<figure><img src="image_47.png" /><figcaption>**Fig. 45** Multi-species fluence spectra of the 29 Nov. 2020 event from (a) PSP, (b) STEREO-A, (c) ACE, and (d) SolO. Figure adapted from Mason et al. (2021a)</figcaption></figure>

O) at 0.32–0.45 MeV/nuc was fairly typical of events this size, with the exception of Fe/O at PSP and ACE where it was depleted by a factor of ~2 (Mason et al. 2021a).

Due to the relative positioning of the source region and PSP, the CME passed directly over the S/C. It was traveling fast enough to overtake a preceding, slower CME in close proximity to PSP, creating a dynamic and evolving shock measured by FIELDS (Giacalone et al. 2021; Nieves-Chinchilla et al. 2022). Coincident with this shock IS⊙IS observed a substantial increase in protons up to at least 1 MeV, likely due to local acceleration. More surprisingly, an increase in energetic electrons was also measured at the shock passage (Fig. 44). Acceleration of electrons by interplanetary shocks is rare (Dresing et al. 2016), thus it is more likely this increase is a consequence of a trapped electron distribution, perhaps caused by the narrowing region between the two CMEs (Cohen et al. 2021b).

The MC of the fast CME followed the shock and sheath region, with a clear rotation seen in the magnetic field components measured by FIELDS (Fig. 46). At the onset of the cloud, the particle intensities dropped as is often seen due to particles being unable to cross into the magnetic structure (Fig. 46). During this period there was a 30 minute interval in which all the particle intensities increased briefly to approximately their pre-cloud levels. This was likely the result of PSP exiting the MC, observing the surrounding environment populated with SEPs, and then returning to the interior of the MC.

Although several of the properties of the SEP event are consistent with those seen in previous studies, the 29 Nov. 2020 event is noteworthy as being observed by four S/C over 230°
<figure><img src="image_48.png" /><figcaption>**Fig. 45** Multi-species fluence spectra of the 29 Nov. 2020 event from (a) PSP, (b) STEREO-A, (c) ACE, and (d) SolO. Figure adapted from Mason et al. (2021a)</figcaption></figure>

O) at 0.32–0.45 MeV/nuc was fairly typical of events this size, with the exception of Fe/O at PSP and ACE where it was depleted by a factor of ~2 (Mason et al. 2021a).

Due to the relative positioning of the source region and PSP, the CME passed directly over the S/C. It was traveling fast enough to overtake a preceding, slower CME in close proximity to PSP, creating a dynamic and evolving shock measured by FIELDS (Giacalone et al. 2021; Nieves-Chinchilla et al. 2022). Coincident with this shock IS⊙IS observed a substantial increase in protons up to at least 1 MeV, likely due to local acceleration. More surprisingly, an increase in energetic electrons was also measured at the shock passage (Fig. 44). Acceleration of electrons by interplanetary shocks is rare (Dresing et al. 2016), thus it is more likely this increase is a consequence of a trapped electron distribution, perhaps caused by the narrowing region between the two CMEs (Cohen et al. 2021b).

The MC of the fast CME followed the shock and sheath region, with a clear rotation seen in the magnetic field components measured by FIELDS (Fig. 46). At the onset of the cloud, the particle intensities dropped as is often seen due to particles being unable to cross into the magnetic structure (Fig. 46). During this period there was a 30 minute interval in which all the particle intensities increased briefly to approximately their pre-cloud levels. This was likely the result of PSP exiting the MC, observing the surrounding environment populated with SEPs, and then returning to the interior of the MC.

Although several of the properties of the SEP event are consistent with those seen in previous studies, the 29 Nov. 2020 event is noteworthy as being observed by four S/C over 230°
<figure><img src="image_48.png" /><figcaption>**Fig. 46** Time profile of energetic protons stopping in the third and fifth detector of HET (top panel, upper and lower traces, respectively) and electrons stopping in the third and fourth detector of HET (middle panel, upper and lower traces, respectively) with the magnetic field in the bottom panel for the 29 Nov. 2020 CME. The over plotted verticle lines illustrate that the variations in the particle count rates occur at the same time as changes in the magnetic field. See Cohen et al. (2021b) for more information</figcaption></figure>

longitude and as the first significant cycle 25 event. The details of many aspects of the event (both from individual S/C and multi-S/C observations) remain to be studied more closely. In addition, modeling of the event has only just begun and will likely yield significant insights regarding the evolution of the CME-associated shock wave (Kouloumvakos et al. 2022) and the acceleration and transport of the SEPs throughout the inner heliosphere (Caplan et al. 2021).

## 10.2 Energetic Electrons

The first observations of energetic electrons by PSP/IS©IS were reported by Mitchell et al. (2020b), who analyzed a series of energetic electron enhancements observed during PSP's second Enc. period, which reached a perihelion of 0.17 AU. Figure 47 shows a series of four electron events that were observed at approximately 03:00, 05:00, 09:00, and 15:40 UT on 2 Apr. 2019. The events are small compared with the background and are only observable due to the small heliocentric distance of PSP during this time. Background subtraction is applied to the electron rates data to help resolve the electron enhancements as well as a second-degree Savitzky-Golay smoothing filter over 7 minutes is applied to reduce random statistical fluctuations (Savitzky and Golay 1964). While the statistics for these events are very low, the fact that they are observed concurrently in both EPI-Hi and EPI-Lo and that they coincide with either abrupt changes in the magnetic field vector or that they can plausibly be connected to type III radio bursts observed by the FIELDS instrument which extend down to $f_{pe}$ suggests that these are real electron events. These are the first energetic electron events which have been observed within 0.2 AU of the Sun and suggest that such small and short-duration electron events may be a common feature close to the Sun that was not previously appreciated since it would not be possible to observe such events farther
<figure><img src="image_49.png" /><figcaption>**Fig. 46** Time profile of energetic protons stopping in the third and fifth detector of HET (top panel, upper and lower traces, respectively) and electrons stopping in the third and fourth detector of HET (middle panel, upper and lower traces, respectively) with the magnetic field in the bottom panel for the 29 Nov. 2020 CME. The over plotted verticle lines illustrate that the variations in the particle count rates occur at the same time as changes in the magnetic field. See Cohen et al. (2021b) for more information</figcaption></figure>

longitude and as the first significant cycle 25 event. The details of many aspects of the event (both from individual S/C and multi-S/C observations) remain to be studied more closely. In addition, modeling of the event has only just begun and will likely yield significant insights regarding the evolution of the CME-associated shock wave (Kouloumvakos et al. 2022) and the acceleration and transport of the SEPs throughout the inner heliosphere (Caplan et al. 2021).

## 10.2 Energetic Electrons

The first observations of energetic electrons by PSP/IS©IS were reported by Mitchell et al. (2020b), who analyzed a series of energetic electron enhancements observed during PSP's second Enc. period, which reached a perihelion of 0.17 AU. Figure 47 shows a series of four electron events that were observed at approximately 03:00, 05:00, 09:00, and 15:40 UT on 2 Apr. 2019. The events are small compared with the background and are only observable due to the small heliocentric distance of PSP during this time. Background subtraction is applied to the electron rates data to help resolve the electron enhancements as well as a second-degree Savitzky-Golay smoothing filter over 7 minutes is applied to reduce random statistical fluctuations (Savitzky and Golay 1964). While the statistics for these events are very low, the fact that they are observed concurrently in both EPI-Hi and EPI-Lo and that they coincide with either abrupt changes in the magnetic field vector or that they can plausibly be connected to type III radio bursts observed by the FIELDS instrument which extend down to $f_{pe}$ suggests that these are real electron events. These are the first energetic electron events which have been observed within 0.2 AU of the Sun and suggest that such small and short-duration electron events may be a common feature close to the Sun that was not previously appreciated since it would not be possible to observe such events farther
<figure><img src="image_49.png" /><figcaption>**Fig. 47** Overview of PSP observations during 2 Apr. 2019. Panels show the following: (a) EPI-Hi electron count rate (0.5–6 MeV) with a background subtraction and 7 minutes Savitzky–Golay smoothing applied and with a dashed line to indicate $2\sigma$ deviation from the mean, (b) EPI-Lo electron count rate (50–500 keV) with a background subtraction and 7 minutes Savitzky–Golay smoothing applied and with a dashed line to indicate $2\sigma$ deviation from the mean, (c) FIELDS high-frequency radio measurements (1.3–19.2 MHz), (d) FIELDS low-frequency radio measurements (10.5 kHz–1.7 MHz), (e) SWEAP solar wind ion density (~5 measurements per second), (f) SWEAP radial solar wind speed (~5 measurements per second), (g) FIELDS 1 minute magnetic field vector in RTN coordinates (with magnetic field strength denoted by the black line). A series of electron events are observed (in the top two panels), occurring at approximately 03:00, 05:00, 09:00, and 15:40 UT, as well as a series of strong type III radio bursts (seen in panels c and d). Figure adapted from Mitchell et al. (2020b)</figcaption></figure>

out from the Sun. This is consistent with previous observations by *Helios* between 0.3 and 1 AU (Wibberenz and Cane 2006). More observations and further analysis are needed to determine what physical acceleration mechanisms may be able to produce such events.

In late Nov. 2020, *PSP* measured an SEP event associated with two CME eruptions, when the S/C was at approximately 0.8 AU. This event is the largest SEP event observed during the first 8 orbits of *PSP*, producing the highest ion fluxes yet observed by IS©IS (Cohen et al. 2021b; Giacalone et al. 2021; Kollhoff et al. 2021; Mason et al. 2021a), and also produced the first energetic electron events capable of producing statistics sufficient to register significant anisotropy measurements by IS©IS as reported by Mitchell et al. (2021). Figure 48 shows an overview of the electron observations during this period along with magnetic field data to provide context. Notable in these observations is the peaking of the EPI-Lo electron count rate at the passage of the second CME shock, which is quite rare, though not unheard of, due to the inefficiency of CME driven shocks in accelerating electrons. This may indicate the importance of quasi-perpendicular shock acceleration in this event, which has been shown to be a more efficient acceleration of electrons (Wu 1984; Krauss-Varban et al. 1989;
<figure><img src="image_50.png" /><figcaption>**Fig. 47** Overview of PSP observations during 2 Apr. 2019. Panels show the following: (a) EPI-Hi electron count rate (0.5–6 MeV) with a background subtraction and 7 minutes Savitzky–Golay smoothing applied and with a dashed line to indicate $2\sigma$ deviation from the mean, (b) EPI-Lo electron count rate (50–500 keV) with a background subtraction and 7 minutes Savitzky–Golay smoothing applied and with a dashed line to indicate $2\sigma$ deviation from the mean, (c) FIELDS high-frequency radio measurements (1.3–19.2 MHz), (d) FIELDS low-frequency radio measurements (10.5 kHz–1.7 MHz), (e) SWEAP solar wind ion density (~5 measurements per second), (f) SWEAP radial solar wind speed (~5 measurements per second), (g) FIELDS 1 minute magnetic field vector in RTN coordinates (with magnetic field strength denoted by the black line). A series of electron events are observed (in the top two panels), occurring at approximately 03:00, 05:00, 09:00, and 15:40 UT, as well as a series of strong type III radio bursts (seen in panels c and d). Figure adapted from Mitchell et al. (2020b)</figcaption></figure>

out from the Sun. This is consistent with previous observations by *Helios* between 0.3 and 1 AU (Wibberenz and Cane 2006). More observations and further analysis are needed to determine what physical acceleration mechanisms may be able to produce such events.

In late Nov. 2020, *PSP* measured an SEP event associated with two CME eruptions, when the S/C was at approximately 0.8 AU. This event is the largest SEP event observed during the first 8 orbits of *PSP*, producing the highest ion fluxes yet observed by IS©IS (Cohen et al. 2021b; Giacalone et al. 2021; Kollhoff et al. 2021; Mason et al. 2021a), and also produced the first energetic electron events capable of producing statistics sufficient to register significant anisotropy measurements by IS©IS as reported by Mitchell et al. (2021). Figure 48 shows an overview of the electron observations during this period along with magnetic field data to provide context. Notable in these observations is the peaking of the EPI-Lo electron count rate at the passage of the second CME shock, which is quite rare, though not unheard of, due to the inefficiency of CME driven shocks in accelerating electrons. This may indicate the importance of quasi-perpendicular shock acceleration in this event, which has been shown to be a more efficient acceleration of electrons (Wu 1984; Krauss-Varban et al. 1989;
<figure><img src="image_50.png" /><figcaption>**Fig. 48** Overview of electron observations associated with the Nov. 29 CMEs Panels are as follows: (a) shows the EPI-Hi electron count rates summed in all 5 apertures (~0.5 – 2 MeV), (b) shows the EPI-Lo electron count rate from wedges 3 and 7 (~57 – 870 keV), (c) shows the FIELDS magnetic field vector in RTN coordinates, and (d) and (e) show the magnetic field vector angles. Vertical lines show the eruption of the second CME and the passage of the shocks associated with both CMEs. Flux-rope like structures are indicated by the shaded grey regions. The decrease in the EPI-Hi electron count rate seen at the passage of the second CME and the overall flat profile are artifacts caused by EPI-Hi dynamic threshold mode changes (explained in detail by Cohen et al. 2021). Figure adapted from Mitchell et al. (2021)</figcaption></figure>

Holman and Pesses 1983; Guo and Giacalone 2010, 2012; Jokipii and Giacalone 2007). The notable dip in the EPI-Hi electron count rate at this time is an artifact associated with dynamic threshold mode changes of the EPI-Hi instrument during this time (for details, see Cohen et al. 2021b).

Figure 49 shows the electron and magnetic field measurements during a three-hour period around the shock crossing associated with the second CME, including the electron PAD. Because of the off-nominal pointing of the S/C during this time, the pitch angle coverage is somewhat limited, however the available data shows that the highest intensities to be in the range of ~40 – 90° at the time of the shock crossing. Distributions with peak intensities at pitch angles of around 90° may be indicative of the shock-drift acceleration mechanism that occurs at quasi-perpendicular shocks (e.g., Jokipii and Giacalone 2007; Sarris and van Allen 1974; Marhavilas et al. 2003). This, along with the peak electron intensities seen at the shock crossing, further supports the proposition that electrons may be efficiently accelerated by quasi-perpendicular shocks associated with CMEs. Other possible explanations are that the peak intensities may be a result of an enhanced electron seed population produced by the preceding CME (similar to observations by Dresing et al. 2016), that energetic electrons may be accelerated as a result of being trapped between the shocks driven by the two CMEs (a mechanism proposed by Dresing et al. 2018), and that enhanced magnetic fluctuations
<figure><img src="image_51.png" /><figcaption>**Fig. 48** Overview of electron observations associated with the Nov. 29 CMEs Panels are as follows: (a) shows the EPI-Hi electron count rates summed in all 5 apertures (~0.5 – 2 MeV), (b) shows the EPI-Lo electron count rate from wedges 3 and 7 (~57 – 870 keV), (c) shows the FIELDS magnetic field vector in RTN coordinates, and (d) and (e) show the magnetic field vector angles. Vertical lines show the eruption of the second CME and the passage of the shocks associated with both CMEs. Flux-rope like structures are indicated by the shaded grey regions. The decrease in the EPI-Hi electron count rate seen at the passage of the second CME and the overall flat profile are artifacts caused by EPI-Hi dynamic threshold mode changes (explained in detail by Cohen et al. 2021). Figure adapted from Mitchell et al. (2021)</figcaption></figure>

Holman and Pesses 1983; Guo and Giacalone 2010, 2012; Jokipii and Giacalone 2007). The notable dip in the EPI-Hi electron count rate at this time is an artifact associated with dynamic threshold mode changes of the EPI-Hi instrument during this time (for details, see Cohen et al. 2021b).

Figure 49 shows the electron and magnetic field measurements during a three-hour period around the shock crossing associated with the second CME, including the electron PAD. Because of the off-nominal pointing of the S/C during this time, the pitch angle coverage is somewhat limited, however the available data shows that the highest intensities to be in the range of ~40 – 90° at the time of the shock crossing. Distributions with peak intensities at pitch angles of around 90° may be indicative of the shock-drift acceleration mechanism that occurs at quasi-perpendicular shocks (e.g., Jokipii and Giacalone 2007; Sarris and van Allen 1974; Marhavilas et al. 2003). This, along with the peak electron intensities seen at the shock crossing, further supports the proposition that electrons may be efficiently accelerated by quasi-perpendicular shocks associated with CMEs. Other possible explanations are that the peak intensities may be a result of an enhanced electron seed population produced by the preceding CME (similar to observations by Dresing et al. 2016), that energetic electrons may be accelerated as a result of being trapped between the shocks driven by the two CMEs (a mechanism proposed by Dresing et al. 2018), and that enhanced magnetic fluctuations
<figure><img src="image_51.png" /><figcaption>**Fig. 49** Electron and magnetic field measurements around the time of the second CME shock crossing (indicated by the vertical red dashed line). Panels show the following: (a) EPI-Lo electron measurements (~130–870 keV) in each of its 8 wedges, (b) FIELDS magnetic field vector in RTN coordinates, (c) azimuthal angle of the magnetic field, (d) polar angle of the magnetic field, (e) pitch angle of the geometric center of each EPI-Lo wedge (each with a width of ~30°), and (f) pitch angle time series for each EPI-Lo wedge (80–870 keV) showing the fraction at each time bin to the total count rate over the entire interval. Figure adapted from Mitchell et al. (2021)</figcaption></figure>

and turbulence created upstream of the shock by the first CME may increase the efficiency of electron acceleration in the shock (as proposed by Guo and Giacalone 2015).

While observations of energetic electron events thus far in the PSP mission have been few, the measurements that have been made have shown IS⊙IS to be quite capable of characterizing energetic electron populations. Because of its close proximity to the Sun during PSP's Enc. phases, IS⊙IS has been shown to be able to measure small, subtle events which are not measurable farther from the Sun, but which may provide new insights into electron acceleration close to the Sun. The demonstrated ability to provide detailed electron anisotropy analyses is also critical for determining the acceleration mechanisms for electrons (particularly close to the Sun where transport effects have not yet influenced these populations) and for providing insight into the magnetic topology of magnetic structures associated with SEP events. As the current trend of increasing solar activity continues, we can expect many more unique observations and discoveries related to energetic electron events in the inner heliosphere from PSP/IS⊙IS.

## 10.3 Coronating/Stream Interaction Region-Associated Energetic Particles

SIRs form where HSSs from coronal holes expand into slower solar wind (Belcher and Davis 1971). As the coronal hole structure corotates on the Sun, the SIR will corotate, as well, and becoming a CIR after one complete solar co-rotation. As the HSS flows radially
<figure><img src="image_52.png" /><figcaption>**Fig. 49** Electron and magnetic field measurements around the time of the second CME shock crossing (indicated by the vertical red dashed line). Panels show the following: (a) EPI-Lo electron measurements (~130–870 keV) in each of its 8 wedges, (b) FIELDS magnetic field vector in RTN coordinates, (c) azimuthal angle of the magnetic field, (d) polar angle of the magnetic field, (e) pitch angle of the geometric center of each EPI-Lo wedge (each with a width of ~30°), and (f) pitch angle time series for each EPI-Lo wedge (80–870 keV) showing the fraction at each time bin to the total count rate over the entire interval. Figure adapted from Mitchell et al. (2021)</figcaption></figure>

and turbulence created upstream of the shock by the first CME may increase the efficiency of electron acceleration in the shock (as proposed by Guo and Giacalone 2015).

While observations of energetic electron events thus far in the PSP mission have been few, the measurements that have been made have shown IS⊙IS to be quite capable of characterizing energetic electron populations. Because of its close proximity to the Sun during PSP's Enc. phases, IS⊙IS has been shown to be able to measure small, subtle events which are not measurable farther from the Sun, but which may provide new insights into electron acceleration close to the Sun. The demonstrated ability to provide detailed electron anisotropy analyses is also critical for determining the acceleration mechanisms for electrons (particularly close to the Sun where transport effects have not yet influenced these populations) and for providing insight into the magnetic topology of magnetic structures associated with SEP events. As the current trend of increasing solar activity continues, we can expect many more unique observations and discoveries related to energetic electron events in the inner heliosphere from PSP/IS⊙IS.

## 10.3 Coronating/Stream Interaction Region-Associated Energetic Particles

SIRs form where HSSs from coronal holes expand into slower solar wind (Belcher and Davis 1971). As the coronal hole structure corotates on the Sun, the SIR will corotate, as well, and becoming a CIR after one complete solar co-rotation. As the HSS flows radially
outward, both forward and reverse shocks can form along the SIR/CIR, often at distances beyond 1 AU (e.g., Jian et al. 2006, 2008; Pizzo 1978; Smith and Wolfe 1976), which act as an important source of energetic ions, particularly during solar minimum. Once accelerated at an SIR/CIR-associated shock, energetic particles can propagate back towards the inner heliosphere along magnetic field lines and are subject to adiabatic deceleration and scattering (Fisk and Lee 1980). The expected result of these transport effects is a softening of the energetic particle spectra and a hardening of the lower-energy suprathermal spectra (see Mason and Sanderson 1999). This spectral variation, however, has not always been captured in observations, motivating the formulation of various other SIR/CIR-associated acceleration processes such as compressive, non-shock related acceleration (e.g., Giacalone et al. 2002; Ebert et al. 2012; Chen et al. 2015) which can accelerate ions into the suprathermal range at lower heliospheric distances. Additionally, footpoint motion and interchange reconnection near the coronal hole boundary has been proposed to lead to more radial magnetic field lines on the HSS side of the SIR/CIR, resulting in more direct access, and less modulation, of energetic particles (Murphy et al. 2002; Schwadron 2002; Schwadron and McComas 2005). Observations within 1 AU, by PSP, are therefore particularly well suited to detangle these acceleration and transport effects as the SIR/CIR-associated suprathermal to energetic ion populations are further from shock-associated acceleration sites that are usually beyond 1 AU.

