# Extended 3He-rich Time Periods Observed by Solar Orbiter: Magnetic Connectivity and Sources

A. Kouloumvakos, G. M. Mason, G. C. Ho, R. C. Allen, Robert F. Wimmer-Schweingruber, A. P. Rouillard, J. Rodriguez-Pacheco

► To cite this version:

A. Kouloumvakos, G. M. Mason, G. C. Ho, R. C. Allen, Robert F. Wimmer-Schweingruber, et al. Extended 3He-rich Time Periods Observed by Solar Orbiter: Magnetic Connectivity and Sources. The Astrophysical Journal, 2023, 956, 10.3847/1538-4357/acf44e. insu-04473145

HAL Id: insu-04473145

https://insu.hal.science/insu-04473145

Submitted on 23 Feb 2024

**HAL** is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire **HAL**, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
OPEN ACCESS

<figure><img src="image_1.png" /></figure>

CrossMark

# Extended ³He-rich Time Periods Observed by Solar Orbiter: Magnetic Connectivity and Sources

A. Kouloumvakos<fnref n="1" />, G. M. Mason<fnref n="1" />, G. C. Ho<fnref n="1" />, R. C. Allen<fnref n="1" />, Robert F. Wimmer-Schweingruber<fnref n="2" />, A. P. Rouillard<fnref n="3" />, and J. Rodriguez-Pacheco<fnref n="4" />

Received 2023 May 15; revised 2023 August 17; accepted 2023 August 24; published 2023 October 16

## Abstract

Observations of Solar Energetic Particles (SEPs) using the Suprathelial Ion Spectrograph (SIS), which is part of the Energetic Particle Detector suite on the Solar Orbiter mission, present an unprecedented opportunity to investigate the composition and evolution of SEPs in close proximity to the Sun. By analyzing data from the SIS instrument, we have compiled a catalog of extended time periods during the first five orbits of the spacecraft around the Sun, which exhibit a significant abundance of ³He. We have identified 33 periods lasting over one day that show a high abundance of ³He. For each period, we examined the SEP characteristics, the magnetic connectivity of the spacecraft, and the magnetically connected regions. Our findings show that these time periods typically span seven days and consist of multiple injections of ³He, and that the peak in ³He flux is observed two days after the time periods begin. The time periods usually start (end) when the spacecraft's magnetic connection changes to (from) an active region (AR). In most cases, we observed a stable magnetic connection between the spacecraft and one or more ARs, with an average connection time of 4.1 ± 1.8 days.

*Unified Astronomy Thesaurus concepts: Solar physics (1476); Solar energetic particles (1491); Solar abundances (1474); Solar magnetic fields (1503)*

## 1. Introduction

The origin of ³He-rich solar energetic particles (SEPs) remains a significant unresolved issue in the field of solar physics. The acceleration of ³He particles and their release into open magnetic field lines has been primarily attributed to magnetic reconnection. However, details regarding these processes are difficult to discern solely from observations at 1 au (astronomical unit). A primary objective of the Solar Orbiter (Müller et al. 2020) mission is to obtain observations close to the Sun, facilitating our comprehension of where SEPs are accelerated and how they escape into the heliosphere. SEP observations obtained from the Suprathelial Ion Spectrograph (SIS), which is part of the Energetic Particle Detector (EPD; Rodríguez-Pacheco et al. 2020; Wimmer-Schweingruber et al. 2021) suite on board the Solar Orbiter mission, offer an unprecedented opportunity to investigate the composition and temporal evolution of SEPs close to the Sun and to shed light on the origins of ³He-rich events (Mason 2007; Reames 2021).

³He-rich SEP events are characterized by an overabundance of the rare isotope of ³He, which is not abundant in the solar corona and solar wind. These ³He-rich events have been associated with small SEP events and also are rich in electrons and heavy ions such as iron (Mason 2007). ³He-rich events have also been associated with type III radio bursts and seem to occur very frequently at active regions (ARs) where jets and flaring activity concurrently occur when ³He is observed (Bučík et al. 2018). Despite the good association with flares,

previous studies have shown that the ³He fluence and ³He:⁴He abundance ratio do not correlate well with the peak flux in soft X-rays of the associated solar flare (Nitta et al. 2006). In addition, the ³He:⁴He does not correlate well with the maximum electron intensity either (Ho et al. 2001). In a few cases, ³He-rich SEP events have been found even without an observed X-ray flare, jet, or small coronal mass ejection (CME) but show only a brightening in extreme ultraviolet (EUV) wavelengths (Mason et al. 2023b).

