spectrograms and magnetic field vectors measured by both instruments. Particularly striking is the remarkable similarity of the magnetic field vector measured by both S/C, suggesting that they both encountered a very similar region of the magnetic structure, contrasted with the dissimilarity of the energetic particle observations, with those at STEREO-A lacking the fine detail and abrupt changes in anisotropy (not shown here) that are seen closer to the Sun. This is likely due to transport effects such as scattering and diffusion which have created a much more uniform distribution of energetic particles by the time the CME has reached 1 AU. This demonstrates the importance of measurements of such events close to the Sun, made possible by PSP/IS⊙IS, when it is still possible to distinguish between different acceleration mechanisms and source regions that contribute to energetic particle populations before these fine distinctions are washed out by transport effects. Such detailed measurements will be critical in determining which mechanisms play an important role in the acceleration of energetic particles close to the Sun.
<figure><img src="image_45.png" /><figcaption>**Fig. 42** Overview of energetic particle anisotropy and magnetic field conditions during the Jan. 2020 CME. Energetic particle measurements are from the TOF-only channel of IS⊙IS/EPI-Lo and magnetic field data are from FIELDS. From top to bottom: omnidirectional ion spectrogram, ion spectrum from Sun (0 – 60° from nominal Parker spiral direction), ion spectrogram toward the Sun (120 – 180°), ion spectrogram in the transverse direction (60 – 120°), and the magnetic field vector in RTN coordinates, with the magnetic field magnitude in black. The period highlighted in yellow shows a strong influx of particles propagating toward the Sun, while periods of energetic particle dropouts are highlighted in orange. Figure adapted from Joyce et al. (2021b)</figcaption></figure>

of stronger energetic particle acceleration, likely closer to the nose of the CME where the compression is likely strongest.

STEREO-A was well-aligned radially with PSP during this time period and observed the same CME also from the western flank. Figure 43 shows the comparison between energetic particle spectrograms and magnetic field vectors measured by both instruments. Particularly striking is the remarkable similarity of the magnetic field vector measured by both S/C, suggesting that they both encountered a very similar region of the magnetic structure, contrasted with the dissimilarity of the energetic particle observations, with those at STEREO-A lacking the fine detail and abrupt changes in anisotropy (not shown here) that are seen closer to the Sun. This is likely due to transport effects such as scattering and diffusion which have created a much more uniform distribution of energetic particles by the time the CME has reached 1 AU. This demonstrates the importance of measurements of such events close to the Sun, made possible by PSP/IS⊙IS, when it is still possible to distinguish between different acceleration mechanisms and source regions that contribute to energetic particle populations before these fine distinctions are washed out by transport effects. Such detailed measurements will be critical in determining which mechanisms play an important role in the acceleration of energetic particles close to the Sun.
<figure><img src="image_45.png" /><figcaption>**Fig. 43** Comparison of energetic particle and magnetic field measurements of the same CME event observed at both PSP and STEREO-A. The data have been lined up by the arrival time of the CME arrival and the PSP data have been stretched in time by a factor of 1.3 to match the magnetic field features seen by both S/C. Gray dotted lines indicate reference points used to line up the measurements from both S/C</figcaption></figure>

10.1.1 The Widespread CME Event on 29 Nov. 2020

The beginning of solar cycle 25 was marked by a significant SEP event in late Nov. 2020. The event has gained substantial attention and study, not only as one of the largest SEP events in several quiet years, but also because it was a circumsolar event spanning at least 230° in longitude and observed by four S/C positioned at or inside of 1 AU (see Fig. 44). Among those S/C was PSP and SolO, providing a first glimpse of coordinated studies that will be possible between the two missions. The solar source was AR 12790 and the associated M4.4 class flare (as observed by GOES at 12:34 UT on 29 Nov.) was at (E99°,S23°) (as viewed from Earth), 2° east of PSP's solar longitude. A CME traveling at ~1700 km s⁻¹ was well observed by SOHO/LASCO and STEREO-A/COR2, both positioned west of PSP (Cohen et al. 2021b). STEREO-A/EUVI also observed an EUV wave propagating away from the source at ~500 km s⁻¹, lasting about an hour and traversing much of the visible disk (Kollhoff et al. 2021).