During the first orbit of PSP, Allen et al. (2020) reported on four HSSs observed by PSP, illustrated in Fig. 50, and compared these to observations of the streams at 1 AU using observations from L1 (ACE and Wind) and STEREO-A. Many of these nascent SIR/CIRs were associated with energetic particle enhancements that were offset from the interface of the SIR/CIR. One of the events also had evidence of local compressive acceleration, which was previously noted by McComas et al. (2019). Cohen et al. (2020) further analyzed energetic particle increases associated with SIR/CIRs during the first two orbits of PSP (Fig. 51). They found He/H abundance ratios similar to previous observations of SIR/CIRs at 1 AU with fast solar wind under 600 km s⁻¹, however the proton spectra power laws, with indices ranging from −4.3 to 6.5, were softer than those often observed at 1 AU. Finally, Desai et al. (2020) investigated the suprathermal-to-energetic (~0.03–3 MeV/nuc) He ions associated with these SIR/CIRs from the first two orbits. They found that the higher energy He ions would arrive further in the rarefaction region than the lower-energy ions. The He spectra behaved as flat power laws modulated by exponential roll overs with an e-folding at energies of ~0.4 MeV/nuc, suggesting acceleration at shocks further out in the heliosphere. Desai et al. (2020) interpreted the tendency for the suprathermal ion peak to be within the rarefaction regions with acceleration further out in the heliosphere as evidence that the rarefaction regions allowed easier access for particles than other regions in the SIR/CIR structure.

One fortuitous CIR passed PSP on 19 Sep. 2019, during the third orbit of PSP, when PSP and STEREO-A were nearly radially aligned and ~0.5 AU apart (Allen et al. 2021a,b). As shown in Fig. 52, while the bulk plasma and magnetic field observations between the two S/C followed expected radial dependencies, the CIR-associated suprathermal ion enhancements were observed at PSP for a longer duration in time than at STEREO-A (Allen et al. 2021b). Additionally, the suprathermal ion spectral slopes between STEREO-A total ions and PSP H⁺ were nearly identical, while the flux at PSP was much smaller, suggesting little to no spectral modulation from transport. Allen et al. (2021b) concluded that the time difference in the CIR-associated suprathermal ion enhancement might be related to the magnetic topology between the slow speed stream ahead of the CIR interface, where the enhancement was first observed, and the HSS rarefaction region, where the suprathermal ions returned to background levels. Wijsen et al. (2021) furthered this investigation by simulating the PSP and STEREO-A observations using the European Heliopheric FORecasting
outward, both forward and reverse shocks can form along the SIR/CIR, often at distances beyond 1 AU (e.g., Jian et al. 2006, 2008; Pizzo 1978; Smith and Wolfe 1976), which act as an important source of energetic ions, particularly during solar minimum. Once accelerated at an SIR/CIR-associated shock, energetic particles can propagate back towards the inner heliosphere along magnetic field lines and are subject to adiabatic deceleration and scattering (Fisk and Lee 1980). The expected result of these transport effects is a softening of the energetic particle spectra and a hardening of the lower-energy suprathermal spectra (see Mason and Sanderson 1999). This spectral variation, however, has not always been captured in observations, motivating the formulation of various other SIR/CIR-associated acceleration processes such as compressive, non-shock related acceleration (e.g., Giacalone et al. 2002; Ebert et al. 2012; Chen et al. 2015) which can accelerate ions into the suprathermal range at lower heliospheric distances. Additionally, footpoint motion and interchange reconnection near the coronal hole boundary has been proposed to lead to more radial magnetic field lines on the HSS side of the SIR/CIR, resulting in more direct access, and less modulation, of energetic particles (Murphy et al. 2002; Schwadron 2002; Schwadron and McComas 2005). Observations within 1 AU, by PSP, are therefore particularly well suited to detangle these acceleration and transport effects as the SIR/CIR-associated suprathermal to energetic ion populations are further from shock-associated acceleration sites that are usually beyond 1 AU.

During the first orbit of PSP, Allen et al. (2020) reported on four HSSs observed by PSP, illustrated in Fig. 50, and compared these to observations of the streams at 1 AU using observations from L1 (ACE and Wind) and STEREO-A. Many of these nascent SIR/CIRs were associated with energetic particle enhancements that were offset from the interface of the SIR/CIR. One of the events also had evidence of local compressive acceleration, which was previously noted by McComas et al. (2019). Cohen et al. (2020) further analyzed energetic particle increases associated with SIR/CIRs during the first two orbits of PSP (Fig. 51). They found He/H abundance ratios similar to previous observations of SIR/CIRs at 1 AU with fast solar wind under 600 km s⁻¹, however the proton spectra power laws, with indices ranging from −4.3 to 6.5, were softer than those often observed at 1 AU. Finally, Desai et al. (2020) investigated the suprathermal-to-energetic (~0.03–3 MeV/nuc) He ions associated with these SIR/CIRs from the first two orbits. They found that the higher energy He ions would arrive further in the rarefaction region than the lower-energy ions. The He spectra behaved as flat power laws modulated by exponential roll overs with an e-folding at energies of ~0.4 MeV/nuc, suggesting acceleration at shocks further out in the heliosphere. Desai et al. (2020) interpreted the tendency for the suprathermal ion peak to be within the rarefaction regions with acceleration further out in the heliosphere as evidence that the rarefaction regions allowed easier access for particles than other regions in the SIR/CIR structure.

One fortuitous CIR passed PSP on 19 Sep. 2019, during the third orbit of PSP, when PSP and STEREO-A were nearly radially aligned and ~0.5 AU apart (Allen et al. 2021a,b). As shown in Fig. 52, while the bulk plasma and magnetic field observations between the two S/C followed expected radial dependencies, the CIR-associated suprathermal ion enhancements were observed at PSP for a longer duration in time than at STEREO-A (Allen et al. 2021b). Additionally, the suprathermal ion spectral slopes between STEREO-A total ions and PSP H⁺ were nearly identical, while the flux at PSP was much smaller, suggesting little to no spectral modulation from transport. Allen et al. (2021b) concluded that the time difference in the CIR-associated suprathermal ion enhancement might be related to the magnetic topology between the slow speed stream ahead of the CIR interface, where the enhancement was first observed, and the HSS rarefaction region, where the suprathermal ions returned to background levels. Wijsen et al. (2021) furthered this investigation by simulating the PSP and STEREO-A observations using the European Heliopheric FORecasting
<figure><img src="image_52.png" /><figcaption>**Fig. 50** Overview of four months around the first perihelion (6 Nov. 2018). Panels show (a) the heliographic distance of PSP; bulk proton (b) radial velocity, (c) density, (d) temperature, and (e) entropy; (f) summation of the magnetic and bulk proton thermal plasma pressure; (g) magnitude of the magnetic field, (h) Θ, and (i) φ angles of the magnetic field; and (j) EPI-Lo ion time-of-flight count rate for energies from 30 to 586 keV. Simulated quantities from two simulations are shown by the yellow and blue lines (see Allen et al. 2020, for more information). The four HSSs investigated in Allen et al. (2020) are indicated by the grey shaded regions, while pink shaded regions denote CMEs. Figure adapted from Allen et al. (2020)</figcaption></figure>

<figure><img src="image_53.png" /><figcaption>**Fig. 51** Summary of EPI-Hi LET ~ 1 – 2 MeV proton observations from the first two orbiter of PSP. SIR-associated energetic particle events studied by Cohen et al. (2020) are denoted by the numbered circles. Figure adapted from Cohen et al. (2020)</figcaption></figure>

Information Asset (EUHFORIA; Pomoell and Poedts 2018) model and the Particle Radiation Asset Directed at Interplanetary Space Exploration (PARADISE; Wijsen et al. 2019,
<figure><img src="image_53.png" /><figcaption>**Fig. 50** Overview of four months around the first perihelion (6 Nov. 2018). Panels show (a) the heliographic distance of PSP; bulk proton (b) radial velocity, (c) density, (d) temperature, and (e) entropy; (f) summation of the magnetic and bulk proton thermal plasma pressure; (g) magnitude of the magnetic field, (h) Θ, and (i) φ angles of the magnetic field; and (j) EPI-Lo ion time-of-flight count rate for energies from 30 to 586 keV. Simulated quantities from two simulations are shown by the yellow and blue lines (see Allen et al. 2020, for more information). The four HSSs investigated in Allen et al. (2020) are indicated by the grey shaded regions, while pink shaded regions denote CMEs. Figure adapted from Allen et al. (2020)</figcaption></figure>

<figure><img src="image_54.png" /><figcaption>**Fig. 51** Summary of EPI-Hi LET ~ 1 – 2 MeV proton observations from the first two orbiter of PSP. SIR-associated energetic particle events studied by Cohen et al. (2020) are denoted by the numbered circles. Figure adapted from Cohen et al. (2020)</figcaption></figure>

Information Asset (EUHFORIA; Pomoell and Poedts 2018) model and the Particle Radiation Asset Directed at Interplanetary Space Exploration (PARADISE; Wijsen et al. 2019,
<figure><img src="image_54.png" /><figcaption>**Fig. 52** Comparison of PSP observations (black) and time-shifted and radially corrected STEREO-A observations (blue) for the CIR that passed over PSP on 19 Sep. 2019. While the bulk solar wind and magnetic field observations match well after typical scaling factors are applied (a-h, see Allen et al. 2021b, for more information), the energetic particle are elevated at PSP (i) for longer than at STEREO-A (j). Figure adapted from Allen et al. (2021b)</figcaption></figure>

2020) model, suggesting that this event provides evidence that CIR-associated acceleration does not always require shock waves.

An SIR that passed over PSP on 15 Nov. 2018 when the S/C was ~0.32 AU from the Sun providing insight into energetic particle acceleration by SIRs in the inner heliosphere and the importance of the magnetic field structures connecting the observer to the acceleration region. Figure 53 shows an overview of the energetic particle, plasma and magnetic field conditions during the passage of the SIR and the energetic particle event that followed it, which started about a day after the passage of the compression and lasted for about four days. The spectral analysis provided by Joyce et al. (2021a), showed that for the first day of the event, the spectra resembled a simple power law, which is commonly associated with local acceleration, despite being well out of the compression region by that point. The spectrum for the remaining three days of the event was shown to be fairly constant, a finding that is inconsistent with the traditional model of SIR energetic particle acceleration provided by
<figure><img src="image_55.png" /><figcaption>**Fig. 52** Comparison of *PSP* observations (black) and time-shifted and radially corrected *STEREO-A* observations (blue) for the CIR that passed over *PSP* on 19 Sep. 2019. While the bulk solar wind and magnetic field observations match well after typical scaling factors are applied (a-h, see Allen et al. 2021b, for more information), the energetic particle are elevated at *PSP* (i) for longer than at *STEREO-A* (j). Figure adapted from Allen et al. (2021b)</figcaption></figure>

2020) model, suggesting that this event provides evidence that CIR-associated acceleration does not always require shock waves.

An SIR that passed over *PSP* on 15 Nov. 2018 when the S/C was ~0.32 AU from the Sun providing insight into energetic particle acceleration by SIRs in the inner heliosphere and the importance of the magnetic field structures connecting the observer to the acceleration region. Figure 53 shows an overview of the energetic particle, plasma and magnetic field conditions during the passage of the SIR and the energetic particle event that followed it, which started about a day after the passage of the compression and lasted for about four days. The spectral analysis provided by Joyce et al. (2021a), showed that for the first day of the event, the spectra resembled a simple power law, which is commonly associated with local acceleration, despite being well out of the compression region by that point. The spectrum for the remaining three days of the event was shown to be fairly constant, a finding that is inconsistent with the traditional model of SIR energetic particle acceleration provided by
<figure><img src="image_55.png" /><figcaption>**Fig. 53** Overview of energetic particle observations associated with the SIR that passed over PSP on 15 Nov. 2018. Plasma data is provided by the SWEAP instrument and magnetic field data by the FIELDS instrument. The compression associated with the passage of the SIR is highlighted in yellow. Figure is updated from figures shown in Schwadron et al. (2021) and Joyce et al. (2021a)</figcaption></figure>

Fisk and Lee (1980), which models energetic particle acceleration at distant regions where SIRs have steepened into shocks and predicts changes in the spectral shape with distance from the source region. Within this paradigm, we would expect that the distance along the magnetic field connecting to the source region would increase during the event and that the observed spectrum would change. This combined with the simple power law spectrum observed on the first day, seems to indicate that the source region is much closer to the observer than is typically thought as we do not see the expected transport effects and that acceleration all along the compression, not only in the distant regions where the SIR may steepen into shocks, may play an important role in energetic particle acceleration associated with SIRs (consistent with previous studies by Chotoo et al. 2000 and Chen et al. 2015).

Schwadron et al. (2021) analyzed the same event, also noting that the long duration of the energetic particle event following the passage of the CME suggests a non-Parker spiral orientation of the magnetic field and proposed that the observations may be explained by a sub-Parker magnetic field structure (Murphy et al. 2002; Schwadron 2002; Schwadron and McComas 2005; Schwadron et al. 2005). The sub-Parker spiral structure forms when magnetic footpoints on the Sun move across coronal hole boundaries, threading the magnetic field between the fast and slow solar wind streams that form the compression and creating a magnetic field structure that is significantly more radial than a nominal Parker spiral. Figure 54a shows a diagram of the sub-Parker spiral and Fig. 54b shows a comparison between the energetic particle fluxes measured by IS○IS in two different energy regimes compared with modeled fluxes for both the Parker and sub-Parker spiral magnetic field orientations. The modeling includes an analytic solution of the distribution function at the SIR reverse
<figure><img src="image_56.png" /><figcaption>**Fig. 53** Overview of energetic particle observations associated with the SIR that passed over PSP on 15 Nov. 2018. Plasma data is provided by the SWEAP instrument and magnetic field data by the FIELDS instrument. The compression associated with the passage of the SIR is highlighted in yellow. Figure is updated from figures shown in Schwadron et al. (2021) and Joyce et al. (2021a)</figcaption></figure>

Fisk and Lee (1980), which models energetic particle acceleration at distant regions where SIRs have steepened into shocks and predicts changes in the spectral shape with distance from the source region. Within this paradigm, we would expect that the distance along the magnetic field connecting to the source region would increase during the event and that the observed spectrum would change. This combined with the simple power law spectrum observed on the first day, seems to indicate that the source region is much closer to the observer than is typically thought as we do not see the expected transport effects and that acceleration all along the compression, not only in the distant regions where the SIR may steepen into shocks, may play an important role in energetic particle acceleration associated with SIRs (consistent with previous studies by Chotoo et al. 2000 and Chen et al. 2015).

Schwadron et al. (2021) analyzed the same event, also noting that the long duration of the energetic particle event following the passage of the CME suggests a non-Parker spiral orientation of the magnetic field and proposed that the observations may be explained by a sub-Parker magnetic field structure (Murphy et al. 2002; Schwadron 2002; Schwadron and McComas 2005; Schwadron et al. 2005). The sub-Parker spiral structure forms when magnetic footpoints on the Sun move across coronal hole boundaries, threading the magnetic field between the fast and slow solar wind streams that form the compression and creating a magnetic field structure that is significantly more radial than a nominal Parker spiral. Figure 54a shows a diagram of the sub-Parker spiral and Fig. 54b shows a comparison between the energetic particle fluxes measured by IS○IS in two different energy regimes compared with modeled fluxes for both the Parker and sub-Parker spiral magnetic field orientations. The modeling includes an analytic solution of the distribution function at the SIR reverse
**Fig. 54 (a)** shows the magnetic field structure associated with an SIR, with the red dashed lines representing the compression region where the fast solar wind overtakes the slow solar wind, the blue lines represent the nominal Parker spiral configuration, and the clack lines represent the sub-Parker spiral field lines that are threaded between the fast and slow solar wind streams as a result of footpoint motion across the coronal hole boundary.

(b) shows a comparison between IS⊙IS energetic particle fluxes in two energy ranges (blue data points) and modeled energetic particle fluxes for both the Parker spiral (blue lines) and sub-Parker spiral (black lines) magnetic field configurations. Figure adapted from Schwadron et al. (2021)

<figure><img src="image_56.png" /></figure>

compression/shock and numerical modeling of the propagation of the particles back to the S/C (details in Schwadron et al. 2021). The modeled fluxes for the sub-Parker spiral match the observed fluxes much better than the nominal Parker spiral, demonstrating that the sub-Parker spiral structure is essential for explaining the extended duration of the energetic particle event associated with the SIR. The sub-Parker spiral is often seen in rarefaction regions, such as those that form behind SIRs, and thus is likely to play a significant role in the observed energetic particle profiles associated with such events. Both the Joyce et al. (2021a) and Schwadron et al. (2021) demonstrate the importance of IS⊙IS observations of SIRs
**Fig. 54 (a)** shows the magnetic field structure associated with an SIR, with the red dashed lines representing the compression region where the fast solar wind overtakes the slow solar wind, the blue lines represent the nominal Parker spiral configuration, and the clack lines represent the sub-Parker spiral field lines that are threaded between the fast and slow solar wind streams as a result of footpoint motion across the coronal hole boundary.

(b) shows a comparison between IS⊙IS energetic particle fluxes in two energy ranges (blue data points) and modeled energetic particle fluxes for both the Parker spiral (blue lines) and sub-Parker spiral (black lines) magnetic field configurations. Figure adapted from Schwadron et al. (2021)

<figure><img src="image_57.png" /></figure>

compression/shock and numerical modeling of the propagation of the particles back to the S/C (details in Schwadron et al. 2021). The modeled fluxes for the sub-Parker spiral match the observed fluxes much better than the nominal Parker spiral, demonstrating that the sub-Parker spiral structure is essential for explaining the extended duration of the energetic particle event associated with the SIR. The sub-Parker spiral is often seen in rarefaction regions, such as those that form behind SIRs, and thus is likely to play a significant role in the observed energetic particle profiles associated with such events. Both the Joyce et al. (2021a) and Schwadron et al. (2021) demonstrate the importance of IS⊙IS observations of SIRs
in understanding the large scale structure of the magnetic field in the inner heliosphere, the motion of magnetic footpoints on the Sun and the propagation of energetic particles, helping us to understand the variability of energetic particles and providing insight into the source of the solar wind.

## 10.4 Inner Heliospheric Anomalous Cosmic Rays

The ability of PSP to measure the energetic particle content at unprecedentedly close radial distances during a deep solar minimum has also allowed for detailed investigations into the radial dependence of ACRs in the inner heliosphere. ACRs are mainly comprised of singly ionized hydrogen, helium, nitrogen, oxygen, neon, and argon, with energies of ~5–50 MeV/nuc (e.g., Garcia-Munoz et al. 1973; Hovestadt et al. 1973; McDonald 1974; Christian et al. 1988; Klecker et al. 1998; Cummings et al. 2002a,b; Potgieter 2013). The source of these particles are neutral interstellar particles that are part of the interstellar wind (McComas et al. 2015) before becoming ionized near the Sun (Fisk et al. 1974). Once the particles become ionized, they become picked-up by the solar wind convective electric field and are convected away from the Sun as pick-up ions. A small portion of these pick-up ions can become accelerated to high energies (tens to hundreds of MeV) further out in the heliosphere before returning into the inner heliosphere, thus becoming ACRs (Jokipii 1996; Mewaldt et al. 1996; Barghouty et al. 2000; Giacalone et al. 2012).

While the acceleration of ACRs is primarily thought to occur at the termination shock (Pesses et al. 1981; Jokipii 1992), neither *Voyager* 1 or *Voyager* 2 (Kohlhase and Penzo 1977) observed a peak in ACR intensity when crossing the termination shock (Stone et al. 2005, 2008). As a result, numerous theories have been proposed to explain this including a “blunt” termination shock geometry in which the ACR acceleration occurs preferentially along the termination shock flanks and tail (McComas and Schwadron 2006; Schwadron et al. 2008) away from the region the *Voyager* S/C crossed, magnetic reconnection at the heliopause (Drake et al. 2010), heliosheath compressive turbulence (Fisk and Gloeckler 2009), and second-order Fermi processes (Strauss and Potgieter 2010).