In general, the acceleration and escape of ³He particles occur within a narrow angular range around regions that are nominally magnetically well-connected with the observer (Nitta et al. 2006). However, certain ³He-rich events exhibit a considerably broader longitudinal distribution of their sources (Wiedenbeck et al. 2013; Bučík et al. 2016). These events appear to originate from regions that are far from the well-connected regions usually derived by the Parker spiral approximation or the potential field source surface (PFSS) model (Schatten et al. 1969; Wang & Sheeley 1992). Consequently, mechanisms such as meandering of the field lines, perpendicular transport, and coronal waves, among others, may contribute to the observed broader distribution of ³He particles. However, dropout events within intervals rich in ³He demonstrate that magnetic connectivity is very important in whether a spacecraft observes ³He particles (Mazur et al. 2000; Ho et al. 2022).

Another noteworthy feature of these events is that they can recurrently appear over a period of several days. As demonstrated by Mason (2007), during these prolonged periods, there can be a nearly continuous presence of energetic ³He (see also Kocharov et al. 2008). Additionally, Bučík et al. (2015) demonstrated using multipoint observations that the associated solar regions can generate ³He-rich events for at

<figure><img src="image_2.png" /></figure>

Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ The Johns Hopkins University Applied Physics Laboratory, 11101 Johns Hopkins Road, Laurel, MD 20723, USA; Athanasios.Kouloumvakos@jhuapl.edu</footnote>

<footnote n="2">² Institute of Experimental and Applied Physics, Christian-Albrechts-University Kiel, D-24118 Kiel, Germany</footnote>

<footnote n="3">³IRAP, Université Toulouse III-Paul Sabatier, CNRS, CNES, Toulouse, France</footnote>

<footnote n="4">⁴Universidad de Alcalá, Space Research Group, E-28805 Alcalá de Henares, Spain</footnote>
<figure><img src="image_2.png" /></figure>

# Extended ³He-rich Time Periods Observed by Solar Orbiter: Magnetic Connectivity and Sources

A. Kouloumvakos<fnref n="1" />, G. M. Mason<fnref n="1" />, G. C. Ho<fnref n="1" />, R. C. Allen<fnref n="1" />, Robert F. Wimmer-Schweingruber<fnref n="2" />, A. P. Rouillard<fnref n="3" />, and J. Rodriguez-Pacheco<fnref n="4" />

Received 2023 May 15; revised 2023 August 17; accepted 2023 August 24; published 2023 October 16

## Abstract

Observations of Solar Energetic Particles (SEPs) using the Suprathelial Ion Spectrograph (SIS), which is part of the Energetic Particle Detector suite on the Solar Orbiter mission, present an unprecedented opportunity to investigate the composition and evolution of SEPs in close proximity to the Sun. By analyzing data from the SIS instrument, we have compiled a catalog of extended time periods during the first five orbits of the spacecraft around the Sun, which exhibit a significant abundance of ³He. We have identified 33 periods lasting over one day that show a high abundance of ³He. For each period, we examined the SEP characteristics, the magnetic connectivity of the spacecraft, and the magnetically connected regions. Our findings show that these time periods typically span seven days and consist of multiple injections of ³He, and that the peak in ³He flux is observed two days after the time periods begin. The time periods usually start (end) when the spacecraft's magnetic connection changes to (from) an active region (AR). In most cases, we observed a stable magnetic connection between the spacecraft and one or more ARs, with an average connection time of 4.1 ± 1.8 days.

Unified Astronomy Thesaurus concepts: Solar physics (1476); Solar energetic particles (1491); Solar abundances (1474); Solar magnetic fields (1503)

## 1. Introduction

The origin of ³He-rich solar energetic particles (SEPs) remains a significant unresolved issue in the field of solar physics. The acceleration of ³He particles and their release into open magnetic field lines has been primarily attributed to magnetic reconnection. However, details regarding these processes are difficult to discern solely from observations at 1 au (astronomical unit). A primary objective of the Solar Orbiter (Müller et al. 2020) mission is to obtain observations close to the Sun, facilitating our comprehension of where SEPs are accelerated and how they escape into the heliosphere. SEP observations obtained from the Suprathelial Ion Spectrograph (SIS), which is part of the Energetic Particle Detector (EPD; Rodríguez-Pacheco et al. 2020; Wimmer-Schweingruber et al. 2021) suite on board the Solar Orbiter mission, offer an unprecedented opportunity to investigate the composition and temporal evolution of SEPs close to the Sun and to shed light on the origins of ³He-rich events (Mason 2007; Reames 2021).

³He-rich SEP events are characterized by an overabundance of the rare isotope of ³He, which is not abundant in the solar corona and solar wind. These ³He-rich events have been associated with small SEP events and also are rich in electrons and heavy ions such as iron (Mason 2007). ³He-rich events have also been associated with type III radio bursts and seem to occur very frequently at active regions (ARs) where jets and flaring activity concurrently occur when ³He is observed (Bučík et al. 2018). Despite the good association with flares,

previous studies have shown that the ³He fluence and ³He:⁴He abundance ratio do not correlate well with the peak flux in soft X-rays of the associated solar flare (Nitta et al. 2006). In addition, the ³He:⁴He does not correlate well with the maximum electron intensity either (Ho et al. 2001). In a few cases, ³He-rich SEP events have been found even without an observed X-ray flare, jet, or small coronal mass ejection (CME) but show only a brightening in extreme ultraviolet (EUV) wavelengths (Mason et al. 2023b).