Protons at energies > 50 MeV and > 1 MeV electrons were observed by PSP, STEREO-A, SOHO, and SolO with onsets and time profiles that were generally organized by the S/C's longitude relative to the source region, as has been seen in multi-S/C events from previous solar cycles (Kollhoff et al. 2021). However, it was clear that intervening solar wind structures such as a slower preceding CME and SIRs affected the temporal evolution of the particle intensities. Analysis of the onset times of the protons and electrons observed at the four S/C yielded solar release times that were compared to the EUV wave propagation. The results were inconsistent with a simplistic scenario of particles being released when the
<figure><img src="image_46.png" /><figcaption>**Fig. 43** Comparison of energetic particle and magnetic field measurements of the same CME event observed at both PSP and STEREO-A. The data have been lined up by the arrival time of the CME arrival and the PSP data have been stretched in time by a factor of 1.3 to match the magnetic field features seen by both S/C. Gray dotted lines indicate reference points used to line up the measurements from both S/C</figcaption></figure>

### 10.1.1 The Widespread CME Event on 29 Nov. 2020

The beginning of solar cycle 25 was marked by a significant SEP event in late Nov. 2020. The event has gained substantial attention and study, not only as one of the largest SEP events in several quiet years, but also because it was a circumsolar event spanning at least 230° in longitude and observed by four S/C positioned at or inside of 1 AU (see Fig. 44). Among those S/C was PSP and SolO, providing a first glimpse of coordinated studies that will be possible between the two missions. The solar source was AR 12790 and the associated M4.4 class flare (as observed by GOES at 12:34 UT on 29 Nov.) was at (E99°,S23°) (as viewed from Earth), 2° east of PSP's solar longitude. A CME traveling at ~1700 km s⁻¹ was well observed by SOHO/LASCO and STEREO-A/COR2, both positioned west of PSP (Cohen et al. 2021b). STEREO-A/EUVI also observed an EUV wave propagating away from the source at ~500 km s⁻¹, lasting about an hour and traversing much of the visible disk (Kollhoff et al. 2021).

Protons at energies > 50 MeV and > 1 MeV electrons were observed by PSP, STEREO-A, SOHO, and SolO with onsets and time profiles that were generally organized by the S/C's longitude relative to the source region, as has been seen in multi-S/C events from previous solar cycles (Kollhoff et al. 2021). However, it was clear that intervening solar wind structures such as a slower preceding CME and SIRs affected the temporal evolution of the particle intensities. Analysis of the onset times of the protons and electrons observed at the four S/C yielded solar release times that were compared to the EUV wave propagation. The results were inconsistent with a simplistic scenario of particles being released when the
<figure><img src="image_46.png" /><figcaption>**Fig. 44** Overview of the widespread CME event on 19 Nov. 2020. Counterclockwise from the top right: *SolO*, *PSP*, *STEREO-A*, and *ACE* energetic particle observations are shown along with the relative location of all S/C. The direction of the CME is given by the black array, while curved lines in the orbit plot indicate nominal Parker Spiral magnetic field lines each S/C would be connected to. Figure adapted from Kollhoff et al. (2021)</figcaption></figure>

EUV wave arrived at the various S/C magnetic footpoints, suggesting more complex particle transport and/or acceleration process.

Heavy ions, including He, O and Fe, were observed by PSP, STEREO-A, ACE and SolO and their event-integrated fluences had longitudinal spreads similar to those obtained from three-S/C events observed in cycle 24 (Mason et al. 2021a). The spectra were all well described by power-laws at low energies followed by an exponential roll-over at higher energies (Fig. 45). The roll-over energy was element dependent such that Fe/O and He/H ratios decreased with increasing energy; a signature of shock-acceleration that is commonly seen in SEP events (Cohen et al. 2021b; Mason et al. 2021a). The overall composition (relative to
<figure><img src="image_47.png" /><figcaption>**Fig. 44** Overview of the widespread CME event on 19 Nov. 2020. Counterclockwise from the top right: *SolO*, *PSP*, *STEREO-A*, and *ACE* energetic particle observations are shown along with the relative location of all S/C. The direction of the CME is given by the black array, while curved lines in the orbit plot indicate nominal Parker Spiral magnetic field lines each S/C would be connected to. Figure adapted from Kollhoff et al. (2021)</figcaption></figure>

EUV wave arrived at the various S/C magnetic footpoints, suggesting more complex particle transport and/or acceleration process.

Heavy ions, including He, O and Fe, were observed by PSP, STEREO-A, ACE and SolO and their event-integrated fluences had longitudinal spreads similar to those obtained from three-S/C events observed in cycle 24 (Mason et al. 2021a). The spectra were all well described by power-laws at low energies followed by an exponential roll-over at higher energies (Fig. 45). The roll-over energy was element dependent such that Fe/O and He/H ratios decreased with increasing energy; a signature of shock-acceleration that is commonly seen in SEP events (Cohen et al. 2021b; Mason et al. 2021a). The overall composition (relative to