After being accelerated, ACR particles penetrate back into the heliosphere, where their intensities decrease due to solar modulation (e.g., Klecker 1999; Cummings et al. 2002a; McComas and Schwadron 2006). The radial gradients of ACRs in the heliosphere have primarily been studied from 1 AU outward through comparing observations at the IMP-8⁸ at 1 AU to observations from Pioneer 10, Pioneer 11, *Voyager* 1, and *Voyager* 2 in the outer heliosphere. These comparisons revealed that the helium ACR intensity varied as $r^{-0.67}$ from 1 to ~41 AU (Cummings et al. 1990). Understanding this modulation provides insight into the various processes that govern global cosmic ray drift paths throughout the heliosphere.

The orbit of PSP is well suited to investigate ACR radial variations due to its sampling of a large range of radial distances near the ecliptic. Additionally, PSP enables investigations into the ACR populations at distances closer to the Sun than previously measured. Rankin et al. (2021) utilized the PSP/IS◎IS/EPI-Hi instrument to study the radial variation of the helium ACR content down to 35.6 R⊕ (0.166 AU) and compare these observations to ACR observations at 1 AU measured the SOHO mission. To ensure that the particles included in the comparisons were ACRs, rather than SEPs, only “quiet-time” periods were used (see the Appendix in Rankin et al. 2021). The resulting quiet-time EPI-Hi and SOHO spectra over the first three orbits of PSP is shown in Fig. 55. The ACR intensity was observed to increase over energies from ~5 to ~40 MeV/nuc, a characteristic feature of ACR spectra.

<footnote n="8">⁸The Interplanetary Monitoring Platform-8</footnote>
in understanding the large scale structure of the magnetic field in the inner heliosphere, the motion of magnetic footpoints on the Sun and the propagation of energetic particles, helping us to understand the variability of energetic particles and providing insight into the source of the solar wind.

## 10.4 Inner Heliospheric Anomalous Cosmic Rays

The ability of PSP to measure the energetic particle content at unprecedentedly close radial distances during a deep solar minimum has also allowed for detailed investigations into the radial dependence of ACRs in the inner heliosphere. ACRs are mainly comprised of singly ionized hydrogen, helium, nitrogen, oxygen, neon, and argon, with energies of ~5–50 MeV/nuc (e.g., Garcia-Munoz et al. 1973; Hovestadt et al. 1973; McDonald 1974; Christian et al. 1988; Klecker et al. 1998; Cummings et al. 2002a,b; Potgieter 2013). The source of these particles are neutral interstellar particles that are part of the interstellar wind (McComas et al. 2015) before becoming ionized near the Sun (Fisk et al. 1974). Once the particles become ionized, they become picked-up by the solar wind convective electric field and are convected away from the Sun as pick-up ions. A small portion of these pick-up ions can become accelerated to high energies (tens to hundreds of MeV) further out in the heliosphere before returning into the inner heliosphere, thus becoming ACRs (Jokipii 1996; Mewaldt et al. 1996; Barghouty et al. 2000; Giacalone et al. 2012).

While the acceleration of ACRs is primarily thought to occur at the termination shock (Pesses et al. 1981; Jokipii 1992), neither *Voyager* 1 or *Voyager* 2 (Kohlhase and Penzo 1977) observed a peak in ACR intensity when crossing the termination shock (Stone et al. 2005, 2008). As a result, numerous theories have been proposed to explain this including a “blunt” termination shock geometry in which the ACR acceleration occurs preferentially along the termination shock flanks and tail (McComas and Schwadron 2006; Schwadron et al. 2008) away from the region the *Voyager* S/C crossed, magnetic reconnection at the heliopause (Drake et al. 2010), heliosheath compressive turbulence (Fisk and Gloeckler 2009), and second-order Fermi processes (Strauss and Potgieter 2010).

After being accelerated, ACR particles penetrate back into the heliosphere, where their intensities decrease due to solar modulation (e.g., Klecker 1999; Cummings et al. 2002a; McComas and Schwadron 2006). The radial gradients of ACRs in the heliosphere have primarily been studied from 1 AU outward through comparing observations at the IMP-8⁸ at 1 AU to observations from Pioneer 10, Pioneer 11, *Voyager* 1, and *Voyager* 2 in the outer heliosphere. These comparisons revealed that the helium ACR intensity varied as $r^{-0.67}$ from 1 to ~41 AU (Cummings et al. 1990). Understanding this modulation provides insight into the various processes that govern global cosmic ray drift paths throughout the heliosphere.

The orbit of PSP is well suited to investigate ACR radial variations due to its sampling of a large range of radial distances near the ecliptic. Additionally, PSP enables investigations into the ACR populations at distances closer to the Sun than previously measured. Rankin et al. (2021) utilized the PSP/IS◎IS/EPI-Hi instrument to study the radial variation of the helium ACR content down to 35.6 R☉ (0.166 AU) and compare these observations to ACR observations at 1 AU measured the SOHO mission. To ensure that the particles included in the comparisons were ACRs, rather than SEPs, only “quiet-time” periods were used (see the Appendix in Rankin et al. 2021). The resulting quiet-time EPI-Hi and SOHO spectra over the first three orbits of PSP is shown in Fig. 55. The ACR intensity was observed to increase over energies from ~5 to ~40 MeV/nuc, a characteristic feature of ACR spectra.

<footnote n="8">⁸The Interplanetary Monitoring Platform-8</footnote>
**Fig. 55** Helium spectra over the first three orbits of PSP after removing transient events (see Rankin et al. 2021, for more information) at PSP (red and blue) and at SOHO (green). A simulated GCR spectrum at 1 AU is included (black) from HelMOD (version 4.0.1, 2021 January; www.helmod.org).

Figure adapted from Rankin et al. (2021)

<figure><img src="image_57.png" /></figure>

Figures 56a and 56b show normalized ACR fluxes from the SOHO Electron Proton He-
lium INstrument (EPHIN; Kunow et al. 1988) and PSP/IS⊙IS/EPI-Hi, respectively. The
ratio of the ACR fluxes (Fig. 56c) correlate well with the heliographic radial distance of
PSP (Fig. 56d). This presents clear evidence of radial-dependent modulation, as expected.
However, the observed radial gradient is stronger (~25 ± 5% AU) than observed beyond
1 AU. Better understanding the radial gradients of ACRs in the inner heliosphere may pro-
vide needed constraints on drift transport and cross-field diffusion models, as cross-field
diffusion will become more dominant in the inner heliosphere (Strauss and Potgieter 2010).
Future studies will also be aided by the addition of ACR measurements by SolO, such as
those reported in Mason et al. (2021c).

**10.5 Open Questions and Future Collaborations**

Over the first four years of the PSP prime mission, large advances have been made regard-
ing our understanding of inner heliospheric energetic particles and solar radio emissions.
Looking forward, as the solar cycle ascends out of solar minimum, and as additional ob-
servatories such as SolO enter into their science phase and provide robust energetic particle
measurements (see Wimmer-Schweingruber et al. 2021), many new opportunities to study
energetic particle populations and dynamics will present themselves.

For example, while PSP has begun exploring the radial evolution of SIRs and associ-
ated energization and transport of particles, future measurements will explore the cause of
known solar cycle dependencies of the SIR/CIR-associated suprathermal ion composition
(e.g., Mason et al. 2008, 2012; Allen et al. 2019). Additionally, future PSP observations
of SIR/CIR-associated ions will be a crucial contribution to studies on the radial gradient
of energetic ions (e.g., Van Hollebeke et al. 1978; Allen et al. 2021c). As SolO begins to
return off-ecliptic observations, the combination of PSP and SolO at different latitudes will
enable needed insight into the latitudinal structuring of SIR/CIRs and associated particle
acceleration.

As the solar cycle progresses, solar activity will increase. This will provide many new
observations of CMEs and SEP events at various intensities and radial distances in the inner
heliosphere, particularly for low energy SEP events that are not measurable at 1 AU (e.g.,
Hill et al. 2020). These PSP observations will further our understanding of CME-associated
shock acceleration and how the energetic content of CMEs evolves with heliographic dis-
tance. The current and future Heliophysics System Observatory (HSO) should also provide
**Fig. 55** Helium spectra over the first three orbits of PSP after removing transient events (see Rankin et al. 2021, for more information) at PSP (red and blue) and at SOHO (green). A simulated GCR spectrum at 1 AU is included (black) from HelMOD (version 4.0.1, 2021 January; www.helmod.org). Figure adapted from Rankin et al. (2021)

<figure><img src="image_58.png" /></figure>

Figures 56a and 56b show normalized ACR fluxes from the SOHO Electron Proton He-
lium INstrument (EPHIN; Kunow et al. 1988) and PSP/IS⊙IS/EPI-Hi, respectively. The
ratio of the ACR fluxes (Fig. 56c) correlate well with the heliographic radial distance of
PSP (Fig. 56d). This presents clear evidence of radial-dependent modulation, as expected.
However, the observed radial gradient is stronger (~25 ± 5% AU) than observed beyond
1 AU. Better understanding the radial gradients of ACRs in the inner heliosphere may pro-
vide needed constraints on drift transport and cross-field diffusion models, as cross-field
diffusion will become more dominant in the inner heliosphere (Strauss and Potgieter 2010).
Future studies will also be aided by the addition of ACR measurements by SolO, such as
those reported in Mason et al. (2021c).

**10.5 Open Questions and Future Collaborations**

Over the first four years of the PSP prime mission, large advances have been made regard-
ing our understanding of inner heliospheric energetic particles and solar radio emissions.
Looking forward, as the solar cycle ascends out of solar minimum, and as additional ob-
servatories such as SolO enter into their science phase and provide robust energetic particle
measurements (see Wimmer-Schweingruber et al. 2021), many new opportunities to study
energetic particle populations and dynamics will present themselves.

For example, while PSP has begun exploring the radial evolution of SIRs and associ-
ated energization and transport of particles, future measurements will explore the cause of
known solar cycle dependencies of the SIR/CIR-associated suprathermal ion composition
(e.g., Mason et al. 2008, 2012; Allen et al. 2019). Additionally, future PSP observations
of SIR/CIR-associated ions will be a crucial contribution to studies on the radial gradient
of energetic ions (e.g., Van Hollebeke et al. 1978; Allen et al. 2021c). As SolO begins to
return off-ecliptic observations, the combination of PSP and SolO at different latitudes will
enable needed insight into the latitudinal structuring of SIR/CIRs and associated particle
acceleration.

As the solar cycle progresses, solar activity will increase. This will provide many new
observations of CMEs and SEP events at various intensities and radial distances in the inner
heliosphere, particularly for low energy SEP events that are not measurable at 1 AU (e.g.,
Hill et al. 2020). These PSP observations will further our understanding of CME-associated
shock acceleration and how the energetic content of CMEs evolves with heliographic dis-
tance. The current and future Heliophysics System Observatory (HSO) should also provide
<figure><img src="image_58.png" /><figcaption>**Fig. 56** ACR normalized flux at (a) 1 AU averaged over 27.27 days and (b) PSP averaged over Carrington longitude. The ratio of intensities (c) has a clear dependence on the radial distance of PSP (d). Figure adapted from Rankin et al. (2021)</figcaption></figure>

additional opportunities to not only study the radial evolution of CMEs (e.g., Freiherr von Forstner et al. 2021), but also the longitudinal variations of these structures, as was done for the 29 Nov. 2020 event (e.g., Cohen et al. 2021b; Kollhoff et al. 2021; Mason et al. 2021a).

As discussed in §10.1, PSP has already expanded our understanding of SEP events in the inner heliosphere. Because *SolO*, which also returns observations of ³He-rich SEP events (e.g., Mason et al. 2021b; Bučík et al. 2021), will soon be taking measurements of SEP events at different latitudes than PSP, the combination of these missions will enable exploration of latitudinal variations in SEP content. Similarly, energetic electron measurements on *SolO* (e.g., Gómez-Herrero et al. 2021), soon to be taken off-ecliptic, will enable future studies into the latitudinal variations of electron events.

In addition to radio observations using multiple S/C, space-based and ground-based multi-wavelength observations enable new types of coordinated analysis of solar activity. Harra et al. (2021) combined *Hinode*, *SDO/AIA*, and RFS observations in a joint analysis of a non-flaring AR and a type III storm observed during PSP Enc. 2, identifying the source of electron beams associated with the storm and using radio measurements to show the evolution of the peak emission height throughout the storm. Cattell et al. (2021b) studied a different storm occurring slightly after Enc. 2 using radio observations from PSP and *Wind*, and solar observations from *SDO/AIA*, *SDO/HMI*, and the Nuclear Spectroscopic Telescope
<figure><img src="image_59.png" /><figcaption>**Fig. 56** ACR normalized flux at (a) 1 AU averaged over 27.27 days and (b) PSP averaged over Carrington longitude. The ratio of intensities (c) has a clear dependence on the radial distance of PSP (d). Figure adapted from Rankin et al. (2021)</figcaption></figure>

additional opportunities to not only study the radial evolution of CMEs (e.g., Freiherr von Forstner et al. 2021), but also the longitudinal variations of these structures, as was done for the 29 Nov. 2020 event (e.g., Cohen et al. 2021b; Kollhoff et al. 2021; Mason et al. 2021a).

As discussed in §10.1, PSP has already expanded our understanding of SEP events in the inner heliosphere. Because *SolO*, which also returns observations of ³He-rich SEP events (e.g., Mason et al. 2021b; Bučík et al. 2021), will soon be taking measurements of SEP events at different latitudes than PSP, the combination of these missions will enable exploration of latitudinal variations in SEP content. Similarly, energetic electron measurements on *SolO* (e.g., Gómez-Herrero et al. 2021), soon to be taken off-ecliptic, will enable future studies into the latitudinal variations of electron events.

In addition to radio observations using multiple S/C, space-based and ground-based multi-wavelength observations enable new types of coordinated analysis of solar activity. Harra et al. (2021) combined *Hinode*, *SDO/AIA*, and RFS observations in a joint analysis of a non-flaring AR and a type III storm observed during PSP Enc. 2, identifying the source of electron beams associated with the storm and using radio measurements to show the evolution of the peak emission height throughout the storm. Cattell et al. (2021b) studied a different storm occurring slightly after Enc. 2 using radio observations from PSP and *Wind*, and solar observations from *SDO/AIA*, *SDO/HMI*, and the Nuclear Spectroscopic Telescope
ARray (NuSTAR; Harrison et al. 2013), finding correlated periodic oscillations in the EUV and radio data indicative of small impulsive electron acceleration.

Additionally, the continuation of the PSP project science team's close relationship with the Whole Heliosphere and Planetary Interactions (WHPI<fnref n="9" />) international initiative, the successor of Whole Sun Month (Galvin and Kohl 1999) and Whole Heliosphere Interval (Gibson et al. 2011; Thompson et al. 2011; Bisi et al. 2011) will allow for multifaceted studies that incorporate ground-based and space-based observatories providing contextual information for the PSP measurements. Many of these studies are beginning now, and should propel our fundamental understanding of the connection of the solar surface to interplanetary space and beyond.

# 11 Dust

## 11.1 Dust Populations in the Inner Heliosphere

The ZDC is one of the largest structures in the heliosphere. It is comprised of cosmic dust particles sourced from comets and asteroids, with most of the material located in the ecliptic plane where the majority of these dust sources reside. The orbits of grains gravitationally bound to the Sun, termed “α-meteoroids”, lose angular momentum from Poynting-Robertson and solar wind drag (e.g., Burns et al. 1979) and subsequently circularize and spiral toward the Sun. Due to the inward transport of zodiacal material, the dust spatial density increases as these grains get closer to the Sun (e.g., Leinert et al. 1981), until they are ultimately collisionally fragmented or sublimated into smaller grains (e.g., Mann et al. 2004). Dust-dust collisions within the cloud are responsible for generating a significant portion of the population of smaller grains. Additionally, a local source for dust particles very near the Sun are the near-Sun comets, “Sunskirters”, that pass the Sun within half of Mercury’s perihelion distance, and sungrazers that reach perihelion within the fluid Roche limit (Jones et al. 2018). Because these comets are in elongated orbits, their dust remains in the vicinity of the Sun only for short time (Mann et al. 2004; Czechowski and Mann 2018).

Sub-micron sized grains, with radii on the order of a few hundred nm, are most susceptible to outward radiation pressure. The orbital characteristics of these submicron-sized “β-meteoroids” are set by the ratio of solar radiation pressure to gravitational force, $\beta = F_R/F_G$, dependent on both grain size and composition (Burns et al. 1979). Grains with $\beta$ above a critical value dependent on their orbital elements have positive orbital energy and follow hyperbolic trajectories escaping the heliosphere. This population of grains represents the highest number flux of micrometeoroids at 1 AU (Grun et al. 1985). For the smallest nanograins ($\lesssim 50$ nm), electromagnetic forces play an important role in their dynamics (e.g., Morfill et al. 1986), where a certain population of grains can become electromagnetically trapped very near the Sun (Czechowski and Mann 2010). Figure 57 summarizes these various processes and dust populations.

When dust particles approach very near to the Sun, they can sublimate rapidly, leaving a region near the Sun relatively devoid of dust. Different estimates of this DFZ have been made based on Fraunhofer-corona (F-corona) observations and model calculations, predicting a DFZ within 2 to 20 solar radii and possible flattened radial profiles before its beginning (Mann et al. 2004). These estimates are based on dust sublimation; however, an additional destruction process recognized in the innermost parts of the solar system is sputtering by

<footnote n="9">⁹https://whpi.hao.ucar.edu/.</footnote>
ARray (NuSTAR; Harrison et al. 2013), finding correlated periodic oscillations in the EUV and radio data indicative of small impulsive electron acceleration.

Additionally, the continuation of the PSP project science team's close relationship with the Whole Heliosphere and Planetary Interactions (WHPI<fnref n="9" />) international initiative, the successor of Whole Sun Month (Galvin and Kohl 1999) and Whole Heliosphere Interval (Gibson et al. 2011; Thompson et al. 2011; Bisi et al. 2011) will allow for multifaceted studies that incorporate ground-based and space-based observatories providing contextual information for the PSP measurements. Many of these studies are beginning now, and should propel our fundamental understanding of the connection of the solar surface to interplanetary space and beyond.

# 11 Dust

## 11.1 Dust Populations in the Inner Heliosphere

The ZDC is one of the largest structures in the heliosphere. It is comprised of cosmic dust particles sourced from comets and asteroids, with most of the material located in the ecliptic plane where the majority of these dust sources reside. The orbits of grains gravitationally bound to the Sun, termed “α-meteoroids”, lose angular momentum from Poynting-Robertson and solar wind drag (e.g., Burns et al. 1979) and subsequently circularize and spiral toward the Sun. Due to the inward transport of zodiacal material, the dust spatial density increases as these grains get closer to the Sun (e.g., Leinert et al. 1981), until they are ultimately collisionally fragmented or sublimated into smaller grains (e.g., Mann et al. 2004). Dust-dust collisions within the cloud are responsible for generating a significant portion of the population of smaller grains. Additionally, a local source for dust particles very near the Sun are the near-Sun comets, “Sunskirters”, that pass the Sun within half of Mercury’s perihelion distance, and sungrazers that reach perihelion within the fluid Roche limit (Jones et al. 2018). Because these comets are in elongated orbits, their dust remains in the vicinity of the Sun only for short time (Mann et al. 2004; Czechowski and Mann 2018).

Sub-micron sized grains, with radii on the order of a few hundred nm, are most susceptible to outward radiation pressure. The orbital characteristics of these submicron-sized “β-meteoroids” are set by the ratio of solar radiation pressure to gravitational force, $\beta = F_R/F_G$, dependent on both grain size and composition (Burns et al. 1979). Grains with $\beta$ above a critical value dependent on their orbital elements have positive orbital energy and follow hyperbolic trajectories escaping the heliosphere. This population of grains represents the highest number flux of micrometeoroids at 1 AU (Grun et al. 1985). For the smallest nanograins ($\lesssim 50$ nm), electromagnetic forces play an important role in their dynamics (e.g., Morfill et al. 1986), where a certain population of grains can become electromagnetically trapped very near the Sun (Czechowski and Mann 2010). Figure 57 summarizes these various processes and dust populations.

When dust particles approach very near to the Sun, they can sublimate rapidly, leaving a region near the Sun relatively devoid of dust. Different estimates of this DFZ have been made based on Fraunhofer-corona (F-corona) observations and model calculations, predicting a DFZ within 2 to 20 solar radii and possible flattened radial profiles before its beginning (Mann et al. 2004). These estimates are based on dust sublimation; however, an additional destruction process recognized in the innermost parts of the solar system is sputtering by

<footnote n="9">⁹https://whpi.hao.ucar.edu/.</footnote>
Dust cloud near sun

<figure><img src="image_59.png" /><figcaption>Fig. 57 The dust environment near the Sun (Mann et al. 2019)</figcaption></figure>

solar wind particles. Baumann et al. (2020) showed that sputtering is more effective during a CME event than during other solar wind conditions and suggested that multiple CMEs can lead to an extension of the DFZ. Dust destruction near the Sun releases molecules and atoms, where photoionization, electron-impact ionization, and charge exchange quickly ionize the atoms and molecules. This process contributes to a population of pickup-ions in the solar wind and provides a seed population for energetic particles (Schwadron et al. 2000; Mann and Czechowski 2005).