In general, the acceleration and escape of ³He particles occur within a narrow angular range around regions that are nominally magnetically well-connected with the observer (Nitta et al. 2006). However, certain ³He-rich events exhibit a considerably broader longitudinal distribution of their sources (Wiedenbeck et al. 2013; Bučík et al. 2016). These events appear to originate from regions that are far from the well-connected regions usually derived by the Parker spiral approximation or the potential field source surface (PFSS) model (Schatten et al. 1969; Wang & Sheeley 1992). Consequently, mechanisms such as meandering of the field lines, perpendicular transport, and coronal waves, among others, may contribute to the observed broader distribution of ³He particles. However, dropout events within intervals rich in ³He demonstrate that magnetic connectivity is very important in whether a spacecraft observes ³He particles (Mazur et al. 2000; Ho et al. 2022).

Another noteworthy feature of these events is that they can recurrently appear over a period of several days. As demonstrated by Mason (2007), during these prolonged periods, there can be a nearly continuous presence of energetic ³He (see also Kocharov et al. 2008). Additionally, Bučík et al. (2015) demonstrated using multipoint observations that the associated solar regions can generate ³He-rich events for at

<figure><img src="image_3.png" /></figure>

Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ The Johns Hopkins University Applied Physics Laboratory, 11101 Johns Hopkins Road, Laurel, MD 20723, USA; Athanasios.Kouloumvakos@jhuapl.edu</footnote>

<footnote n="2">² Institute of Experimental and Applied Physics, Christian-Albrechts-University Kiel, D-24118 Kiel, Germany</footnote>

<footnote n="3">³IRAP, Université Toulouse III-Paul Sabatier, CNRS, CNES, Toulouse, France</footnote>

<footnote n="4">⁴Universidad de Alcalá, Space Research Group, E-28805 Alcalá de Henares, Spain</footnote>
least a quarter of a solar rotation. It is probable that, during these periods, there is a continuous acceleration and release of energetic $^3$He particles, which have access to open field lines and can be observed in situ, provided that the observing spacecraft maintains magnetic connectivity to the source region for an extended period. In this case, the source region of these events can be the same for the full period. Bučík et al. (2021) analyzed an extended $^3$He-rich time period that lasted almost 7 days and was observed by Solar Orbiter in 2020 November when the spacecraft was near 0.9 au from the Sun. This study showed that the extended $^3$He-rich period is related to the longitudinal extent of the sunspot group and the activity there during the analyzed time period.

In this study, we search for extended $^3$He-rich time periods (i.e., $^3$He fluxes three times above the background and time periods lasting longer than one day) that were observed by the SIS on board Solar Orbiter, we establish a catalog of these cases, and we examine the connectivity and source regions. We first identify the extended $^3$He-rich time periods and construct a catalog of these cases in Section 3.1. For each time period, we examine the $^3$He and other SEP characteristics in Section 3.2, compute and analyze the magnetic connectivity of the spacecraft during these periods in Section 3.3, and we determine the sources of magnetic connection in Section 3.3.3. We discuss and conclude with the results of our study in Section 4.

## 2. Instrumentation and Observations

In this study, we employed energetic particle measurements obtained from the SIS instrument, which is part of the EPD suite on board the Solar Orbiter spacecraft. The EPD suite provides precise measurements of energetic electrons, protons, and heavy ions spanning a wide energy range (Rodríguez-Pacheco et al. 2020). SIS instrument design is based on the high mass resolution Ultra Low Energy Isotope Spectrometer (ULEIS; Mason et al. 1998) instrument on the Advanced Composition Explorer (ACE), and it is capable of observing the elemental and isotopic composition of ions in the energy range of ~100 keV nuc⁻¹ to ~10 MeV nuc⁻¹, with two distinct viewing directions and a very high mass resolution (σ < 0.02 amu for $^4$He). We present SIS observations in Figure 1. The top row of Figure 1 displays $^3$He observations from SIS, starting from the commencement of nominal observations after the launch of Solar Orbiter on 2020 February 9 in the energy range of ~0.5–2.0 MeV nuc⁻¹. The bottom panels of the figure depicts Solar Orbiter’s first to fifth orbits (2020 April to 2022 December). Bilateral to the spacecraft trajectory line, we display $^3$He and $^4$He observations from SIS in the same energy range as in the top panel. Additionally, we utilized remote-sensing EUV observations from the Extreme Ultraviolet Imager (EUI; Rochus et al. 2020) on board Solar Orbiter to aid our analysis when that was necessary. We also utilized observations from various other instruments on board different spacecraft. These are used to examine the solar activity during the $^3$He-rich periods observed by Solar Orbiter. We used remote-sensing observations from the Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamics Observatory (SDO; Pesnell et al. 2012) and the Extreme Ultraviolet Imager (EUVI; Wuelser et al. 2004) that is part of the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI; Howard et al. 2008) instrument suite on board Solar TERRrestrial Relations Observatory-Ahead (STEREO-A; Kaiser et al. 2008).

# 3. The Catalog of $^3$He-rich Periods

## 3.1. Identification of the Time Periods

Previous studies of extended periods of $^3$He emission have shown that, when the spacecraft presumably connects magnetically to an emitting region, $^3$He may be present for a considerable period even though separate injections are not observed (e.g., see Figure 3 of Bučík et al. 2014). Due to the small numbers of counts for $^3$He, identifying the onset and termination of such periods is necessarily somewhat subjective, but the high resolution and low background of the Solar Orbiter SIS instrument enables reasonable identifications. In addition to considering the presence of $^3$He counts themselves, the selection of periods also took account of the intensities of H, $^4$He, O, and Fe.

As an example, we present Figure 2 for the period 2021 May 1–31. Looking at the lower panel of the figure, May 1–7 show no $^3$He counts, with $^3$He appearing on May 8–12, and with injections on May 9–10. This defines one of the periods in the study (#7) as May 8–12 inclusive. Continuing, there is a single $^3$He count on May 13, then none until a few late on May 18. There is clear $^3$He presence from May 19 to 28, with multiple injections May 22–24, and $^3$He persisting until approximately through May 28, when a gradual composition injection on May 29 terminates the period. The period May 19–28 is period #8 in the study. Period 8 alternatively might have been started on May 18, where there were a few $^3$He counts, and this is an indication of the uncertainty in definition of the periods. For this reason, the start and end times are day boundaries, with a few exceptions (#17, #19, and #29) that were terminated at mid-day due to the sharp onset of a gradual SEP event. Uncertainties of ± 1 day in the boundaries of the periods have little or no effect on the analysis below.

During some of these extended periods, such as that of 2022 February 5, it is possible that the spacecraft’s magnetic connection might move from one active region to a nearby one that is also emitting $^3$He, and such a change in connection might have no obvious signature in the particle data at the spacecraft. Some of the longer time periods (e.g., #21, #22, and #23) almost certainly are due to successive connections to multiple active regions that were present during these periods. Specifically, the magnetic connectivity tool (Rouillard et al. 2020) shows that Solar Orbiter was connected to multiple active regions during period 23, the longest in our study.

The analysis reveals a total of 33 extended time periods rich in $^3$He, which are presented in Table 1. It is found that the median duration of the time periods is seven days with a standard deviation of 5.1 days (see column 2 in Table 1), as shown in the blue histogram in Figure 3(a). The 33 extended time periods are marked with the red shaded areas in Figure 1 (top), where Solar Orbiter’s SIS $^3$He observations from 2020 April to the end of 2022 are displayed. It should be noted that many short-duration transient events and a few large SEP events have been excluded from the analysis. The criterion that their duration should be more than one day has filtered out the short-duration events. Some major SEP events with high intensities of $^4$He have been left out due to fact that the broadening of the mass spectrum distribution overwhelms the detection of the $^3$He, whose signal is buried under the wings of the much higher $^4$He mass peak. In less intense periods such as those shown by the red points in Figure 1 (top) identification of $^3$He periods is done by examining each individual ion event’s
least a quarter of a solar rotation. It is probable that, during these periods, there is a continuous acceleration and release of energetic $^3$He particles, which have access to open field lines and can be observed in situ, provided that the observing spacecraft maintains magnetic connectivity to the source region for an extended period. In this case, the source region of these events can be the same for the full period. Bučík et al. (2021) analyzed an extended $^3$He-rich time period that lasted almost 7 days and was observed by Solar Orbiter in 2020 November when the spacecraft was near 0.9 au from the Sun. This study showed that the extended $^3$He-rich period is related to the longitudinal extent of the sunspot group and the activity there during the analyzed time period.

In this study, we search for extended $^3$He-rich time periods (i.e., $^3$He fluxes three times above the background and time periods lasting longer than one day) that were observed by the SIS on board Solar Orbiter, we establish a catalog of these cases, and we examine the connectivity and source regions. We first identify the extended $^3$He-rich time periods and construct a catalog of these cases in Section 3.1. For each time period, we examine the $^3$He and other SEP characteristics in Section 3.2, compute and analyze the magnetic connectivity of the spacecraft during these periods in Section 3.3, and we determine the sources of magnetic connection in Section 3.3.3. We discuss and conclude with the results of our study in Section 4.