The inner heliosphere's dust populations within a few AU have been observed both with *in situ* dust impact detections and remotely via scattered light observations. Due to their higher number densities, dust grains with radii on the order of ~ µm and smaller can be observed with *in situ* impact measurements. Dedicated dust measurements within this size range have been taken in the inner solar system with *Pioneers* 8 and 9 (Berg and Grün 1973), *HEOS-2*<fnref n="10" /> (Hoffmann et al. 1975b,a), *Helios* (Leinert et al. 1978, 1981; Gruen et al. 1980; Altobelli et al. 2006; Krüger et al. 2020), *Ulysses* (Wehry and Mann 1999; Wehry et al. 2004; Landgraf et al. 2003; Sterken et al. 2015; Strub et al. 2019), and *Galileo* (Grün et al. 1997). These observations identified three populations of dust: α-meteoroids, β-meteoroids, and interstellar grains transiting the solar system (Grun et al. 1993). Before *PSP*, the innermost dust measurements were made by *Helios* as close as 0.3 AU from the Sun.

For grains on the order of several µm and larger, astronomical observations of the F-corona and ZL (Leinert et al. 1981) provide important constraints on their density distributions. Unlike the broader zodiacal cloud (ZC) structure, which is most concentrated near the ecliptic plane, the solar F-corona has a more spherical shape, with the transition from one to the other following a super-ellipsoidal shape according to the radial variation of a flattening index using observations from the *STEREO/SECCHI* instrument (Stauffer et al. 2018). Measurements from *Helios* 1 and *Helios* 2 at locations between 0.3 to 1 AU showed that the brightness profile at the symmetry axis of the ZL falls off as a power law of solar distance, with exponent 2.3 (Leinert et al. 1981), which is consistent with a derived dust

<footnote n="10">¹⁰The Highly Eccentric Orbit Satellite 2</footnote>
Dust cloud near sun

<figure><img src="image_60.png" /><figcaption>Fig. 57 The dust environment near the Sun (Mann et al. 2019)</figcaption></figure>

solar wind particles. Baumann et al. (2020) showed that sputtering is more effective during a CME event than during other solar wind conditions and suggested that multiple CMEs can lead to an extension of the DFZ. Dust destruction near the Sun releases molecules and atoms, where photoionization, electron-impact ionization, and charge exchange quickly ionize the atoms and molecules. This process contributes to a population of pickup-ions in the solar wind and provides a seed population for energetic particles (Schwadron et al. 2000; Mann and Czechowski 2005).

The inner heliosphere's dust populations within a few AU have been observed both with *in situ* dust impact detections and remotely via scattered light observations. Due to their higher number densities, dust grains with radii on the order of ~ µm and smaller can be observed with *in situ* impact measurements. Dedicated dust measurements within this size range have been taken in the inner solar system with *Pioneers* 8 and 9 (Berg and Grün 1973), *HEOS-2*<fnref n="10" /> (Hoffmann et al. 1975b,a), *Helios* (Leinert et al. 1978, 1981; Gruen et al. 1980; Altobelli et al. 2006; Krüger et al. 2020), *Ulysses* (Wehry and Mann 1999; Wehry et al. 2004; Landgraf et al. 2003; Sterken et al. 2015; Strub et al. 2019), and *Galileo* (Grün et al. 1997). These observations identified three populations of dust: α-meteoroids, β-meteoroids, and interstellar grains transiting the solar system (Grun et al. 1993). Before *PSP*, the innermost dust measurements were made by *Helios* as close as 0.3 AU from the Sun.

For grains on the order of several µm and larger, astronomical observations of the F-corona and ZL (Leinert et al. 1981) provide important constraints on their density distributions. Unlike the broader zodiacal cloud (ZC) structure, which is most concentrated near the ecliptic plane, the solar F-corona has a more spherical shape, with the transition from one to the other following a super-ellipsoidal shape according to the radial variation of a flattening index using observations from the *STEREO/SECCHI* instrument (Stauffer et al. 2018). Measurements from *Helios* 1 and *Helios* 2 at locations between 0.3 to 1 AU showed that the brightness profile at the symmetry axis of the ZL falls off as a power law of solar distance, with exponent 2.3 (Leinert et al. 1981), which is consistent with a derived dust

<footnote n="10">¹⁰The Highly Eccentric Orbit Satellite 2</footnote>
density profile of the form $n(r) = n_0 r^{-1.3}$. This dust density dependence is well-reproduced by the dust produced by Jupiter-family comets (Pokorný and Kuchner 2019). Additionally, there were a number of discussions on the influence of excess dust in circumsolar rings near the Sun (Kimura and Mann 1998) on the observed F-corona brightness (Kimura et al. 1998). Later on, (Mann et al. 2004) showed that no prominent dust rings exist near the Sun.

More recently, Lamy et al. (2021). Lamy et al. (2021) analyzed images obtained with the SOHO/LASCO-C3 between 1996 and 2019. Based on a polarimetric analysis of the LASCO-C3 images, they separated the F- and K-corona components and derived the electron-density distribution. In addition, they reported the likely increasing polarization of the F-corona with increasing solar elongation. They do not discuss, however, the dust distribution near the Sun. They further discuss in detail the properties of the F-corona in Lamy et al. (2022).

To date, our understanding of the near-Sun dust environment is built on both *in situ* and remote measurements outside 0.3 AU, or 65 $R_\odot$. PSP, with its eccentric and progressively low perihelion orbit, provides the only *in situ* measurements and remote sensing observations of interplanetary dust in the near-Sun environment inside 0.3 AU. In the first six orbits alone, PSP has transited as close as 20 $R_\odot$ from the center of the Sun, offering an unprecedented opportunity to understand heliospheric dust in the densest part of the ZC and provide critical insight into long-standing fundamental science questions concerning the stellar processing of dust debris discs. Key questions PSP is well-posed to address are: How is the ZC eroded in the very near-Sun environment?; which populations of material are able to survive in this intense environment?; how do the near-Sun dust populations interact with the solar wind?, among others.

## 11.2 Dust Detection on PSP

A number of sensors on PSP are capable of detecting interplanetary dust in the inner heliosphere, each by a different mechanism. The FIELDS instrument detects perturbations to the S/C potential that result from transient plasma clouds formed when dust grains strike the S/C at high velocities, vaporizing and ionizing the impacting grain and some fraction of the S/C surface (Szalay et al. 2020a; Page et al. 2020; Malaspina et al. 2020c). WISPR detects solar photons scattered by dust in the ZC (Howard et al. 2019; Stenborg et al. 2021b), and ISOCIS is sensitive to dust penetration of its collimator foils by dust (Szalay et al. 2020a). Dust detection by these mechanisms has lead to advances in our understanding of the structure and evolution of the ZDC, and we describe these observations in the context of *in situ* and remote-based measurements separately below.

## 11.3 In Situ Impact Detection

### 11.3.1 FIELDS

As PSP traverses the inner heliosphere, its orbital trajectory results in high relative velocities between the S/C and interplanetary dust grains. Relative velocities for α-meteoroids can approach $100 \text{ km s}^{-1}$ and exceed 100's $\text{km s}^{-1}$ for β-meteoroids and retrograde impactors (Szalay et al. 2020a). The impact-generated plasma cloud perturbs the S/C surface potential, creating a signal detectable by the FIELDS electric field sensors. This method of *in situ* dust detection was first demonstrated on the *Voyager* S/C by Gurnett et al. (1983) and has subsequently been reported on numerous other S/C. See the review by Mann et al. (2019) and references therein.
density profile of the form $n(r) = n_0 r^{-1.3}$. This dust density dependence is well-reproduced by the dust produced by Jupiter-family comets (Pokorný and Kuchner 2019). Additionally, there were a number of discussions on the influence of excess dust in circumsolar rings near the Sun (Kimura and Mann 1998) on the observed F-corona brightness (Kimura et al. 1998). Later on, (Mann et al. 2004) showed that no prominent dust rings exist near the Sun.

More recently, Lamy et al. (2021). Lamy et al. (2021) analyzed images obtained with the SOHO/LASCO-C3 between 1996 and 2019. Based on a polarimetric analysis of the LASCO-C3 images, they separated the F- and K-corona components and derived the electron-density distribution. In addition, they reported the likely increasing polarization of the F-corona with increasing solar elongation. They do not discuss, however, the dust distribution near the Sun. They further discuss in detail the properties of the F-corona in Lamy et al. (2022).

To date, our understanding of the near-Sun dust environment is built on both *in situ* and remote measurements outside 0.3 AU, or 65 $R_\odot$. PSP, with its eccentric and progressively low perihelion orbit, provides the only *in situ* measurements and remote sensing observations of interplanetary dust in the near-Sun environment inside 0.3 AU. In the first six orbits alone, PSP has transited as close as 20 $R_\odot$ from the center of the Sun, offering an unprecedented opportunity to understand heliospheric dust in the densest part of the ZC and provide critical insight into long-standing fundamental science questions concerning the stellar processing of dust debris discs. Key questions PSP is well-posed to address are: How is the ZC eroded in the very near-Sun environment?; which populations of material are able to survive in this intense environment?; how do the near-Sun dust populations interact with the solar wind?, among others.

## 11.2 Dust Detection on PSP

A number of sensors on PSP are capable of detecting interplanetary dust in the inner heliosphere, each by a different mechanism. The FIELDS instrument detects perturbations to the S/C potential that result from transient plasma clouds formed when dust grains strike the S/C at high velocities, vaporizing and ionizing the impacting grain and some fraction of the S/C surface (Szalay et al. 2020a; Page et al. 2020; Malaspina et al. 2020c). WISPR detects solar photons scattered by dust in the ZC (Howard et al. 2019; Stenborg et al. 2021b), and ISOCIS is sensitive to dust penetration of its collimator foils by dust (Szalay et al. 2020a). Dust detection by these mechanisms has lead to advances in our understanding of the structure and evolution of the ZDC, and we describe these observations in the context of *in situ* and remote-based measurements separately below.

## 11.3 In Situ Impact Detection

### 11.3.1 FIELDS

As PSP traverses the inner heliosphere, its orbital trajectory results in high relative velocities between the S/C and interplanetary dust grains. Relative velocities for α-meteoroids can approach $100 \text{ km s}^{-1}$ and exceed 100's $\text{km s}^{-1}$ for β-meteoroids and retrograde impactors (Szalay et al. 2020a). The impact-generated plasma cloud perturbs the S/C surface potential, creating a signal detectable by the FIELDS electric field sensors. This method of *in situ* dust detection was first demonstrated on the *Voyager* S/C by Gurnett et al. (1983) and has subsequently been reported on numerous other S/C. See the review by Mann et al. (2019) and references therein.
While there is agreement that electric field sensors detect impact ionization of dust, the physical mechanism by which potential perturbations are generated continues to be an active topic of research, with a range of competing theories (Oberc 1996; Zaslavsky 2015; Kellogg et al. 2016; Meyer-Vernet et al. 2017; Mann et al. 2019; Shen et al. 2021b), and rapidly advancing lines of inquiry using controlled laboratory experiments (e.g., Collette et al. 2014, 2015, 2016; Nouzák et al. 2018; Shen et al. 2021a).

On PSP, the vast majority of dust impacts ionization events produce high amplitude (> 10 mV), brief (µs to ms) voltage spikes. These can be detected in various FIELDS data products, including peak detector data, bandpass filter data, high cadence time-series burst data, and lower cadence continuous time-series data (Bale et al. 2016; Malaspina et al. 2016).

Impact plasma clouds often produce asymmetric responses on electric field antennas (e.g., Malaspina et al. 2014). By comparing the relative dust signal amplitude on each FIELDS antenna for a given impact, the location of the impact on the S/C body can be inferred. From the impact location, and constraints imposed by dust population dynamics, one can deduce the pre-impact directionality of the dust that struck the S/C (Malaspina et al. 2020c; Pusack et al. 2021).

PSP data have revealed new physical processes active in the impact ionization of dust. Dudok de Wit et al. (2022) presented the first observations of magnetic signatures associated with the escape of electrons during dust impact ionization. Malaspina et al. (2022) demonstrated strong connection between the plasma signatures of dust impact ionization and subsequent debris clouds observed by WISPR and the PSP star trackers. This study also demonstrated that long-duration S/C potential perturbations, which follow some dust impacts, are consistent with theoretical expectations for clouds of S/C debris that electrostatically charge in the solar wind (Shen et al. 2021a). These perturbations can persist up to 60 seconds, much longer than the brief (µs to ms) voltage spikes generated by the vast majority of dust impacts.

## 11.3.2 Data-Model Comparisons

Since FIELDS can detect impacts over the entire S/C surface area, in the range of 4 – 7 m² (Page et al. 2020), PSP provides a robust observation of the total impact rate to the S/C. Figure 58 shows the impact rates as a function of heliocentric distance and in ecliptic J2000 coordinates (Szalay et al. 2021). There are a number of features that have been observed in the impact rate profiles. For the first three orbits, all with very similar orbits, a single pre-perihelion peak was observed. For the subsequent two orbit groups, orbits 4 – 5, and orbit 6, a post-perihelion peak was also observed, where a local minimum in impact rate was present near perihelion. As shown in Fig. 58c, the substructure in observed impact rate occurs inside the previous inner limit of in situ dust detections by Helios.

While PSP registers a large number of impacts due to its effective area, determining impactor speed, mass, and directionality is not straightforward. To interpret these impact rates into meaningful conclusions about inner zodiacal dust requires data-model comparisons. Analysis of PSP dust impact data from the first three orbits found the orbital variation in dust count rates detected by FIELDS during the first three solar Encs. were consistent with primarily sub-micron β-meteoroids (Szalay et al. 2020a; Page et al. 2020; Malaspina et al. 2020c). From the first three orbits, it was determined that the flux of β-meteoroids varies by approximately 50%, suggesting the inner solar system's collisional environment varies on timescales of 100's of days (Malaspina et al. 2020c). Additionally, nanograins with radii below 100 nm were not found to appreciably contribute to the observed impact rates from these first orbits (Mann and Czechowski 2021).
While there is agreement that electric field sensors detect impact ionization of dust, the physical mechanism by which potential perturbations are generated continues to be an active topic of research, with a range of competing theories (Oberc 1996; Zaslavsky 2015; Kellogg et al. 2016; Meyer-Vernet et al. 2017; Mann et al. 2019; Shen et al. 2021b), and rapidly advancing lines of inquiry using controlled laboratory experiments (e.g., Collette et al. 2014, 2015, 2016; Nouzák et al. 2018; Shen et al. 2021a).

On PSP, the vast majority of dust impacts ionization events produce high amplitude (> 10 mV), brief (µs to ms) voltage spikes. These can be detected in various FIELDS data products, including peak detector data, bandpass filter data, high cadence time-series burst data, and lower cadence continuous time-series data (Bale et al. 2016; Malaspina et al. 2016).

Impact plasma clouds often produce asymmetric responses on electric field antennas (e.g., Malaspina et al. 2014). By comparing the relative dust signal amplitude on each FIELDS antenna for a given impact, the location of the impact on the S/C body can be inferred. From the impact location, and constraints imposed by dust population dynamics, one can deduce the pre-impact directionality of the dust that struck the S/C (Malaspina et al. 2020c; Pusack et al. 2021).

PSP data have revealed new physical processes active in the impact ionization of dust. Dudok de Wit et al. (2022) presented the first observations of magnetic signatures associated with the escape of electrons during dust impact ionization. Malaspina et al. (2022) demonstrated strong connection between the plasma signatures of dust impact ionization and subsequent debris clouds observed by WISPR and the PSP star trackers. This study also demonstrated that long-duration S/C potential perturbations, which follow some dust impacts, are consistent with theoretical expectations for clouds of S/C debris that electrostatically charge in the solar wind (Shen et al. 2021a). These perturbations can persist up to 60 seconds, much longer than the brief (µs to ms) voltage spikes generated by the vast majority of dust impacts.

## 11.3.2 Data-Model Comparisons

Since FIELDS can detect impacts over the entire S/C surface area, in the range of 4 – 7 m² (Page et al. 2020), PSP provides a robust observation of the total impact rate to the S/C. Figure 58 shows the impact rates as a function of heliocentric distance and in ecliptic J2000 coordinates (Szalay et al. 2021). There are a number of features that have been observed in the impact rate profiles. For the first three orbits, all with very similar orbits, a single pre-perihelion peak was observed. For the subsequent two orbit groups, orbits 4 – 5, and orbit 6, a post-perihelion peak was also observed, where a local minimum in impact rate was present near perihelion. As shown in Fig. 58c, the substructure in observed impact rate occurs inside the previous inner limit of in situ dust detections by Helios.

While PSP registers a large number of impacts due to its effective area, determining impactor speed, mass, and directionality is not straightforward. To interpret these impact rates into meaningful conclusions about inner zodiacal dust requires data-model comparisons. Analysis of PSP dust impact data from the first three orbits found the orbital variation in dust count rates detected by FIELDS during the first three solar Encs. were consistent with primarily sub-micron β-meteoroids (Szalay et al. 2020a; Page et al. 2020; Malaspina et al. 2020c). From the first three orbits, it was determined that the flux of β-meteoroids varies by approximately 50%, suggesting the inner solar system’s collisional environment varies on timescales of 100’s of days (Malaspina et al. 2020c). Additionally, nanograins with radii below 100 nm were not found to appreciably contribute to the observed impact rates from these first orbits (Mann and Czechowski 2021).
<figure><img src="image_60.png" /><figcaption>**Fig. 58** Daily averaged impact rates as a function of radial distance for orbits 1 – 6, separated by inbound (a) and outbound (b). (c) Impact rates overlaid on the PSP trajectory in the ecliptic J2000 frame, averaged over orbits 1 – 3, 4 – 5, and individually shown for orbit 6. Color and width of the color strip represents the impact rate. Figure adapted from Szalay et al. (2021)</figcaption></figure>

Subsequent analysis which included the first six orbits (Szalay et al. 2021) compared PSP data to a two-component analytic dust model to conclude PSP's dust impact rates are consistent with at least three distinct populations: (α) bound zodiacal α-meteoroids on eliptic orbits, (β) unbound β-meteoroids on hyperbolic orbits, and a distinct third population of impactors. Unlike during the first three orbits of dust impact data, which were dominated by escaping β-meteoroids, larger grains have been inferred to dominate FIELDS detections for sections of each orbit (Szalay et al. 2021) during orbits 4 – 6. Data-model comparisons from the first six orbits have already provided important insight on the near-Sun dust environment. First, they placed quantitative constraints on the zodiacal collisional erosion rate of greater than 100 kg s⁻¹. This material, in the form of outgoing β-meteoroids, was found to be predominantly produced within 10−20 $R_⊕$. It was also determined that β-meteoroids are unlikely to be the inner source of pickup ions, instead suggesting the population of trapped nanograins (Czechowski and Mann 2010) with radii ≲ 50 nm is likely this source. The flux of β-meteoroids at 1 AU was also estimated to be in the range of 0.4−0.8 × 10⁻⁴ m⁻² s⁻¹.

From the data-model comparisons, orbits 4 – 6 exhibited a post-perihelion peak in the impact rate profile that was not well-described using the two-component model of nominal α-meteoroids and β-meteoroids (Szalay et al. 2021). Two hypotheses were provided to explain this post-perihelion impact rate enhancements: (a) PSP directly transited and observed grains within a meteoroid stream or (b) PSP flew through the collisional by-products produced when a meteoroid stream collides with the nominal ZC, termed a β-stream. The timing and total flux observed during this time favors the latter explanation, and more specifically, a β-stream from the Geminids meteoroid stream was suggested to be the most likely candidate (Szalay et al. 2021). A separate analysis focusing on the directionality and amplitude distribution during the orbit 4 post-perihelion impact rate enhancement also supports the Geminids β-stream hypothesis (Pusack et al. 2021). Figure 59 shows the amplitude and directionality trends observed during orbit 4, where the two impact rate peaks exhibit different behaviors. For the pre-perihelion peak, predicted by the two-component model, impact rates for multiple separate amplitude ranges all peak at similar times (Fig. 59b) and impact the S/C from similar locations (Fig. 59c). The post-perihelion peak exhibits a clear amplitude dispersion, where impacts producing smaller amplitudes peak in rate ahead of the impacts that produce larger amplitudes. Additionally, the ram/anti-ram ratio is significantly different from the pre-perihelion peak. As further described in Pusack et al. (2021), these differences
<figure><img src="image_61.png" /><figcaption>**Fig. 58** Daily averaged impact rates as a function of radial distance for orbits 1–6, separated by inbound (a) and outbound (b). (c) Impact rates overlaid on the PSP trajectory in the ecliptic J2000 frame, averaged over orbits 1–3, 4–5, and individually shown for orbit 6. Color and width of the color strip represents the impact rate. Figure adapted from Szalay et al. (2021)</figcaption></figure>

Subsequent analysis which included the first six orbits (Szalay et al. 2021) compared PSP data to a two-component analytic dust model to conclude PSP's dust impact rates are consistent with at least three distinct populations: (α) bound zodiacal α-meteoroids on eliptic orbits, (β) unbound β-meteoroids on hyperbolic orbits, and a distinct third population of impactors. Unlike during the first three orbits of dust impact data, which were dominated by escaping β-meteoroids, larger grains have been inferred to dominate FIELDS detections for sections of each orbit (Szalay et al. 2021) during orbits 4–6. Data-model comparisons from the first six orbits have already provided important insight on the near-Sun dust environment. First, they placed quantitative constraints on the zodiacal collisional erosion rate of greater than 100 kg s⁻¹. This material, in the form of outgoing β-meteoroids, was found to be predominantly produced within 10−20 $R_⊕$. It was also determined that β-meteoroids are unlikely to be the inner source of pickup ions, instead suggesting the population of trapped nanograins (Czechowski and Mann 2010) with radii ≲ 50 nm is likely this source. The flux of β-meteoroids at 1 AU was also estimated to be in the range of 0.4−0.8 × 10⁻⁴ m⁻² s⁻¹.