## 2. Instrumentation and Observations

In this study, we employed energetic particle measurements obtained from the SIS instrument, which is part of the EPD suite on board the Solar Orbiter spacecraft. The EPD suite provides precise measurements of energetic electrons, protons, and heavy ions spanning a wide energy range (Rodríguez-Pacheco et al. 2020). SIS instrument design is based on the high mass resolution Ultra Low Energy Isotope Spectrometer (ULEIS; Mason et al. 1998) instrument on the Advanced Composition Explorer (ACE), and it is capable of observing the elemental and isotopic composition of ions in the energy range of ~100 keV nuc⁻¹ to ~10 MeV nuc⁻¹, with two distinct viewing directions and a very high mass resolution (σ < 0.02 amu for $^4$He). We present SIS observations in Figure 1. The top row of Figure 1 displays $^3$He observations from SIS, starting from the commencement of nominal observations after the launch of Solar Orbiter on 2020 February 9 in the energy range of ~0.5–2.0 MeV nuc⁻¹. The bottom panels of the figure depicts Solar Orbiter’s first to fifth orbits (2020 April to 2022 December). Bilateral to the spacecraft trajectory line, we display $^3$He and $^4$He observations from SIS in the same energy range as in the top panel. Additionally, we utilized remote-sensing EUV observations from the Extreme Ultraviolet Imager (EUI; Rochus et al. 2020) on board Solar Orbiter to aid our analysis when that was necessary. We also utilized observations from various other instruments on board different spacecraft. These are used to examine the solar activity during the $^3$He-rich periods observed by Solar Orbiter. We used remote-sensing observations from the Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamics Observatory (SDO; Pesnell et al. 2012) and the Extreme Ultraviolet Imager (EUVI; Wuelser et al. 2004) that is part of the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI; Howard et al. 2008) instrument suite on board Solar TERRrestrial Relations Observatory-Ahead (STEREO-A; Kaiser et al. 2008).

# 3. The Catalog of $^3$He-rich Periods

## 3.1. Identification of the Time Periods

Previous studies of extended periods of $^3$He emission have shown that, when the spacecraft presumably connects magnetically to an emitting region, $^3$He may be present for a considerable period even though separate injections are not observed (e.g., see Figure 3 of Bučík et al. 2014). Due to the small numbers of counts for $^3$He, identifying the onset and termination of such periods is necessarily somewhat subjective, but the high resolution and low background of the Solar Orbiter SIS instrument enables reasonable identifications. In addition to considering the presence of $^3$He counts themselves, the selection of periods also took account of the intensities of H, $^4$He, O, and Fe.

As an example, we present Figure 2 for the period 2021 May 1–31. Looking at the lower panel of the figure, May 1–7 show no $^3$He counts, with $^3$He appearing on May 8–12, and with injections on May 9–10. This defines one of the periods in the study (#7) as May 8–12 inclusive. Continuing, there is a single $^3$He count on May 13, then none until a few late on May 18. There is clear $^3$He presence from May 19 to 28, with multiple injections May 22–24, and $^3$He persisting until approximately through May 28, when a gradual composition injection on May 29 terminates the period. The period May 19–28 is period #8 in the study. Period 8 alternatively might have been started on May 18, where there were a few $^3$He counts, and this is an indication of the uncertainty in definition of the periods. For this reason, the start and end times are day boundaries, with a few exceptions (#17, #19, and #29) that were terminated at mid-day due to the sharp onset of a gradual SEP event. Uncertainties of ±1 day in the boundaries of the periods have little or no effect on the analysis below.

During some of these extended periods, such as that of 2022 February 5, it is possible that the spacecraft’s magnetic connection might move from one active region to a nearby one that is also emitting $^3$He, and such a change in connection might have no obvious signature in the particle data at the spacecraft. Some of the longer time periods (e.g., #21, #22, and #23) almost certainly are due to successive connections to multiple active regions that were present during these periods. Specifically, the magnetic connectivity tool (Rouillard et al. 2020) shows that Solar Orbiter was connected to multiple active regions during period 23, the longest in our study.