From the data-model comparisons, orbits 4–6 exhibited a post-perihelion peak in the impact rate profile that was not well-described using the two-component model of nominal α-meteoroids and β-meteoroids (Szalay et al. 2021). Two hypotheses were provided to explain this post-perihelion impact rate enhancements: (a) PSP directly transited and observed grains within a meteoroid stream or (b) PSP flew through the collisional by-products produced when a meteoroid stream collides with the nominal ZC, termed a β-stream. The timing and total flux observed during this time favors the latter explanation, and more specifically, a β-stream from the Geminids meteoroid stream was suggested to be the most likely candidate (Szalay et al. 2021). A separate analysis focusing on the directionality and amplitude distribution during the orbit 4 post-perihelion impact rate enhancement also supports the Geminids β-stream hypothesis (Pusack et al. 2021). Figure 59 shows the amplitude and directionality trends observed during orbit 4, where the two impact rate peaks exhibit different behaviors. For the pre-perihelion peak, predicted by the two-component model, impact rates for multiple separate amplitude ranges all peak at similar times (Fig. 59b) and impact the S/C from similar locations (Fig. 59c). The post-perihelion peak exhibits a clear amplitude dispersion, where impacts producing smaller amplitudes peak in rate ahead of the impacts that produce larger amplitudes. Additionally, the ram/anti-ram ratio is significantly different from the pre-perihelion peak. As further described in Pusack et al. (2021), these differences
<figure><img src="image_61.png" /><figcaption>**Fig. 59** (a) Orbit 4 count rates vs. time for antennas V1, V2, V3, and V4 using the 50–1000 mV amplitude window with darker gray lines corresponding to the ram direction of the S/C and lighter gray lines corresponding to the anti-ram direction. (b) Count rates vs. time for each amplitude window on all four planar antennas. Gray shaded region indicates the anomaly duration. (c) carries with it the same gradation as (a). of tone to the various color families depicted, where each color family corresponds to a different amplitude window: blues for 50–100 mV, orange–yellows for 100–200 mV, pinks for 200–400 mV, and greens for 400–1000 mV. (d) Amplitude window ram/anti-ram rates vs. time with the same color families as (b). Figure adapted from Pusack et al. (2021)</figcaption></figure>

<figure><img src="image_62.png" /><figcaption>**Fig. 60** PSP detects impacts due to α-meteoroids, β-meteoroids, and likely from discrete meteoroid streams. Left: Impact rates and model fits from orbit 3 (inbound) and orbit 4 (outbound). Right: Sources for the multiple populations observed by PSP. Figure adapted from Szalay et al. (2021)</figcaption></figure>
<figure><img src="image_62.png" /><figcaption>**Fig. 59** (a) Orbit 4 count rates vs. time for antennas V1, V2, V3, and V4 using the 50–1000 mV amplitude window with darker gray lines corresponding to the ram direction of the S/C and lighter gray lines corresponding to the anti-ram direction. (b) Count rates vs. time for each amplitude window on all four planar antennas. Gray shaded region indicates the anomaly duration. (c) carries with it the same gradation as (a) of tone to the various color families depicted, where each color family corresponds to a different amplitude window: blues for 50–100 mV, orange–yellows for 100–200 mV, pinks for 200–400 mV, and greens for 400–1000 mV. (d) Amplitude window ram/anti-ram rates vs. time with the same color families as (b). Figure adapted from Pusack et al. (2021)</figcaption></figure>

<figure><img src="image_63.png" /><figcaption>**Fig. 60** PSP detects impacts due to α-meteoroids, β-meteoroids, and likely from discrete meteoroid streams. Left: Impact rates and model fits from orbit 3 (inbound) and orbit 4 (outbound). Right: Sources for the multiple populations observed by PSP. Figure adapted from Szalay et al. (2021)</figcaption></figure>
are also suggestive of a Geminids β-stream. We note that grains that are smaller than the detected β-meteoroids and affected by electromagnetic forces have a much larger flux close to the orbital perihelia than at other parts of the orbit (Mann and Czechowski 2021), yet their detection is difficult with PSP/FIELDS due to the low expected impact charge generated by such small mass grains (Szalay et al. 2021).

Figure 60 summarizes the dust populations PSP is likely directly encountering. From the data-model comparisons, the relative fluxes and densities of bound α-meteoroids and unbound β-meteoroids has been quantitatively constrained. PSP's dust impact measurements have been able to directly inform on the intense near-Sun dust environment. Furthermore, the existence of a third dust population suggests collisions between material along asteroid or cometary orbits can be a significant source of near-sun collisional grinding and β-meteoroid production in the form of β-stream (Szalay et al. 2021), and is a fundamental physical process occurring at all stellar dust clouds.

## 11.4 Remote Sensing

### 11.4.1 Near-Sun Dust Density Radial Dependence

In anticipation of the PSP observations, several studies of the ZL/F-corona based on observations from the STEREO/SECCHI instrument were carried out (Stenborg and Howard 2017a,b; Stenborg et al. 2018a; Stauffer et al. 2018). These studies established a baseline of F-corona properties from 1 AU to help identify any differences that may arise due to the varying heliocentric distance of the corresponding WISPR observations.

The question of whether a DFZ (Russell 1929) exists close to the Sun is long-standing and has not been answered by pre-PSP observations of the ZL/F-corona. White light observations obtained from distances between 0.3 – 1 AU (e.g., Stenborg et al. 2018a; Leinert et al. 1981) do not reveal any break in the radial gradient of the brightness along of the symmetry plane of the ZDC, which was found to follow a power law $I(r) \sim r^{-2.3}$ to at least below the theoretically predicted start of the DFZ at $\approx 4 - 5R_\odot$.

The WISPR instrument has recorded the intensities of the ZL/F-corona from ever decreasing observing distances, down to about 0.074 AU (~15.9 $R_\odot$) at the last executed perihelion (by the time of this writing). This unprecedented observer distance corresponds to an inner limit of the FOV of WISPR-i of about 3.7 $R_\odot$ (0.017 AU). A striking result from the WISPR observations obtained during the first five orbits, was the departure of the radial dependence of the F-corona brightness profile along the symmetry axis of the ZDC from the previously-established power law (Howard et al. 2019; Stenborg et al. 2021b, hereafter referred to as HS). In the left panel of Fig. 61, we show a sample of WISPR brightness profiles along the symmetry axis obtained during orbits 1, 2, 4, and 5 (in black color), along with the fitting of an empirical model comprising a linear and an exponential function (red dashed line). The linear portion of the empirical model is delineated with the light-blue dashed line. We note that the linear behavior (i.e., constant gradient) continues down to $\sim 20 R_\odot$ with the same slope as observed in former studies (i.e., $\propto r^{-2.3}$). Below that elongation distance, the radial brightness gradient becomes less steep. The modeled brightness measurements depart by about 35% at the inner limit of 7.65 $R_\odot$ (0.036 AU) from the extrapolation of the linear part the model (see inset in Fig. 61). The brightness decrease is quite smooth down to 7.65 $R_\odot$, i.e., it does not show discrete brightness enhancements due to sublimation effects of any particular dust species (Kobayashi et al. 2009).

The brightness profile was forward-modeled using RAYTRACE (Thernisien and Howard 2006),<fnref n="11" /> which was adapted to integrate the product of an empirical volume scattering func-

<footnote n="11">¹¹RAYTRACE is available in the SOHO Solarsoft library, http://www.lmsal.com/solarsoft/.</footnote>
are also suggestive of a Geminids β-stream. We note that grains that are smaller than the detected β-meteoroids and affected by electromagnetic forces have a much larger flux close to the orbital perihelia than at other parts of the orbit (Mann and Czechowski 2021), yet their detection is difficult with PSP/FIELDS due to the low expected impact charge generated by such small mass grains (Szalay et al. 2021).

Figure 60 summarizes the dust populations PSP is likely directly encountering. From the data-model comparisons, the relative fluxes and densities of bound α-meteoroids and unbound β-meteoroids has been quantitatively constrained. PSP's dust impact measurements have been able to directly inform on the intense near-Sun dust environment. Furthermore, the existence of a third dust population suggests collisions between material along asteroid or cometary orbits can be a significant source of near-sun collisional grinding and β-meteoroid production in the form of β-stream (Szalay et al. 2021), and is a fundamental physical process occurring at all stellar dust clouds.

## 11.4 Remote Sensing

### 11.4.1 Near-Sun Dust Density Radial Dependence

In anticipation of the PSP observations, several studies of the ZL/F-corona based on observations from the STEREO/SECCHI instrument were carried out (Stenborg and Howard 2017a,b; Stenborg et al. 2018a; Stauffer et al. 2018). These studies established a baseline of F-corona properties from 1 AU to help identify any differences that may arise due to the varying heliocentric distance of the corresponding WISPR observations.

The question of whether a DFZ (Russell 1929) exists close to the Sun is long-standing and has not been answered by pre-PSP observations of the ZL/F-corona. White light observations obtained from distances between 0.3 – 1 AU (e.g., Stenborg et al. 2018a; Leinert et al. 1981) do not reveal any break in the radial gradient of the brightness along of the symmetry plane of the ZDC, which was found to follow a power law $I(r) \sim r^{-2.3}$ to at least below the theoretically predicted start of the DFZ at $\approx 4 - 5R_\odot$.

The WISPR instrument has recorded the intensities of the ZL/F-corona from ever decreasing observing distances, down to about 0.074 AU (~15.9 $R_\odot$) at the last executed perihelion (by the time of this writing). This unprecedented observer distance corresponds to an inner limit of the FOV of WISPR-i of about 3.7 $R_\odot$ (0.017 AU). A striking result from the WISPR observations obtained during the first five orbits, was the departure of the radial dependence of the F-corona brightness profile along the symmetry axis of the ZDC from the previously-established power law (Howard et al. 2019; Stenborg et al. 2021b, hereafter referred to as HS). In the left panel of Fig. 61, we show a sample of WISPR brightness profiles along the symmetry axis obtained during orbits 1, 2, 4, and 5 (in black color), along with the fitting of an empirical model comprising a linear and an exponential function (red dashed line). The linear portion of the empirical model is delineated with the light-blue dashed line. We note that the linear behavior (i.e., constant gradient) continues down to $\sim 20 R_\odot$ with the same slope as observed in former studies (i.e., $\propto r^{-2.3}$). Below that elongation distance, the radial brightness gradient becomes less steep. The modeled brightness measurements depart by about 35% at the inner limit of 7.65 $R_\odot$ (0.036 AU) from the extrapolation of the linear part the model (see inset in Fig. 61). The brightness decrease is quite smooth down to 7.65 $R_\odot$, i.e., it does not show discrete brightness enhancements due to sublimation effects of any particular dust species (Kobayashi et al. 2009).

The brightness profile was forward-modeled using RAYTRACE (Thernisien and Howard 2006),<fnref n="11" /> which was adapted to integrate the product of an empirical volume scattering func-

<footnote n="11">¹¹RAYTRACE is available in the SOHO Solarsoft library, http://www.lmsal.com/solarsoft/.</footnote>
<figure><img src="image_63.png" /><figcaption>**Fig. 61** (a) Left panel: Sample of radial brightness gradients along the symmetry axis of the F-corona (black) and data fit with an empirical model (red dashed line). The linear portion of the model is delineated with the light-blue dashed line. The inset shows the percentage departure of the empirical model from the linear trend. Upper Right panel: Comparison of the empirical model (in black color) and the forward modeling of the ZL brightness along the symmetry axis considering a DDZ between 2–20 $R_\odot$ (in green color) and 3–19 $R_\odot$ (in red color). The inset shows the relative error of the simulations compared to the empirical model (same color code). Bottom Right panel: Dust density model used in the simulations relative to the outermost edge of the DDZ between 3–19 $R_\odot$ assuming a linear decrease in the multiplier of the nominal density. The dashed, vertical line in a red color indicates the innermost distance of the WISPR FOV in this study. Figure adapted from Stenborg et al. (2021b)</figcaption></figure>

tion (VSF) and a dust density at each point along any given LOS. The VSF was given by Lamy and Perrin (1986), which condenses all the physics of the scattering process into an empirical function of a single parameter, the scattering angle. The dust density along the symmetry axis of the ZDC was taken from Leinert et al. (1981) ($n(r) \propto r^{-1.3}$). The intensity decrease observed in WISPR results was ascribed to the presence of a dust depletion zone (DDZ), which appears to begin somewhere between 19 and 20 $R_\odot$ and extends sunward to the beginning of the DFZ. To model the density decrease in the DDZ, we used a multiplier in the density model defined as a linear factor that varied between 1 at the outer boundary of the DDZ and 0 at the inner boundary. The extent of the DDZ was determined empirically by matching the RAYTRACE calculation with the empirical model of the radial brightness profile.

In the upper right panel of Fig. 61 we show in green and red colors (the red is fully underneath the green) the forward modeling of the brightness with two different boundaries for the DDZ along with the empirical model (in black). The inner boundary was a free parameter to give the best match to the empirical model. The 3–19 $R_\odot$ range (green) for the DDZ yields a slightly better fit to the observation than in the 2–20 $R_\odot$ range (red). Note that the behavior below the observational limit of 7.65 $R_\odot$ is only an extrapolation. The inset shows the difference between the two forward models. We thus choose 19 $R_\odot$ as the upper limit of the DDZ, although depletion could start beyond 19 $R_\odot$ but doesn't cause a noticeable change in the intensities until about 19 $R_\odot$.

In the bottom right panel of Fig. 61, we show the radial profile of the dust density relative to the density at 19 $R_\odot$ for the best fit to the intensity profile. Note that from ~10 to 19 $R_\odot$, the density appears to be approximately constant. In future orbits, WISPR will observe the corona down to 2.3 $R_\odot$, which will help establish more accurately the actual limit of the DFZ.
<figure><img src="image_64.png" /><figcaption>**Fig. 61** (a) Left panel: Sample of radial brightness gradients along the symmetry axis of the F-corona (black) and data fit with an empirical model (red dashed line). The linear portion of the model is delineated with the light-blue dashed line. The inset shows the percentage departure of the empirical model from the linear trend. Upper Right panel: Comparison of the empirical model (in black color) and the forward modeling of the ZL brightness along the symmetry axis considering a DDZ between 2–20 $R_\odot$ (in green color) and 3–19 $R_\odot$ (in red color). The inset shows the relative error of the simulations compared to the empirical model (same color code). Bottom Right panel: Dust density model used in the simulations relative to the outermost edge of the DDZ between 3–19 $R_\odot$ assuming a linear decrease in the multiplier of the nominal density. The dashed, vertical line in a red color indicates the innermost distance of the WISPR FOV in this study. Figure adapted from Stenborg et al. (2021b)</figcaption></figure>

tion (VSF) and a dust density at each point along any given LOS. The VSF was given by Lamy and Perrin (1986), which condenses all the physics of the scattering process into an empirical function of a single parameter, the scattering angle. The dust density along the symmetry axis of the ZDC was taken from Leinert et al. (1981) ($n(r) \propto r^{-1.3}$). The intensity decrease observed in WISPR results was ascribed to the presence of a dust depletion zone (DDZ), which appears to begin somewhere between 19 and 20 $R_\odot$ and extends sunward to the beginning of the DFZ. To model the density decrease in the DDZ, we used a multiplier in the density model defined as a linear factor that varied between 1 at the outer boundary of the DDZ and 0 at the inner boundary. The extent of the DDZ was determined empirically by matching the RAYTRACE calculation with the empirical model of the radial brightness profile.

In the upper right panel of Fig. 61 we show in green and red colors (the red is fully underneath the green) the forward modeling of the brightness with two different boundaries for the DDZ along with the empirical model (in black). The inner boundary was a free parameter to give the best match to the empirical model. The 3–19 $R_\odot$ range (green) for the DDZ yields a slightly better fit to the observation than in the 2–20 $R_\odot$ range (red). Note that the behavior below the observational limit of 7.65 $R_\odot$ is only an extrapolation. The inset shows the difference between the two forward models. We thus choose 19 $R_\odot$ as the upper limit of the DDZ, although depletion could start beyond 19 $R_\odot$ but doesn't cause a noticeable change in the intensities until about 19 $R_\odot$.

In the bottom right panel of Fig. 61, we show the radial profile of the dust density relative to the density at 19 $R_\odot$ for the best fit to the intensity profile. Note that from ~10 to 19 $R_\odot$, the density appears to be approximately constant. In future orbits, WISPR will observe the corona down to 2.3 $R_\odot$, which will help establish more accurately the actual limit of the DFZ.
### 11.4.2 Implications for Collisions and/or Sublimation

The smooth behavior of the radial brightness profile of the F-corona along its symmetry axis from 35 $R_\odot$ down to 7.65 $R_\odot$ is suggestive of a smooth and continuous process of dust removal. No evidence is seen of dust depletion at a particular distance due to the sublimation of a particular species. Thus the dust remaining at these distances is probably similar to quartz or obsidian, which are fairly resistant to sublimation (e.g., Mann et al. 2004).

### 11.4.3 Dust Density Enhancement Along the Inner Planets' Orbits

In addition to measurements of the broad ZC structure, discrete dust structures have also been observed by WISPR. A dust density enhancement nearby Earth's orbit was theoretically predicted in the late eighties by Jackson and Zook (1989) and observationally confirmed by Dermott et al. (1994) using observations from the Infrared Astronomy Satellite (IRAS; Neugebauer et al. 1984a). Reach et al. (1995) confirmed the predicted structure of the dust ring near Earth using observations from the Diffuse Infrared Background Experiment (DIRBE; Silverberg et al. 1993) on the Cosmic Background Explorer mission (COBE; Boggess et al. 1992). More recently, in a reanalysis of white light observations from the *Helios* mission (Porsche 1981), Leinert and Moster (2007) found evidence of a brightness enhancement nearby Venus' orbit, which was later confirmed by Jones et al. (2013, 2017) using *STEREO/SECCHI* observations. Finally, in spite of the lack of a theoretical prediction, a very faint, circumsolar dust ring associated with Mercury was indirectly inferred from 6+ years of white-light observations (Stenborg et al. 2018b) obtained with the *STEREO*-Ahead/HI-1 instrument. In all the observational cases mentioned above, only particular viewing geometries allowed the detection of just a small portion of the dust rings.

The *Helios* measurements were carried out with the 90° photometer of the Zodiacal Light Experiment (ZLE; Leinert et al. 1975), which looked perpendicular to the ecliptic plane. The observations reported a 2% increase in brightness as *Helios* crossed just outside of Venus's orbit (Leinert and Moster 2007). On the other hand, the *STEREO* observations were obtained with the SECCHI/HI-2 telescopes, which image the interplanetary medium about ±20° above and below the ecliptic plane. In the latter, the brightness enhancements observed were detected only when the viewing geometry was tangent to the orbit of Venus. The findings were interpreted, via theoretical modeling, as due to the presence of a resonant dust ring slightly beyond Venus' orbit (Jones et al. 2013, 2017). However, in a more recent work, the dust environment near Venus' orbit was modeled by coalescing the orbital paths of more than 10,000,000 dust particles of different provenance under the influence of gravitational and non-gravitational forces (Pokorný and Kuchner 2019). According to this model, an hypothetical population of dust particles released by Venus co-orbital asteroids could be stable enough to produce enough signal to match the observations. So far, twilight telescopic surveys have not found any long-term stable Venus co-orbital asteroids (Pokorný et al. 2020); however, their existence cannot be ruled out.

At visible wavelengths, the high density and scattering properties of the dust particles in the ZC (e.g., Lamy and Perrin 1986), makes it difficult to detect localized density structures embedded in it from 1 AU. However, as shown in Stenborg et al. (2021a), the PSP mission traveling through regions never visited before by any man-made probe, allows the comprehensive visualization of discrete dust density enhancements in the ZDC. As with other white-light heliospheric imagers, the scene recorded in WISPR observations is dominated by the ZL (or F-corona close to the Sun; see, e.g., Howard et al. 2019). To reveal discrete, stationary F-corona features in the FOV of the WISPR instrument, it is necessary to estimate
### 11.4.2 Implications for Collisions and/or Sublimation

The smooth behavior of the radial brightness profile of the F-corona along its symmetry axis from 35 $R_\odot$ down to 7.65 $R_\odot$ is suggestive of a smooth and continuous process of dust removal. No evidence is seen of dust depletion at a particular distance due to the sublimation of a particular species. Thus the dust remaining at these distances is probably similar to quartz or obsidian, which are fairly resistant to sublimation (e.g., Mann et al. 2004).

### 11.4.3 Dust Density Enhancement Along the Inner Planets' Orbits

In addition to measurements of the broad ZC structure, discrete dust structures have also been observed by WISPR. A dust density enhancement nearby Earth's orbit was theoretically predicted in the late eighties by Jackson and Zook (1989) and observationally confirmed by Dermott et al. (1994) using observations from the Infrared Astronomy Satellite (IRAS; Neugebauer et al. 1984a). Reach et al. (1995) confirmed the predicted structure of the dust ring near Earth using observations from the Diffuse Infrared Background Experiment (DIRBE; Silverberg et al. 1993) on the Cosmic Background Explorer mission (COBE; Boggess et al. 1992). More recently, in a reanalysis of white light observations from the *Helios* mission (Porsche 1981), Leinert and Moster (2007) found evidence of a brightness enhancement nearby Venus' orbit, which was later confirmed by Jones et al. (2013, 2017) using *STEREO/SECCHI* observations. Finally, in spite of the lack of a theoretical prediction, a very faint, circumsolar dust ring associated with Mercury was indirectly inferred from 6+ years of white-light observations (Stenborg et al. 2018b) obtained with the *STEREO*-Ahead/HI-1 instrument. In all the observational cases mentioned above, only particular viewing geometries allowed the detection of just a small portion of the dust rings.

The *Helios* measurements were carried out with the 90° photometer of the Zodiacal Light Experiment (ZLE; Leinert et al. 1975), which looked perpendicular to the ecliptic plane. The observations reported a 2% increase in brightness as *Helios* crossed just outside of Venus's orbit (Leinert and Moster 2007). On the other hand, the *STEREO* observations were obtained with the SECCHI/HI-2 telescopes, which image the interplanetary medium about ±20° above and below the ecliptic plane. In the latter, the brightness enhancements observed were detected only when the viewing geometry was tangent to the orbit of Venus. The findings were interpreted, via theoretical modeling, as due to the presence of a resonant dust ring slightly beyond Venus' orbit (Jones et al. 2013, 2017). However, in a more recent work, the dust environment near Venus' orbit was modeled by coalescing the orbital paths of more than 10,000,000 dust particles of different provenance under the influence of gravitational and non-gravitational forces (Pokorný and Kuchner 2019). According to this model, an hypothetical population of dust particles released by Venus co-orbital asteroids could be stable enough to produce enough signal to match the observations. So far, twilight telescopic surveys have not found any long-term stable Venus co-orbital asteroids (Pokorný et al. 2020); however, their existence cannot be ruled out.

At visible wavelengths, the high density and scattering properties of the dust particles in the ZC (e.g., Lamy and Perrin 1986), makes it difficult to detect localized density structures embedded in it from 1 AU. However, as shown in Stenborg et al. (2021a), the PSP mission traveling through regions never visited before by any man-made probe, allows the comprehensive visualization of discrete dust density enhancements in the ZDC. As with other white-light heliospheric imagers, the scene recorded in WISPR observations is dominated by the ZL (or F-corona close to the Sun; see, e.g., Howard et al. 2019). To reveal discrete, stationary F-corona features in the FOV of the WISPR instrument, it is necessary to estimate
<figure><img src="image_64.png" /></figure>

**Fig. 62** Combined WISPR observations of a circumsolar dust ring near Venus's orbit on 25 Aug. 2019. Images are projected onto the surface of a sphere with observer at the center (PSP S/C) and radius equal to the heliocentric distance of the observer. The Sun is not to scale. The gray areas surrounding the bright point-like objects (Mercury, Venus, and Earth) are artifacts of the image processing due to the saturation caused by their excessive brightness. The odd oval-shaped object and its surrounding area are caused by reflections in the optics of the very bright Venus. The red dots delineate Venus's orbital path, the dashed orange line the ecliptic, and the yellow dotted line the invariable plane. Figure adapted from Stenborg et al. (2021a)

the F-corona background component (for its subsequent removal from the images) with images where the stationary feature is present at a different location in the FOV. By exploiting the different rolls while the S/C was between 0.5 and 0.25 AU, Stenborg et al. (2021a) revealed the first comprehensive, white light observation of a brightness enhancement across a 345° longitudinal extension along the Venus' orbit.

Figure 62 shows a composite panorama of the Venusian dust ring in WISPR images acquired during the inbound segment of orbit 3 while the PSP S/C was performing roll maneuvers (as extracted from Stenborg et al. 2021a). The study showed that the latitudinal extension of the brightness enhancement corresponds to a dust ring extending 0.043 AU ± 0.004 AU, co-spatial with Venus' orbital path. Likewise, the median excess brightness of the band w.r.t. the background (of about 1%), was shown to correspond to a dust density enhancement relative to the local density of the ZC of about 10%. Both, the latitudinal extension and density estimates is in general agreement with the findings of Leinert and Moster (2007) and Jones et al. (2013, 2017). The viewing geometry only allowed a measure of the inclination and projected height of the ring, not of its radial position or extent. Therefore, no detailed information on the distance of the dust ring from the orbit of Venus could be extracted.

### 11.4.4 Dust Trail of 3200 Phaethon

Discovered in 1983 (Green and Kowal 1983), asteroid (3200) Phaethon is one of the most widely-studied inner solar system minor bodies, by virtue of a 1.434 year orbit, its large size for a near-Earth object (6 km in diameter, Taylor et al. 2019), and a low 0.0196 AU Earth minimum orbit intersection distance (MOID) favorable to ground-based optical and radar observations (Jewitt 1991; Jewitt and Li 2010). Phaethon is recognized as the parent of the Geminid meteor shower and is associated with the Phaethon-Geminid meteoroid
<figure><img src="image_65.png" /></figure>

**Fig. 62** Combined WISPR observations of a circumsolar dust ring near Venus's orbit on 25 Aug. 2019. Images are projected onto the surface of a sphere with observer at the center (PSP S/C) and radius equal to the heliocentric distance of the observer. The Sun is not to scale. The gray areas surrounding the bright point-like objects (Mercury, Venus, and Earth) are artifacts of the image processing due to the saturation caused by their excessive brightness. The odd oval-shaped object and its surrounding area are caused by reflections in the optics of the very bright Venus. The red dots delineate Venus's orbital path, the dashed orange line the ecliptic, and the yellow dotted line the invariable plane. Figure adapted from Stenborg et al. (2021a)

the F-corona background component (for its subsequent removal from the images) with images where the stationary feature is present at a different location in the FOV. By exploiting the different rolls while the S/C was between 0.5 and 0.25 AU, Stenborg et al. (2021a) revealed the first comprehensive, white light observation of a brightness enhancement across a 345° longitudinal extension along the Venus' orbit.

Figure 62 shows a composite panorama of the Venusian dust ring in WISPR images acquired during the inbound segment of orbit 3 while the PSP S/C was performing roll maneuvers (as extracted from Stenborg et al. 2021a). The study showed that the latitudinal extension of the brightness enhancement corresponds to a dust ring extending 0.043 AU ± 0.004 AU, co-spatial with Venus' orbital path. Likewise, the median excess brightness of the band w.r.t. the background (of about 1%), was shown to correspond to a dust density enhancement relative to the local density of the ZC of about 10%. Both, the latitudinal extension and density estimates is in general agreement with the findings of Leinert and Moster (2007) and Jones et al. (2013, 2017). The viewing geometry only allowed a measure of the inclination and projected height of the ring, not of its radial position or extent. Therefore, no detailed information on the distance of the dust ring from the orbit of Venus could be extracted.

#### 11.4.4 Dust Trail of 3200 Phaethon

Discovered in 1983 (Green and Kowal 1983), asteroid (3200) Phaethon is one of the most widely-studied inner solar system minor bodies, by virtue of a 1.434 year orbit, its large size for a near-Earth object (6 km in diameter, Taylor et al. 2019), and a low 0.0196 AU Earth minimum orbit intersection distance (MOID) favorable to ground-based optical and radar observations (Jewitt 1991; Jewitt and Li 2010). Phaethon is recognized as the parent of the Geminid meteor shower and is associated with the Phaethon-Geminid meteoroid
<figure><img src="image_65.png" /><figcaption>**Fig. 63** WISPR-i observations recorded on 5 Nov. 2018, 14:14 UT, 6 Nov. 1:43 UT and 6 Nov. 14:54 UT. Plotted symbols indicate the imaginary position of Phaethon along the orbit in 60 minute increments, both pre-perihelion (blue) and post-perihelion (white). Symbols are excluded in the region where the trail is most easily visible. The white diamond indicates the perihelion position of the orbit in the FOV. Figure adapted from Battams et al. (2020)</figcaption></figure>

stream complex including likely relationships with asteroids 2005 UD and 1999 YC (e.g., Whipple 1983; Gustafson 1989; Williams and Wu 1993; Ohtsuka et al. 2006, 2008). Due to a small 0.14 AU perihelion distance, observations of Phaethon near the Sun are impossible from traditional ground-based telescopes. The first detections of Phaethon at perihelion were made by STEREO/SECCHI (Jewitt et al. 2013). While Phaethon is active near perihelion and experiences an intense impact environment near the Sun, the mass-loss rates from cometary-like activity (Jewitt et al. 2013) and impact ejecta (Szalay et al. 2019) were both found to be orders of magnitude too low to sustain the Geminids.

As presented in Battams et al. (2020), an unexpected white-light feature revealed in the WISPR background-corrected data was the presence of a faint extended dust trail following the orbit of Phaethon. In Fig. 63, we show three WISPR-i telescope observations that highlight the most visible portion of this dust trail as detected during PSP Enc. 1, which is seen following the projection of Phaethon's orbital path perfectly.

Despite the dust trail being close to the instrument noise floor, the mean brightness along the trail was determined to be $8.2 \times 10^{-15} B_{\odot}$ (where $B_{\odot}$ is the mean solar brightness), which equates to a visual magnitude of $15.8 \pm 0.3$ per pixel. This result, coupled with the $70 \ \text{arcsec}^{-2}$ per pixel resolution of WISPR-i, yields an estimated surface brightness of $25.0 \ \text{mag arcsec}^{-2}$ for the dust trail, which in turn is shown to yield a total mass of dust in the entire trail of $\sim (0.4 - 1.3) \times 10^{12} \ \text{kg}$. This mass estimate is inconsistent with dust by Phaethon at perihelion, but is plausibly in-line (slightly below) mass estimates of the Geminids. The difference is attributed primarily to the faintness of the detection.

This detection highlights the remarkable sensitivity of WISPR to white-light dust structures. Recent ground- and space-based surveys have failed to detect a dust trail in the orbit of 3200 Phaethon (Ye et al. 2018). The WISPR observation explains this as, when factors such as heliocentric distance and orbital spreading/clustering of the dust are considered, it can be shown that the surface brightness of the trail as seen from a terrestrial viewpoint is less than $30 \ \text{mag arcsec}^{-2}$, which constitutes an extremely challenging target even for deep sky surveys.

The Phaethon dust trail continues to be clearly observed in the WISPR data in every PSP orbit, and remains under continued investigation. The dust trail of comet 2P/Encke is also quite clearly visible in the WISPR data, again highlighting the instrument's ability to detect faint dust features. The inner solar system is rich with fragmenting comets and comet
<figure><img src="image_66.png" /><figcaption>**Fig. 63** WISPR-i observations recorded on 5 Nov. 2018, 14:14 UT, 6 Nov. 1:43 UT and 6 Nov. 14:54 UT. Plotted symbols indicate the imaginary position of Phaethon along the orbit in 60 minute increments, both pre-perihelion (blue) and post-perihelion (white). Symbols are excluded in the region where the trail is most easily visible. The white diamond indicates the perihelion position of the orbit in the FOV. Figure adapted from Battams et al. (2020)</figcaption></figure>

stream complex including likely relationships with asteroids 2005 UD and 1999 YC (e.g., Whipple 1983; Gustafson 1989; Williams and Wu 1993; Ohtsuka et al. 2006, 2008). Due to a small 0.14 AU perihelion distance, observations of Phaethon near the Sun are impossible from traditional ground-based telescopes. The first detections of Phaethon at perihelion were made by STEREO/SECCHI (Jewitt et al. 2013). While Phaethon is active near perihelion and experiences an intense impact environment near the Sun, the mass-loss rates from cometary-like activity (Jewitt et al. 2013) and impact ejecta (Szalay et al. 2019) were both found to be orders of magnitude too low to sustain the Geminids.

As presented in Battams et al. (2020), an unexpected white-light feature revealed in the WISPR background-corrected data was the presence of a faint extended dust trail following the orbit of Phaethon. In Fig. 63, we show three WISPR-i telescope observations that highlight the most visible portion of this dust trail as detected during PSP Enc. 1, which is seen following the projection of Phaethon's orbital path perfectly.

Despite the dust trail being close to the instrument noise floor, the mean brightness along the trail was determined to be $8.2 \times 10^{-15} B_{\odot}$ (where $B_{\odot}$ is the mean solar brightness), which equates to a visual magnitude of $15.8 \pm 0.3$ per pixel. This result, coupled with the $70 \ \text{arcsec}^{-2}$ per pixel resolution of WISPR-i, yields an estimated surface brightness of $25.0 \ \text{mag arcsec}^{-2}$ for the dust trail, which in turn is shown to yield a total mass of dust in the entire trail of $\sim (0.4 - 1.3) \times 10^{12} \ \text{kg}$. This mass estimate is inconsistent with dust by Phaethon at perihelion, but is plausibly in-line (slightly below) mass estimates of the Geminids. The difference is attributed primarily to the faintness of the detection.

This detection highlights the remarkable sensitivity of WISPR to white-light dust structures. Recent ground- and space-based surveys have failed to detect a dust trail in the orbit of 3200 Phaethon (Ye et al. 2018). The WISPR observation explains this as, when factors such as heliocentric distance and orbital spreading/clustering of the dust are considered, it can be shown that the surface brightness of the trail as seen from a terrestrial viewpoint is less than $30 \ \text{mag arcsec}^{-2}$, which constitutes an extremely challenging target even for deep sky surveys.

The Phaethon dust trail continues to be clearly observed in the WISPR data in every PSP orbit, and remains under continued investigation. The dust trail of comet 2P/Encke is also quite clearly visible in the WISPR data, again highlighting the instrument's ability to detect faint dust features. The inner solar system is rich with fragmenting comets and comet
families, yielding the potential for the discovery of additional dust features as the mission orbit evolves.

### 11.4.5 Mass Loading of the Solar Wind by Charged Interplanetary Dust

If charged dust grains reach sufficient density, they are theoretically capable of impacting solar wind plasma dynamics, primarily through mass-loading the wind (e.g., Rasca et al. 2014a). As the solar wind flows over charged dust grains, the Lorentz force attempts to accelerate these grains up to the solar wind velocity. The resulting momentum exchange can slow the solar wind and distort solar wind magnetic fields (Lai et al. 2015). In practice, a high enough density of dust grains with sufficiently large charge-to-mass ratio to distort the solar wind flow is most likely to be found near localized dust sources, like comets (Rasca et al. 2014b). The Solar Probe data so far have yielded one such potential comet-solar wind interaction, and a study of this event was inconclusive with regard to whether mass loading created an observable impact on the solar wind (He et al. 2021a).

## 11.5 Summary of Dust Observations and Future Prospects for PSP Dust Measurements

Summarizing our understanding of the inner heliosphere's dust environment after 3 years of PSP dust data:

1. Impact rates from the first six orbits are produced by three dust sources: α-meteoroids on bound elliptic orbits, β-meteoroids on unbound, hyperbolic orbits, and a third dust source likely related to meteoroid streams.

2. The flux of β-meteoroids varies by at least 50% on year-long timescales.

3. Directionality analysis and data-model comparisons suggests the third source detected during PSP's first six orbits is a β-stream.

4. A zodiacal erosion rate of at least ~100 kg s⁻¹ is consistent with observed impact rates.

5. The flux of β-meteoroids at 1 AU is estimated to be in the range of 0.4 – 0.8 × 10⁻⁴ m⁻² s⁻¹.

6. The majority of zodiacal collisions production β-meteoroids occur in a region from ~10–20 $R_⊕$.

7. If the inner source of pickup ions is due to dust, it must be from nanograins with radii ≲ 50 nm.

8. The zodiacal dust density is expected to maintain a constant value in the range of 10–19 $R_⊕$.

9. A dust ring along the orbit of Venus' orbit has been directly observed.

10. Multiple meteoroid streams have been directly observed, including the Geminids meteoroid stream.

There are a number of ongoing and recently open questions in the PSP-era of ZC exploration. For example, it is not yet determined why the FIELDS dust count rate rises within each orbital group. Increases among orbital groups are expected because, as the S/C moves closer to the Sun, its relative velocity to zodiacal dust populations increases and the zodiacal dust density increases closer to the Sun (Szalay et al. 2020a). While this effect is observed, it is also observed (Pusack et al. 2021; Szalay et al. 2021) that successive orbits with the same perihelion distance show increasing dust count rates (e.g., high dust count rates on orbit 5 compared to 4). Additionally, can FIELDS dust detections be used to differentiate between existing theories for the generation of voltage spikes by impact-generated plasma?
families, yielding the potential for the discovery of additional dust features as the mission orbit evolves.

### 11.4.5 Mass Loading of the Solar Wind by Charged Interplanetary Dust

If charged dust grains reach sufficient density, they are theoretically capable of impacting solar wind plasma dynamics, primarily through mass-loading the wind (e.g., Rasca et al. 2014a). As the solar wind flows over charged dust grains, the Lorentz force attempts to accelerate these grains up to the solar wind velocity. The resulting momentum exchange can slow the solar wind and distort solar wind magnetic fields (Lai et al. 2015). In practice, a high enough density of dust grains with sufficiently large charge-to-mass ratio to distort the solar wind flow is most likely to be found near localized dust sources, like comets (Rasca et al. 2014b). The Solar Probe data so far have yielded one such potential comet-solar wind interaction, and a study of this event was inconclusive with regard to whether mass loading created an observable impact on the solar wind (He et al. 2021a).

## 11.5 Summary of Dust Observations and Future Prospects for PSP Dust Measurements

Summarizing our understanding of the inner heliosphere's dust environment after 3 years of PSP dust data:

1. Impact rates from the first six orbits are produced by three dust sources: α-meteoroids on bound elliptic orbits, β-meteoroids on unbound, hyperbolic orbits, and a third dust source likely related to meteoroid streams.

2. The flux of β-meteoroids varies by at least 50% on year-long timescales.

3. Directionality analysis and data-model comparisons suggests the third source detected during PSP's first six orbits is a β-stream.

4. A zodiacal erosion rate of at least ~100 kg s⁻¹ is consistent with observed impact rates.

5. The flux of β-meteoroids at 1 AU is estimated to be in the range of 0.4 – 0.8 × 10⁻⁴ m⁻² s⁻¹.

6. The majority of zodiacal collisions production β-meteoroids occur in a region from ~10–20 $R_⊕$.

7. If the inner source of pickup ions is due to dust, it must be from nanograins with radii ≲ 50 nm.

8. The zodiacal dust density is expected to maintain a constant value in the range of 10–19 $R_⊕$.

9. A dust ring along the orbit of Venus' orbit has been directly observed.

10. Multiple meteoroid streams have been directly observed, including the Geminids meteoroid stream.

There are a number of ongoing and recently open questions in the PSP-era of ZC exploration. For example, it is not yet determined why the FIELDS dust count rate rises within each orbital group. Increases among orbital groups are expected because, as the S/C moves closer to the Sun, its relative velocity to zodiacal dust populations increases and the zodiacal dust density increases closer to the Sun (Szalay et al. 2020a). While this effect is observed, it is also observed (Pusack et al. 2021; Szalay et al. 2021) that successive orbits with the same perihelion distance show increasing dust count rates (e.g., high dust count rates on orbit 5 compared to 4). Additionally, can FIELDS dust detections be used to differentiate between existing theories for the generation of voltage spikes by impact-generated plasma?
<figure><img src="image_66.png" /><figcaption>**Fig. 64** β-meteoroid fluxes observed by multiple S/C and detection schemes</figcaption></figure>

PSP traverses a wide range of thermal plasma, photon flux, magnetic field, and dust impact velocity conditions, enabling new tests of impact-plasma behavior as a function of these parameters.

The WISPR remote measurements have provided an unparalleled look at the dust environment with a few 10's of $R_⊙$. Upcoming orbits will reveal whether the DFZ indicated by WISPR data (Howard et al. 2019; Stenborg et al. 2021b) will be directly observable *in situ* with FIELDS, and if the observed trend from WISPR for larger grains holds in the micron-sized regime. While PSP will directly transit the region of constant radial density profile inferred by WISPR in the range of 10–19 $R_⊙$, the decrease of this profile towards a DFZ occurs inside 10 $R_⊙$ where PSP will not transit.