The analysis reveals a total of 33 extended time periods rich in $^3$He, which are presented in Table 1. It is found that the median duration of the time periods is seven days with a standard deviation of 5.1 days (see column 2 in Table 1), as shown in the blue histogram in Figure 3(a). The 33 extended time periods are marked with the red shaded areas in Figure 1 (top), where Solar Orbiter’s SIS $^3$He observations from 2020 April to the end of 2022 are displayed. It should be noted that many short-duration transient events and a few large SEP events have been excluded from the analysis. The criterion that their duration should be more than one day has filtered out the short-duration events. Some major SEP events with high intensities of $^4$He have been left out due to fact that the broadening of the mass spectrum distribution overwhelms the detection of the $^3$He, whose signal is buried under the wings of the much higher $^4$He mass peak. In less intense periods such as those shown by the red points in Figure 1 (top) identification of $^3$He periods is done by examining each individual ion event’s
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Solar Orbiter's SIS observations of ³He from when the SIS instrument turned on after the launch of Solar Orbiter to 2022 December. The top panel shows the ³He flux (blue line) summed over the energy range from 0.5 to 2.0 MeV nucleon⁻¹ and averaged over 30 minute time intervals. The red points show more accurate ³He intensities where the ³He high intensities are blanked out when the measurements are dominated by spillover from ⁴He. The 33 extended ³He-rich time periods are labeled and marked in the panel with horizontal pink shaded areas. The black line depicts the spacecraft's radial distance, where the ³He-rich time periods are marked with the black circles and the five first perihelia are labeled. Bottom panels show Solar Orbiter's orbits 1 to 5 (2020 April to 2022 December). The spacecraft orbit is projected onto a view of the ecliptic plane from the north. The color-coded data plotted bilaterally to the spacecraft trajectory line are measurements of ³He and ⁴He from SIS, illustrated as the data outside and inside of the trajectory, respectively. The data are summed over the same energy range as the above panel and averaged over 12 hour time intervals. At the center of the plot, we depict the Sun (not to scale) and give the orbit number.</figcaption></figure>

two independent mass determinations and requiring consistency—this detailed procedure is done with on-ground processing, and it is not possible for the onboard computed rates (shown as blue lines in the top panel of Figure 1), due to

the limited computing power of the onboard processor. For example, this is the case for some high-activity periods such as 2020 November (see Kollhoff et al. 2021; Kouloumvakos et al. 2022), 2021 October (see Lario et al. 2022; Jebaraj et al. 2023),
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Solar Orbiter's SIS observations of <sup>3</sup>He from when the SIS instrument turned on after the launch of Solar Orbiter to 2022 December. The top panel shows the <sup>3</sup>He flux (blue line) summed over the energy range from 0.5 to 2.0 MeV nucleon<sup>-1</sup> and averaged over 30 minute time intervals. The red points show more accurate <sup>3</sup>He intensities where the <sup>3</sup>He high intensities are blanked out when the measurements are dominated by spillover from <sup>4</sup>He. The 33 extended <sup>3</sup>He-rich time periods are labeled and marked in the panel with horizontal pink shaded areas. The black line depicts the spacecraft's radial distance, where the <sup>3</sup>He-rich time periods are marked with the black circles and the five first perihelia are labeled. Bottom panels show Solar Orbiter's orbits 1 to 5 (2020 April to 2022 December). The spacecraft orbit is projected onto a view of the ecliptic plane from the north. The color-coded data plotted bilaterally to the spacecraft trajectory line are measurements of <sup>3</sup>He and <sup>4</sup>He from SIS, illustrated as the data outside and inside of the trajectory, respectively. The data are summed over the same energy range as the above panel and averaged over 12 hour time intervals. At the center of the plot, we depict the Sun (not to scale) and give the orbit number.</figcaption></figure>

two independent mass determinations and requiring consistency—this detailed procedure is done with on-ground processing, and it is not possible for the onboard computed rates (shown as blue lines in the top panel of Figure 1), due to

the limited computing power of the onboard processor. For example, this is the case for some high-activity periods such as 2020 November (see Kollhoff et al. 2021; Kouloumvakos et al. 2022), 2021 October (see Lario et al. 2022; Jebaraj et al. 2023),
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Top: hourly intensities of ~230–320 keV⁻¹ nucleon SIS-a H, ³He, ⁴He, O, and Fe. Bottom: mass and arrival times of 0.4–10 MeV nucleon⁻¹ ions. Dashed lines enclose intervals for Periods 7 and 8 in Table 1.</figcaption></figure>

and 2022 April (see Wimmer-Schweingruber et al. 2023). Furthermore, there is an increasing trend of the ³He flux with time, showing a correlation with the solar activity as seen in numerous previous studies. We registered 19 extended ³He-rich time periods during orbits 4 and 5 (e.g., from 2021 December to 2023 December), with 10 and 9 periods, respectively. The analysis also indicates no correlation (Pearson's correlation coefficient (cc): -0.09, *p*-value: 0.62) between the total duration and Solar Orbiter's distance at the beginnings of the time periods, as depicted in Figure 3(a).

Moreover, it is observed from the top panel of Figure 1 that there is no obvious dependence on the occurrence of the time periods with the phase of the Solar Orbiter's orbit. Some of the ³He-rich events took place when Solar Orbiter was at a distance below 0.65 au (see column 3 in Table 1), with around one-third of the extended ³He-rich time periods occurring at such distances. This can also be seen in the red histogram of Figure 3(a). Solar Orbiter's SEP observations near the Sun are of great importance because the timings become more accurate, interplanetary transport effects lessen, and magnetic connectivity presumably becomes less ambiguous than near 1 au. During the extended ³He-rich time periods, the ⁴He (and solar particle fluxes of heavier ions) can also be elevated for many days, which is evident in the orbit plots of Figure 1.