Finally, PSP's long mission duration will enable it to be a long-term observation platform for β-meteoroid fluxes inside 1 AU. The flux of β-meteoroids directly encodes the collisional erosion occurring in the inner heliosphere, therefore a determination of their flux provides an important window into the dynamics and evolution of the ZC. Furthermore, the flux of β-meteoroids is the largest impactor source by number flux at 1 AU, and may be responsible for sustaining a significant portion of the Moon's impact ejecta cloud (Szalay et al. 2020b). Hence, β-meteoroids may play a more important role in space weathering airless bodies than previously considered, and constraining their fluxes and variations with time can provide key insight on the space weathering of airless bodies which transit inside 1 AU. Figure 64 highlights the multiple β-meteoroid flux estimates from dedicated dust instruments onboard Pioneers 8 & 9 (Berg and Grün 1973), *Ulysses* (Wehry and Mann 1999; Wehry et al. 2004), as well electric field-based observations from *STEREO* (Zaslavsky et al. 2012), *Solo* (Zaslavsky et al. 2021), and *PSP* (Szalay et al. 2020a, 2021; Mann and Czechowski 2021). As shown in this figure, the dedicated dust observations indicated a higher flux of β-meteoroids than the more recent estimates derived from electric field observations taken decades later. The extent to which the flux of β-meteoroids varies over time is a quantity PSP will be uniquely posed to answer in its many years of upcoming operations.
<figure><img src="image_67.png" /><figcaption>**Fig. 64** β-meteoroid fluxes observed by multiple S/C and detection schemes</figcaption></figure>

PSP traverses a wide range of thermal plasma, photon flux, magnetic field, and dust impact velocity conditions, enabling new tests of impact-plasma behavior as a function of these parameters.

The WISPR remote measurements have provided an unparalleled look at the dust environment with a few 10's of $R_{⊙}$. Upcoming orbits will reveal whether the DFZ indicated by WISPR data (Howard et al. 2019; Stenborg et al. 2021b) will be directly observable *in situ* with FIELDS, and if the observed trend from WISPR for larger grains holds in the micron-sized regime. While PSP will directly transit the region of constant radial density profile inferred by WISPR in the range of 10–19 $R_{⊙}$, the decrease of this profile towards a DFZ occurs inside 10 $R_{⊙}$ where PSP will not transit.

Finally, PSP's long mission duration will enable it to be a long-term observation platform for β-meteoroid fluxes inside 1 AU. The flux of β-meteoroids directly encodes the collisional erosion occurring in the inner heliosphere, therefore a determination of their flux provides an important window into the dynamics and evolution of the ZC. Furthermore, the flux of β-meteoroids is the largest impactor source by number flux at 1 AU, and may be responsible for sustaining a significant portion of the Moon's impact ejecta cloud (Szalay et al. 2020b). Hence, β-meteoroids may play a more important role in space weathering airless bodies than previously considered, and constraining their fluxes and variations with time can provide key insight on the space weathering of airless bodies which transit inside 1 AU. Figure 64 highlights the multiple β-meteoroid flux estimates from dedicated dust instruments onboard Pioneers 8 & 9 (Berg and Grün 1973), *Ulysses* (Wehry and Mann 1999; Wehry et al. 2004), as well electric field-based observations from *STEREO* (Zaslavsky et al. 2012), *Solo* (Zaslavsky et al. 2021), and PSP (Szalay et al. 2020a, 2021; Mann and Czechowski 2021). As shown in this figure, the dedicated dust observations indicated a higher flux of β-meteoroids than the more recent estimates derived from electric field observations taken decades later. The extent to which the flux of β-meteoroids varies over time is a quantity PSP will be uniquely posed to answer in its many years of upcoming operations.
<figure><img src="image_67.png" /><figcaption>**Fig. 65** (a) WISPR-i image of the nightside of Venus from VGA3, showing thermal emission from the surface on the disk and O₂ nightglow emission at the limb. (b) Topographical map from Magellan, using an inverse black and white scale to match the WISPR image, with bright regions being low elevation and dark regions being high elevation. (c) WISPR-i and -o images of Venus from VGA4. The same part of the Venusian surface is observed as in (a). Red numbers in all panels mark common features for ease of reference. Figure adapted from Wood et al. (2022)</figcaption></figure>

## 12 Venus

Putting PSP into an orbit that reaches within $10 R_⊙$ of the Sun requires a series of VGA flybys to push the orbital perihelion closer and closer to the Sun. A total of seven such flybys are planned, five of which have already occurred as of this writing. These visits to Venus naturally provide an opportunity for PSP to study Venus and its interactions with the solar wind. In this section, we review results of observations made during these flybys.

Direct images of Venus have been obtained by the WISPR imagers on board PSP. The first attempt to image Venus with WISPR was during the third flyby (VGA3) on 11 Jul. 2020. The dayside of Venus is much too bright for WISPR to image. With no shutter mechanism, the effective minimum exposure time with WISPR is the image readout time of about 2 s, much too long for Venus to be anything other than highly overexposed in WISPR images made close to the planet. Furthermore, the VGA3 sequence of images demonstrated that if any part of dayside Venus is in the FOV, not only is the planet highly overexposed, but there are scattered light artifacts that contaminate the entire image.

Fortunately, there were a couple VGA3 images that only contained nightside Venus in the FOV, and these images proved surprisingly revelatory. One of these images is shown in Fig. 65a (Wood et al. 2022). Structure is clearly seen on the disk. Furthermore, comparison with a topographical map of Venus from the Magellan mission (see Fig. 65b) makes it clear that we are actually seeing the surface of the planet. This was unexpected, as the surface of Venus had never before been imaged at optical wavelengths. Viewing the planetary surface is impossible on the dayside due to the blinding presence of scattered sunlight from the very thick Venusian atmosphere.

However, on the nightside there are windows in the near infrared (NIR) where the surface of the planet had been imaged before, particularly by the *Venus Express* (VEX; Titov et al.
<figure><img src="image_68.png" /><figcaption>**Fig. 65** (a) WISPR-i image of the nightside of Venus from VGA3, showing thermal emission from the surface on the disk and O₂ nightglow emission at the limb. (b) Topographical map from Magellan, using an inverse black and white scale to match the WISPR image, with bright regions being low elevation and dark regions being high elevation. (c) WISPR-i and -o images of Venus from VGA4. The same part of the Venusian surface is observed as in (a). Red numbers in all panels mark common features for ease of reference. Figure adapted from Wood et al. (2022)</figcaption></figure>

## 12 Venus

Putting PSP into an orbit that reaches within $10 R_⊙$ of the Sun requires a series of VGA flybys to push the orbital perihelion closer and closer to the Sun. A total of seven such flybys are planned, five of which have already occurred as of this writing. These visits to Venus naturally provide an opportunity for PSP to study Venus and its interactions with the solar wind. In this section, we review results of observations made during these flybys.

Direct images of Venus have been obtained by the WISPR imagers on board PSP. The first attempt to image Venus with WISPR was during the third flyby (VGA3) on 11 Jul. 2020. The dayside of Venus is much too bright for WISPR to image. With no shutter mechanism, the effective minimum exposure time with WISPR is the image readout time of about 2 s, much too long for Venus to be anything other than highly overexposed in WISPR images made close to the planet. Furthermore, the VGA3 sequence of images demonstrated that if any part of dayside Venus is in the FOV, not only is the planet highly overexposed, but there are scattered light artifacts that contaminate the entire image.

Fortunately, there were a couple VGA3 images that only contained nightside Venus in the FOV, and these images proved surprisingly revelatory. One of these images is shown in Fig. 65a (Wood et al. 2022). Structure is clearly seen on the disk. Furthermore, comparison with a topographical map of Venus from the Magellan mission (see Fig. 65b) makes it clear that we are actually seeing the surface of the planet. This was unexpected, as the surface of Venus had never before been imaged at optical wavelengths. Viewing the planetary surface is impossible on the dayside due to the blinding presence of scattered sunlight from the very thick Venusian atmosphere.

However, on the nightside there are windows in the near infrared (NIR) where the surface of the planet had been imaged before, particularly by the *Venus Express* (VEX; Titov et al.
2006) and AKATSUKI (Nakamura 2011) missions (Helbert et al. 2008; Mueller et al. 2008; Iwagami et al. 2018). This is not reflected light but thermal emission from the surface, which even on the nightside of Venus is about 735 K. The WISPR imagers are sensitive enough to detect this thermal emission within their optical bandpass. Because surface temperature decreases with altitude on Venus, as it does on Earth, dark areas in the PSP/WISPR images correspond to cooler highland areas while bright areas correspond to hotter lowland regions. The dark oval-shaped region dominating the WISPR image near the equator is the Ovda Regio plateau at the western end of Aphrodite Terra, the largest highland region on Venus.

In addition to the thermal emission from the disk of the planet, a bright rim of emission is seen at the limb of the planet. This is O₂ nightglow emission from the upper atmosphere of the planet, which had been observed by previous missions, particularly VEX (García Muñoz et al. 2009, 2013). This emission is believed to be excited by winds of material flowing in the upper atmosphere from the dayside to the nightside.

The experience with the VGA3 images allowed for better planning for VGA4, and during this fourth flyby on 21 Feb. 2021 a much more extensive series of images was taken of the Venusian nightside, using both the WISPR-i and WISPR-o imagers. Figure 65c shows a view from VGA4, combining the WISPR-i and WISPR-o images. It so happened that VGA4 occurred with essentially the same part of Venus on the nightside as VGA3, so the VGA3 and VGA4 images in Figs. 65a and 65c are of roughly the same part of the planet, with the Ovda Regio area dominating both.

An initial analysis of the WISPR images has been presented by Wood et al. (2022). A model spectrum of the surface thermal emission was computed, propagated through a model Venusian atmosphere. This model, assuming a 735 K surface temperature, was able to reproduce the count rates observed by WISPR. A long-term goal will be to compare the WISPR observations with NIR images. Ratios of the two could potentially provide a diagnostic for surface composition. However, before such mineralogy can be done, a more detailed analysis of the WISPR data must be performed to correct the images for scattered light, disk O₂ nightglow, and the effects of spatially variable atmospheric opacity.

Finally, additional WISPR observations should be coming in the future. Although the Enc. geometry of VGA5 was not favorable for nightside imaging, and VGA6 will likewise be unfavorable, the final flyby (VGA7) on 6 Nov. 2024 should provide an opportunity for new images to be made. Furthermore, for VGA7 we will be viewing the side of Venus not observed in VGA3 and VGA4.

PSP also made extensive particle and fields measurements during the Venus flybys. Such measurements are rare at Venus, particularly high cadence electric and magnetic field measurements (Futaana et al. 2017). Therefore, PSP data recorded near Venus has the potential to yield new physical insights.

Several studies examined the interaction between the induced magnetosphere of Venus and the solar wind. Bowen et al. (2021) explored kinetic-scale turbulence in the Venusian magnetosheath, quantifying properties of the shock and demonstrating developed sub-ion kinetic turbulence. Malaspina et al. (2020a) identified kinetic-scale electric field structures in the Venusian bow shock, including electron phase space holes and double layers. The occurrence rate of double layers was suggested to be greater than at Earth's bow shock, hinting at a potential significant difference in the kinetic properties of bow shocks at induced magnetospheres vs. intrinsic magnetospheres. Goodrich et al. (2021) identified subproton scale magnetic holes in the Venusian magnetosheath, one of the few observations of such structures beyond Earth's magnetosphere.

Other studies used the closest portions of the flybys to examine the structure and properties of the Venusian ionosphere. Collinson et al. (2021) examined the ionospheric density
2006) and AKATSUKI (Nakamura 2011) missions (Helbert et al. 2008; Mueller et al. 2008; Iwagami et al. 2018). This is not reflected light but thermal emission from the surface, which even on the nightside of Venus is about 735 K. The WISPR imagers are sensitive enough to detect this thermal emission within their optical bandpass. Because surface temperature decreases with altitude on Venus, as it does on Earth, dark areas in the PSP/WISPR images correspond to cooler highland areas while bright areas correspond to hotter lowland regions. The dark oval-shaped region dominating the WISPR image near the equator is the Ovda Regio plateau at the western end of Aphrodite Terra, the largest highland region on Venus.

In addition to the thermal emission from the disk of the planet, a bright rim of emission is seen at the limb of the planet. This is O₂ nightglow emission from the upper atmosphere of the planet, which had been observed by previous missions, particularly VEX (García Muñoz et al. 2009, 2013). This emission is believed to be excited by winds of material flowing in the upper atmosphere from the dayside to the nightside.

The experience with the VGA3 images allowed for better planning for VGA4, and during this fourth flyby on 21 Feb. 2021 a much more extensive series of images was taken of the Venusian nightside, using both the WISPR-i and WISPR-o imagers. Figure 65c shows a view from VGA4, combining the WISPR-i and WISPR-o images. It so happened that VGA4 occurred with essentially the same part of Venus on the nightside as VGA3, so the VGA3 and VGA4 images in Figs. 65a and 65c are of roughly the same part of the planet, with the Ovda Regio area dominating both.

An initial analysis of the WISPR images has been presented by Wood et al. (2022). A model spectrum of the surface thermal emission was computed, propagated through a model Venusian atmosphere. This model, assuming a 735 K surface temperature, was able to reproduce the count rates observed by WISPR. A long-term goal will be to compare the WISPR observations with NIR images. Ratios of the two could potentially provide a diagnostic for surface composition. However, before such mineralogy can be done, a more detailed analysis of the WISPR data must be performed to correct the images for scattered light, disk O₂ nightglow, and the effects of spatially variable atmospheric opacity.

Finally, additional WISPR observations should be coming in the future. Although the Enc. geometry of VGA5 was not favorable for nightside imaging, and VGA6 will likewise be unfavorable, the final flyby (VGA7) on 6 Nov. 2024 should provide an opportunity for new images to be made. Furthermore, for VGA7 we will be viewing the side of Venus not observed in VGA3 and VGA4.

PSP also made extensive particle and fields measurements during the Venus flybys. Such measurements are rare at Venus, particularly high cadence electric and magnetic field measurements (Futaana et al. 2017). Therefore, PSP data recorded near Venus has the potential to yield new physical insights.

Several studies examined the interaction between the induced magnetosphere of Venus and the solar wind. Bowen et al. (2021) explored kinetic-scale turbulence in the Venusian magnetosheath, quantifying properties of the shock and demonstrating developed sub-ion kinetic turbulence. Malaspina et al. (2020a) identified kinetic-scale electric field structures in the Venusian bow shock, including electron phase space holes and double layers. The occurrence rate of double layers was suggested to be greater than at Earth's bow shock, hinting at a potential significant difference in the kinetic properties of bow shocks at induced magnetospheres vs. intrinsic magnetospheres. Goodrich et al. (2021) identified subproton scale magnetic holes in the Venusian magnetosheath, one of the few observations of such structures beyond Earth's magnetosphere.

Other studies used the closest portions of the flybys to examine the structure and properties of the Venusian ionosphere. Collinson et al. (2021) examined the ionospheric density
at 1,100 km altitude, demonstrating consistency with solar cycle predictions for ionospheric variability. Collinson et al. (2022) used PSP observations of cold plasma filaments extending from the Venus ionosphere (tail rays) to reconcile previously inconsistent observations of tail rays by Pioneer 12 (also named Pioneer Venus Orbiter) and VEX.

Finally, Pulupa et al. (2021) examined radio frequency data recorded during PSP Venus flybys, searching for evidence of lightning-generated plasma waves. No such waves were found, supporting results from earlier Cassini flyby observations (Gurnett et al. 2001).

# 13 Summary and Conclusions

PSP has completed 13 of its 24 scheduled orbits around the Sun over a 7-year nominal mission duration. The S/C flew by Venus for the fifth time on 16 Oct. 2021, followed by the closest perihelion of 13.28 $R_\odot$. Generally, the S/C has performed well within the expectations. The science data returned is a true treasure trove, revealing new aspects of the young solar wind and phenomena that we did not know much about. The following is a summary of the findings of the PSP mission during its four years of operations. We, however, refer the readers to the corresponding sections for more details.

**Switchbacks** The magnetic field switchbacks observed by the PSP are a fundamental phenomenon of the young solar wind. SBs show an impressive effect; they turn the ambient slow solar wind into fast for the crossing duration without changing the connection to the source. These structures are Alfvénic, show little changes in the density, and display a slight preference to deflect in the tangential direction. The duration of the observed switchbacks is related to how S/C cross through the structure, which is in turn associated with the deflection, dimensions, orientation, and the S/C velocity. Most studies implied that these structures are long and thin along the flow direction. SB patches have shown the local modulation in the alpha fraction observed in-situ, which could be a direct signature of spatial modulation in solar sources. They also have shown large-scale heating of protons in the parallel direction to the magnetic field, indicating the preferential heating of the plasmas inside the switchbacks. Observations provided a clue that switchbacks might have relevance to the heating and acceleration of the solar wind. Therefore it is essential to understand their generation and propagation mechanism. Some aspects of these features point toward ex-situ processes (e.g., interchange reconnection and other solar-surface processes) and others toward in-situ mechanisms (covering stream interactions, AWs, and turbulence) in which switchbacks result from processes within the solar wind as it propagates outwards. The various flavors of interchange-reconnection-based models have several attractive features, in particular their natural explanation of the likely preferred tangential deflections of large switchbacks, the bulk tangential flow, and the possible observed temperature enhancements. However, some important features remain unclear, such as the Alfvénicity of the structures and how they evolve as they propagate to PSP altitudes.

While, AW models naturally recover the Alfvénicity and radial elongation of switchbacks seen in PSP observations, but can struggle with some other features. In particular, it remains unclear whether the preferred tangential deflections of large switchbacks can be recovered and also struggle to reproduce the high switchback fractions observed by PSP. When radially stratified environment conditions are considered for AW models, studies showed that before propagating any significant distance, a switchback will have deformed significantly, either changing shape or unfolding depending on the background profile. This blurs the line
at 1,100 km altitude, demonstrating consistency with solar cycle predictions for ionospheric variability. Collinson et al. (2022) used PSP observations of cold plasma filaments extending from the Venus ionosphere (tail rays) to reconcile previously inconsistent observations of tail rays by *Pioneer 12* (also named Pioneer Venus Orbiter) and *VEX*.

Finally, Pulupa et al. (2021) examined radio frequency data recorded during PSP Venus flybys, searching for evidence of lightning-generated plasma waves. No such waves were found, supporting results from earlier Cassini flyby observations (Gurnett et al. 2001).

# 13 Summary and Conclusions

PSP has completed 13 of its 24 scheduled orbits around the Sun over a 7-year nominal mission duration. The S/C flew by Venus for the fifth time on 16 Oct. 2021, followed by the closest perihelion of 13.28 $R_\odot$. Generally, the S/C has performed well within the expectations. The science data returned is a true treasure trove, revealing new aspects of the young solar wind and phenomena that we did not know much about. The following is a summary of the findings of the PSP mission during its four years of operations. We, however, refer the readers to the corresponding sections for more details.

**Switchbacks** The magnetic field switchbacks observed by the PSP are a fundamental phenomenon of the young solar wind. SBs show an impressive effect; they turn the ambient slow solar wind into fast for the crossing duration without changing the connection to the source. These structures are Alfvénic, show little changes in the density, and display a slight preference to deflect in the tangential direction. The duration of the observed switchbacks is related to how S/C cross through the structure, which is in turn associated with the deflection, dimensions, orientation, and the S/C velocity. Most studies implied that these structures are long and thin along the flow direction. SB patches have shown the local modulation in the alpha fraction observed in-situ, which could be a direct signature of spatial modulation in solar sources. They also have shown large-scale heating of protons in the parallel direction to the magnetic field, indicating the preferential heating of the plasmas inside the switchbacks. Observations provided a clue that switchbacks might have relevance to the heating and acceleration of the solar wind. Therefore it is essential to understand their generation and propagation mechanism. Some aspects of these features point toward ex-situ processes (e.g., interchange reconnection and other solar-surface processes) and others toward in-situ mechanisms (covering stream interactions, AWs, and turbulence) in which switchbacks result from processes within the solar wind as it propagates outwards. The various flavors of interchange-reconnection-based models have several attractive features, in particular their natural explanation of the likely preferred tangential deflections of large switchbacks, the bulk tangential flow, and the possible observed temperature enhancements. However, some important features remain unclear, such as the Alfvénicity of the structures and how they evolve as they propagate to PSP altitudes.

While, AW models naturally recover the Alfvénicity and radial elongation of switchbacks seen in PSP observations, but can struggle with some other features. In particular, it remains unclear whether the preferred tangential deflections of large switchbacks can be recovered and also struggle to reproduce the high switchback fractions observed by PSP. When radially stratified environment conditions are considered for AW models, studies showed that before propagating any significant distance, a switchback will have deformed significantly, either changing shape or unfolding depending on the background profile. This blurs the line
between ex-situ and in-situ formation scenarios. There are interrelationships and the coexistence of different mechanisms in some of the proposed models; moving forward, we must keep all the models in mind as we attempt to distinguish observationally between different mechanisms. Further understanding of switchback formation will require constant collaboration between observers and theorists.

**Solar Wind Sources** A central question in heliophysics is connecting the solar wind to its sources. A broad range of coronal and heliospheric modeling efforts have supported all the PSP Encs. PSP has mainly observed only slow solar wind with a few exceptions. The first Enc. proved unique where all models pointed to a distinct equatorial coronal hole at perihelion as the dominant solar wind source. The flow was predominantly slow and highly Alfvénic. During the subsequent Encs., the S/C was connected to polar coronal hole boundaries and a flatter HCS. However, what has been a surprise is that the slow solar wind streams were seen to have turbulence and fluctuation properties, including the presence of the SBs, typical of Alfvénic fluctuations usually associated with HSSs. That slow wind interval appeared to have much of the same characteristics of the fast wind, including the presence of predominantly outwards Alfvénic fluctuations, except for the overall speed. The consensus is that the slow Alfvénic solar wind observed by PSP originates from coronal holes or coronal hole boundaries. It is still unclear how the Alfvénic slow wind emerge: (1) does it always arise from small isolated coronal holes with large expansion factors within the subsonic/supersonic critical point? Or is it born at the boundaries of large, polar coronal holes? There is, however, one possible implication of the overall high Alfvénicity observed by PSP in the deep inner heliosphere. All solar wind might be born Alfvénic, or rather that Alfvénic fluctuations be a universal initial condition of solar wind outflow. Whether this is borne out by PSP measurements closer to the Sun remains to be seen.

Quiet periods typically separate the SBs-dominated patches. These quiet periods are at odds with theories relating to slow wind formation and continual reconfiguration of the coronal magnetic field lines due to footpoint exchange. This should drive strong wind variability continually (e.g., Fisk 1996). Another interesting finding from the PSP data is the well-known open flux problem persists down to 0.13 AU, suggesting there exist solar wind sources which are not yet captured accurately by modeling.

**Kinetic Physics** PSP measurements show interesting kinetic physics phenomena. The plasma data reveal the prevalence of electromagnetic ion-scale waves in the inner heliosphere for the first time. The statistical analysis of these waves shows that a near-radial magnetic field is favorable for their observation and that they mainly propagate anti-sunward. PSP observed for the first time a series of proton beams with the so-called hammerhead velocity distributions that is an excessively broadened VDF in the direction perpendicular to the mean magnetic field vector. These distributions coincide with intense, circularly polarized, FM/W waves. These findings suggest that the hammerhead distributions arise when field-aligned proton beams excite FM/W waves and subsequently scatter off these waves. PSP waveform data has also provided the first definitive evidence of sunward propagating whistler-mode waves. This is an important discovery because sunward-propagating waves can interact with the anti-sunward propagating strahl only if the wave vector is parallel to the background magnetic field.

**Turbulence** Turbulence often refers to the energy cascade process that describes the energy transfer across scales. In solar wind turbulence, the energy is presumably injected at a very large scale (e.g., with a period of a few days). It cascades then down to smaller scales until it
between ex-situ and in-situ formation scenarios. There are interrelationships and the coexistence of different mechanisms in some of the proposed models; moving forward, we must keep all the models in mind as we attempt to distinguish observationally between different mechanisms. Further understanding of switchback formation will require constant collaboration between observers and theorists.

**Solar Wind Sources** A central question in heliophysics is connecting the solar wind to its sources. A broad range of coronal and heliospheric modeling efforts have supported all the PSP Encs. PSP has mainly observed only slow solar wind with a few exceptions. The first Enc. proved unique where all models pointed to a distinct equatorial coronal hole at perihelion as the dominant solar wind source. The flow was predominantly slow and highly Alfvénic. During the subsequent Encs., the S/C was connected to polar coronal hole boundaries and a flatter HCS. However, what has been a surprise is that the slow solar wind streams were seen to have turbulence and fluctuation properties, including the presence of the SBs, typical of Alfvénic fluctuations usually associated with HSSs. That slow wind interval appeared to have much of the same characteristics of the fast wind, including the presence of predominantly outwards Alfvénic fluctuations, except for the overall speed. The consensus is that the slow Alfvénic solar wind observed by PSP originates from coronal holes or coronal hole boundaries. It is still unclear how the Alfvénic slow wind emerge: (1) does it always arise from small isolated coronal holes with large expansion factors within the sub-sonic/supersonic critical point? Or is it born at the boundaries of large, polar coronal holes? There is, however, one possible implication of the overall high Alfvénicity observed by PSP in the deep inner heliosphere. All solar wind might be born Alfvénic, or rather that Alfvénic fluctuations be a universal initial condition of solar wind outflow. Whether this is borne out by PSP measurements closer to the Sun remains to be seen.

Quiet periods typically separate the SBs-dominated patches. These quiet periods are at odds with theories relating to slow wind formation and continual reconfiguration of the coronal magnetic field lines due to footpoint exchange. This should drive strong wind variability continually (e.g., Fisk 1996). Another interesting finding from the PSP data is the well-known open flux problem persists down to 0.13 AU, suggesting there exist solar wind sources which are not yet captured accurately by modeling.

**Kinetic Physics** PSP measurements show interesting kinetic physics phenomena. The plasma data reveal the prevalence of electromagnetic ion-scale waves in the inner heliosphere for the first time. The statistical analysis of these waves shows that a near-radial magnetic field is favorable for their observation and that they mainly propagate anti-sunward. PSP observed for the first time a series of proton beams with the so-called hammerhead velocity distributions that is an excessively broadened VDF in the direction perpendicular to the mean magnetic field vector. These distributions coincide with intense, circularly polarized, FM/W waves. These findings suggest that the hammerhead distributions arise when field-aligned proton beams excite FM/W waves and subsequently scatter off these waves. PSP waveform data has also provided the first definitive evidence of sunward propagating whistler-mode waves. This is an important discovery because sunward-propagating waves can interact with the anti-sunward propagating strahl only if the wave vector is parallel to the background magnetic field.

**Turbulence** Turbulence often refers to the energy cascade process that describes the energy transfer across scales. In solar wind turbulence, the energy is presumably injected at a very large scale (e.g., with a period of a few days). It cascades then down to smaller scales until it
dissipates at scales near the ion and electron scales. The intermediate scale range between the injection scale and dissipation (or the kinetic) range is known as the inertial range. The PSP observations shed light on the properties of the turbulence at various scales (i.e., outer scale, inertial-range scale, and kinetic scales) at the closest distances to the Sun. This includes the sub-Alfvénic region where the solar wind speed becomes smaller than the typical Alfvén speed. Several recent studies using PSP data reveal the significance of solar wind turbulence on the overall heating and acceleration of the solar wind plasma. For instance, magnetic field switchbacks are associated with turbulent structures, which mainly follow the field’s kink. Turbulence features such as the intermittency, the Alfvénicity, and the compressibility have also been investigated. Overall, the data show that solar wind turbulence is mostly highly Alfvénic with less degree of compressibility even in the slow solar wind. Other studies used PSP measurements to examine the typical plasma scale at which the energy spectrum breaks. However, it remains challenging to interpret the appropriate plasma scales corresponding to the empirical timescales using the standard frozen-in-flow Taylor hypothesis as the solar wind speed and the local Alfvén speed becomes comparable.

**Large Scale** Due to its low heliographic latitude orbit, PSP crossed the HCS multiple times in each Enc. and observed many LFRs and SFRs. The observed locations of HCS crossings and PFSS model predictions were compared. An irregular source surface with a variable radius is utilized to minimize the timing and location differences. The internal structure of the HCS near the Sun is very complex, comprising structures with magnetic field magnitude depressions, increased solar wind proton bulk speeds, and associated suprathermal electron strahl dropouts, likely indicating 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. PSP measurements also show that, despite being the site of frequent magnetic reconnection, the near-Sun HCS is much thicker than expected. HCS observations at 1 AU reveal significantly different magnetic and plasma signatures implying that the near-Sun HCS is the location of active evolution of the internal structures. In addition, 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 by providing valuable information that will allow us to distinguish the evidence of the early transition from CME to ICME.

PSP has also observed a multitude of events, both large- and small-scale, connected to flux ropes. For instance, at least one SBO event showed a flux rope 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. PSP also observed a significant number of SFRs. Several tens of SFRs were analyzed, suggesting that the SFRs are primarily found in the slow solar wind and that their possible source is MHD turbulence. Other SFRs seem to be the result of magnetic reconnection.

From WISPR imaging data, the most striking features (in addition to CMEs) are the small-scale features observed when the S/C crosses the HCS. The imaging of the young solar wind plasma is revealing. The internal structure of CMEs is observed in ways not accessible before the PSP era. Also, features such as the fine structure of coronal streamers indicate the highly-dynamic nature of the solar wind close to the Sun. An excellent example of the feature identified by WISPR are bright and narrow streamer rays located at the core of the streamer belt.

**Radio Emissions and Energetic Particles** The first four years of the PSP mission enabled an essential understanding of the variability of solar radio emissions and provided critical insights into the acceleration and transport of energetic particles in the inner heliosphere. PSP
dissipates at scales near the ion and electron scales. The intermediate scale range between the injection scale and dissipation (or the kinetic) range is known as the inertial range. The PSP observations shed light on the properties of the turbulence at various scales (i.e., outer scale, inertial-range scale, and kinetic scales) at the closest distances to the Sun. This includes the sub-Alfvénic region where the solar wind speed becomes smaller than the typical Alfvén speed. Several recent studies using PSP data reveal the significance of solar wind turbulence on the overall heating and acceleration of the solar wind plasma. For instance, magnetic field switchbacks are associated with turbulent structures, which mainly follow the field’s kink. Turbulence features such as the intermittency, the Alfvénicity, and the compressibility have also been investigated. Overall, the data show that solar wind turbulence is mostly highly Alfvénic with less degree of compressibility even in the slow solar wind. Other studies used PSP measurements to examine the typical plasma scale at which the energy spectrum breaks. However, it remains challenging to interpret the appropriate plasma scales corresponding to the empirical timescales using the standard frozen-in-flow Taylor hypothesis as the solar wind speed and the local Alfvén speed becomes comparable.

**Large Scale** Due to its low heliographic latitude orbit, PSP crossed the HCS multiple times in each Enc. and observed many LFRs and SFRs. The observed locations of HCS crossings and PFSS model predictions were compared. An irregular source surface with a variable radius is utilized to minimize the timing and location differences. The internal structure of the HCS near the Sun is very complex, comprising structures with magnetic field magnitude depressions, increased solar wind proton bulk speeds, and associated suprathermal electron strahl dropouts, likely indicating 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. PSP measurements also show that, despite being the site of frequent magnetic reconnection, the near-Sun HCS is much thicker than expected. HCS observations at 1 AU reveal significantly different magnetic and plasma signatures implying that the near-Sun HCS is the location of active evolution of the internal structures. In addition, 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 by providing valuable information that will allow us to distinguish the evidence of the early transition from CME to ICME.

PSP has also observed a multitude of events, both large- and small-scale, connected to flux ropes. For instance, at least one SBO event showed a flux rope 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. PSP also observed a significant number of SFRs. Several tens of SFRs were analyzed, suggesting that the SFRs are primarily found in the slow solar wind and that their possible source is MHD turbulence. Other SFRs seem to be the result of magnetic reconnection.

From WISPR imaging data, the most striking features (in addition to CMEs) are the small-scale features observed when the S/C crosses the HCS. The imaging of the young solar wind plasma is revealing. The internal structure of CMEs is observed in ways not accessible before the PSP era. Also, features such as the fine structure of coronal streamers indicate the highly-dynamic nature of the solar wind close to the Sun. An excellent example of the feature identified by WISPR are bright and narrow streamer rays located at the core of the streamer belt.

**Radio Emissions and Energetic Particles** The first four years of the PSP mission enabled an essential understanding of the variability of solar radio emissions and provided critical insights into the acceleration and transport of energetic particles in the inner heliosphere. PSP
observed many solar radio emissions, SEP events, CMEs, CIRs and SIRs, inner heliospheric ACRs, and energetic electron events, which are critical to exploring the fundamental physics of particle acceleration and transport in the near-Sun environment.

The PSP/FIELDS RFS measures electric fields from 10 kHz to 19.2 MHz, enabling radio observations. Only Enc. 2 featured multiple strong type III radio bursts and a type III storm during the first four Encs. As the solar activity began rising with Encs. 5 and beyond, the occurrence of radio bursts has also increased. The PSP radio measurements enabled several critical studies, e.g.: (1) Searching for evidence of heating of the corona by small-scale nanoflares; (2) Measurement of the circular polarization near the start of several type III bursts in Enc. 2; (3) Characterization of 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; (4) Finding evidence for emission generated via the electron cyclotron maser instability over the several-MHz frequency range corresponding to solar distances where $f_{ce} > f_{pe}$; and (5) and determine the directivity of individual type III radio bursts using data from other missions, which was only possible using statistical analysis of large numbers of bursts.

PSP observed many SEP events from different sources (e.g., SBOs, jets, surges, CMEs, flares, etc.) and with various properties that are key to characterizing the acceleration and transport of particles in the inner heliosphere. Cohen et al. (2021a) investigated the helium content of six SEP events from May to Jun. 2020 during the fifth orbit. At least three of these six events originated from the same AR. Yet, they have significantly different $^3$He/$^4$He and He/H ratios. In addition, Chhiber et al. (2021b) found that the path length of these events greatly exceeded that of the Parker spiral. They attributed that to the random walk of magnetic field lines.

Most of the CMEs observed by PSP were slow and did not produce clear events at 1 AU. They nonetheless produced particle events that were observed by PSP closer to Sun. Giacalone et al. (2020) and Mitchell et al. (2020a) reported on a particular CME event observed by PSP shortly after the first perihelion pass that produced a significant enhancement in SEPs with energies below a few hundred keV/nuc, which also showed a clear velocity dispersion. The PSP plasma measurement did not show any shock evidence, and the particle flux decayed before the CME crossed the S/C. Two different interpretations were proposed for this event. Giacalone et al. (2020) suggested diffusive transport of particles accelerated by the CME starting about the time it was $7.5 R_\odot$ as observations suggest that very weak shocks, or even non-shock plasma compressions driven by a slow CME, are capable of accelerating particles. Mitchell et al. (2020a) proposed an alternative based on the “pressure cooker” mechanism observed in the magnetosphere, where 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 highest-energy particles overcome this barrier earlier and arrive at the S/C earlier than low-energy particles, presumably released much later when the CME has erupted from the Sun. The other interesting aspect is that the “pressure cooker” mechanism produces maximum energy that depends on the charge of the species. Although the event was relatively 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.

SIRs/CIRs are known to be regions where energetic particles are accelerated. Therefore, PSP observations within 1 AU are particularly well suited to detangle these acceleration and transport effects as the SIR/CIR-associated suprathermal to energetic ion populations are further from shock-associated acceleration sites that are usually beyond 1 AU. Many of these nascent SIR/CIRs were associated with energetic particle enhancements offset from
observed many solar radio emissions, SEP events, CMEs, CIRs and SIRs, inner heliospheric ACRs, and energetic electron events, which are critical to exploring the fundamental physics of particle acceleration and transport in the near-Sun environment.

The PSP/FIELDS RFS measures electric fields from 10 kHz to 19.2 MHz, enabling radio observations. Only Enc. 2 featured multiple strong type III radio bursts and a type III storm during the first four Encs. As the solar activity began rising with Encs. 5 and beyond, the occurrence of radio bursts has also increased. The PSP radio measurements enabled several critical studies, e.g.: (1) Searching for evidence of heating of the corona by small-scale nanoflares; (2) Measurement of the circular polarization near the start of several type III bursts in Enc. 2; (3) Characterization of 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; (4) Finding evidence for emission generated via the electron cyclotron maser instability over the several-MHz frequency range corresponding to solar distances where $f_{ce} > f_{pe}$; and (5) and determine the directivity of individual type III radio bursts using data from other missions, which was only possible using statistical analysis of large numbers of bursts.

PSP observed many SEP events from different sources (e.g., SBOs, jets, surges, CMEs, flares, etc.) and with various properties that are key to characterizing the acceleration and transport of particles in the inner heliosphere. Cohen et al. (2021a) investigated the helium content of six SEP events from May to Jun. 2020 during the fifth orbit. At least three of these six events originated from the same AR. Yet, they have significantly different $^3$He/$^4$He and He/H ratios. In addition, Chhiber et al. (2021b) found that the path length of these events greatly exceeded that of the Parker spiral. They attributed that to the random walk of magnetic field lines.

Most of the CMEs observed by PSP were slow and did not produce clear events at 1 AU. They nonetheless produced particle events that were observed by PSP closer to Sun. Giacalone et al. (2020) and Mitchell et al. (2020a) reported on a particular CME event observed by PSP shortly after the first perihelion pass that produced a significant enhancement in SEPs with energies below a few hundred keV/nuc, which also showed a clear velocity dispersion. The PSP plasma measurement did not show any shock evidence, and the particle flux decayed before the CME crossed the S/C. Two different interpretations were proposed for this event. Giacalone et al. (2020) suggested diffusive transport of particles accelerated by the CME starting about the time it was $7.5 R_\odot$ as observations suggest that very weak shocks, or even non-shock plasma compressions driven by a slow CME, are capable of accelerating particles. Mitchell et al. (2020a) proposed an alternative based on the “pressure cooker” mechanism observed in the magnetosphere, where 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 highest-energy particles overcome this barrier earlier and arrive at the S/C earlier than low-energy particles, presumably released much later when the CME has erupted from the Sun. The other interesting aspect is that the “pressure cooker” mechanism produces maximum energy that depends on the charge of the species. Although the event was relatively 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.

SIRs/CIRs are known to be regions where energetic particles are accelerated. Therefore, PSP observations within 1 AU are particularly well suited to detangle these acceleration and transport effects as the SIR/CIR-associated suprathermal to energetic ion populations are further from shock-associated acceleration sites that are usually beyond 1 AU. Many of these nascent SIR/CIRs were associated with energetic particle enhancements offset from
the SIR/CIR interface. At least one of these events had evidence of local compressive acceleration, suggesting that this event provides evidence that CIR-associated acceleration does not always require shock waves.

PSP also observed ACRs with intensities increasing over energies from ~5 to ~40 MeV/nuc, a characteristic feature of ACR spectra. However, the observed radial gradient is stronger (~25 ± 5% AU) than observed beyond 1 AU. Understanding the radial gradients of ACRs in the inner heliosphere provides constraints on drift transport and cross-field diffusion models.

**Dust in the Inner Heliosphere** The zodiacal dust cloud is one of the most significant structures in the heliosphere. To date, our understanding of the near-Sun dust environment is built on both in-situ and remote measurements outside 0.3 AU. PSP provides the only in-situ measurements and remote sensing observations of interplanetary dust in the near-Sun environment inside 0.3 AU. PSP provides the total dust impact rate to the S/C. The FIELDS instrument detects perturbations to the S/C potential that result from transient plasma clouds formed when dust grains strike the S/C at high velocities, vaporizing and ionizing the impacting grain and some fraction of the S/C surface. Several features have been observed in the impact rate profiles. For the first three orbits, a single pre-perihelion peak was observed. Another post-perihelion peak also marks the subsequent orbits. Between these two peaks, a local minimum in impact rate was present near the perihelion.

Comparing the PSP data to a two-component analytic dust model shows that PSP's dust impact rates are consistent with at least three distinct populations: a bound zodiacal α-meteoroids on elliptic orbits; an unbound β-meteoroids on hyperbolic orbits; and a distinct third population of impactors. The data-model comparison indicates that the β-meteoroids are predominantly produced within $10 - 20 R_⊕$ and are unlikely to be the inner source of pickup ions, instead suggesting the population of trapped nanograins is likely this source. The post-perihelion peak is like the result of PSP flying through the collisional by-products produced when a meteoroid stream (i.e., the Geminids meteoroid stream) collides with the nominal zodiacal cloud.

At about 19 $R_⊕$, WISPR white-light observations revealed a lower increase of F-corona brightness compared to observations obtained between 0.3 AU and 1 AU. This marks the outer boundary of the DDZ. The radius of the DFZ itself is found to be about 4 $R_⊕$. The PSP imaging observations confirm a nine-decade prediction of stellar DFZs by Russell (1929).

**Venus** WISPR Images of the Venusian night side during VGAs 3 and 4 proved surprisingly revelatory, clearly showing structures on the disk. This was unexpected, as the surface of Venus had never before been imaged at optical wavelengths. The WISPR imagers are sensitive enough to detect this thermal emission within their optical bandpass. The WISPR images show the Ovda Regio plateau at the western end of Aphrodite Terra, the most extensive highland region on Venus. In addition to the thermal emission from the planet's disk, the data show an O_2 night glow emission from the planet's upper atmosphere, which previous missions had observed. Another important planetary discovery is that of the dust ring along the orbit of Venus (see §11.4.3).

PSP is over four years into its prime mission. It uncovered numerous phenomena that were unknown to us so far and which are about phenomena occurring during the solar cycle minimum, where the Sun is not very active. The activity level is rising as we approach the maximum of solar cycle 25. We will undoubtedly discover other aspects of the solar corona and inner heliosphere. For instance, we pine for the S/C to fly through many of the most violent solar explosions and tell us how particles are accelerated to extreme levels.