### 3.2. ³He and Other SEP Characteristics

We examine the SEP characteristics during the extended ³He time periods. Through a visual inspection of the particle spectrograms, we observe that the majority of these periods consist of a combination of single or multiple ³He injections and always include a continuum (flux values close to the background without a clear velocity dispersion or a peak in flux). To further our analysis, we identify the individual ³He injections for each time period, categorizing them into three

groups based on their injection characteristics: “C” for continuum, “S” for single injection, and “M” for multiple injections. For some cases, the superposition of multiple injections happening in quick succession makes it difficult to distinguish the individual injections. Our findings show that, in 63% of the cases (21 out of 33), there is more than one injection of ³He particles during the extended ³He time periods. Single ³He injections were registered only in five cases. Of the cases with multiple injections, we identify more than two injections in 17 time periods. We were not able to detect any ³He injection in seven cases, where only a ³He continuum close but above three times the background levels is present. These results are presented in Table 1 (see column 4d).

Additionally, for each time period under investigation, we determined the peak flux of ³He, which we refer to as F³He hereafter, in the energy range 0.2–10 MeV nucleon⁻¹ at 30 minute averaged time series. The results are presented in Table 1 (see column 4a). For our statistical sample the F³He varies from 0.20 to 1848 fu.<fnref n="5" /> The minimum and maximum F³He were found for the time periods of 2022 August 7 and 2022 March 11, respectively. The median F³He for our sample was calculated to be 8.91 fu, as shown by the red histogram in Figure 3(b). We find that there is a moderate correlation (cc: 0.55, *p*-value: 9.6 × 10⁻⁴) between the F³He and the duration of the extended ³He time periods, which can be observed in Figure 3(b). Typically, the peak in ³He flux is observed two days after the start of the time periods. We find no correlation (cc: -0.34, *p*-value: 5.43 × 10⁻²) between the F³He and the distance of Solar Orbiter, although a few cases exhibited the highest F³He below 0.65 au. In Figure 3(c), we also show the distribution of F³He (and duration) for the three categories for the injection characterization. We find that the time periods with multiple injections exhibit greater F³He than those with a

<footnote n="5">⁵ Here, 1 fu = 1 particle cm⁻² sr⁻¹ s⁻¹ (MeV/n)⁻¹.</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Top: hourly intensities of ~230–320 keV⁻¹ nucleon SIS-a H, ³He, ⁴He, O, and Fe. Bottom: mass and arrival times of 0.4–10 MeV nucleon⁻¹ ions. Dashed lines enclose intervals for Periods 7 and 8 in Table 1.</figcaption></figure>

and 2022 April (see Wimmer-Schweingruber et al. 2023). Furthermore, there is an increasing trend of the ³He flux with time, showing a correlation with the solar activity as seen in numerous previous studies. We registered 19 extended ³He-rich time periods during orbits 4 and 5 (e.g., from 2021 December to 2023 December), with 10 and 9 periods, respectively. The analysis also indicates no correlation (Pearson's correlation coefficient (cc): -0.09, p-value: 0.62) between the total duration and Solar Orbiter's distance at the beginnings of the time periods, as depicted in Figure 3(a).

Moreover, it is observed from the top panel of Figure 1 that there is no obvious dependence on the occurrence of the time periods with the phase of the Solar Orbiter's orbit. Some of the ³He-rich events took place when Solar Orbiter was at a distance below 0.65 au (see column 3 in Table 1), with around one-third of the extended ³He-rich time periods occurring at such distances. This can also be seen in the red histogram of Figure 3(a). Solar Orbiter's SEP observations near the Sun are of great importance because the timings become more accurate, interplanetary transport effects lessen, and magnetic connectivity presumably becomes less ambiguous than near 1 au. During the extended ³He-rich time periods, the ⁴He (and solar particle fluxes of heavier ions) can also be elevated for many days, which is evident in the orbit plots of Figure 1.

### 3.2. ³He and Other SEP Characteristics

We examine the SEP characteristics during the extended ³He time periods. Through a visual inspection of the particle spectrograms, we observe that the majority of these periods consist of a combination of single or multiple ³He injections and always include a continuum (flux values close to the background without a clear velocity dispersion or a peak in flux). To further our analysis, we identify the individual ³He injections for each time period, categorizing them into three

groups based on their injection characteristics: “C” for continuum, “S” for single injection, and “M” for multiple injections. For some cases, the superposition of multiple injections happening in quick succession makes it difficult to distinguish the individual injections. Our findings show that, in 63% of the cases (21 out of 33), there is more than one injection of ³He particles during the extended ³He time periods. Single ³He injections were registered only in five cases. Of the cases with multiple injections, we identify more than two injections in 17 time periods. We were not able to detect any ³He injection in seven cases, where only a ³He continuum close but above three times the background levels is present. These results are presented in Table 1 (see column 4d).

Additionally, for each time period under investigation, we determined the peak flux of ³He, which we refer to as F³He hereafter, in the energy range 0.2–10 MeV nucleon⁻¹ at 30 minute averaged time series. The results are presented in Table 1 (see column 4a). For our statistical sample the F³He varies from 0.20 to 1848 fu.<fnref n="5" /> The minimum and maximum F³He were found for the time periods of 2022 August 7 and 2022 March 11, respectively. The median F³He for our sample was calculated to be 8.91 fu, as shown by the red histogram in Figure 3(b). We find that there is a moderate correlation (cc: 0.55, p-value: 9.6 × 10⁻⁴) between the F³He and the duration of the extended ³He time periods, which can be observed in Figure 3(b). Typically, the peak in ³He flux is observed two days after the start of the time periods. We find no correlation (cc: -0.34, p-value: 5.43 × 10⁻²) between the F³He and the distance of Solar Orbiter, although a few cases exhibited the highest F³He below 0.65 au. In Figure 3(c), we also show the distribution of F³He (and duration) for the three categories for the injection characterization. We find that the time periods with multiple injections exhibit greater F³He than those with a

<footnote n="5">⁵ Here, 1 fu = 1 particle cm⁻² sr⁻¹ s⁻¹ (MeV/n)⁻¹.</footnote>
<table><caption>**Table 1**
List of the Extended ³He-rich Time Periods</caption><thead><tr><th rowspan="2">No.</th><th rowspan="2">Date<br>(1)</th><th rowspan="2">Duration<br>(Days)<br>(2)</th><th rowspan="2">Dist.<br>(au)<br>(3)</th><th colspan="4">SIS</th><th colspan="4">Magnetic Connectivity</th></tr><tr><th><sup>3</sup>He<br>(4a)</th><th><sup>3</sup>He:<sup>4</sup>He<br>(4b)</th><th>Fe:O<br>(4c)</th><th>Inj.<br>(4d)</th><th>Type<br>(5a)</th><th>Source<br>(5b)</th><th>Vis.<br>(5c)</th><th>#ARs<br>(5d)</th></tr></thead><tbody><tr><td>1</td><td>2020-07-19</td><td>5</td><td>0.66</td><td>9.07</td><td>0.14</td><td>1.5</td><td>M</td><td>D</td><td>CH<sup>a</sup></td><td>FS</td><td>...</td></tr><tr><td>2</td><td>2020-11-17</td><td>10</td><td>0.93</td><td>12.10</td><td>0.07</td><td>0.18</td><td>M</td><td>D (C)</td><td>AR<sup>a</sup> (CH)</td><td>FS</td><td>2</td></tr><tr><td>3</td><td>2020-12-09</td><td>10</td><td>0.83</td><td>6.83</td><td>0.45</td><td>2.97</td><td>M</td><td>S (+)</td><td>AR</td><td>FS</td><td>2</td></tr><tr><td>4</td><td>2021-02-13</td><td>3</td><td>0.50</td><td>0.46</td><td>1.65</td><td>...</td><td>S</td><td>S (+)</td><td>AR<sup>a</sup> (CH)</td><td>FS</td><td>1</td></tr><tr><td>5</td><td>2021-03-04</td><td>4</td><td>0.57</td><td>0.41</td><td>0.2</td><td>...</td><td>S</td><td>D (C)</td><td>CH<sup>a</sup> (AR)</td><td>VD</td><td>...</td></tr><tr><td>6</td><td>2021-04-19</td><td>8</td><td>0.85</td><td>11.38</td><td>0.06</td><td>0.22</td><td>S</td><td>C</td><td>CH (AR)</td><td>VD</td><td>...</td></tr><tr><td>7</td><td>2021-05-08</td><td>5</td><td>0.92</td><td>8.47</td><td>0.05</td><td>0.33</td><td>C</td><td>D</td><td>AR (CH)</td><td>VD</td><td>2</td></tr><tr><td>8</td><td>2021-05-19</td><td>10</td><td>0.94</td><td>27.25</td><td>0.58</td><td>0.46</td><td>M</td><td>S (C)</td><td>AR (CH)</td><td>VD</td><td>1</td></tr><tr><td>9</td><td>2021-06-03</td><td>8</td><td>0.95</td><td>0.59</td><td>0.25</td><td>0.36</td><td>M</td><td>D (C)</td><td>CH<sup>a</sup></td><td>VD</td><td>...</td></tr><tr><td>10</td><td>2021-07-23</td><td>7</td><td>0.82</td><td>3.21</td><td>0.08</td><td>0.33</td><td>S</td><td>C</td><td>AR (CH)</td><td>VD</td><td>3</td></tr><tr><td>11</td><td>2021-08-23</td><td>11</td><td>0.64</td><td>36.75</td><td>0.06</td><td>0.68</td><td>M</td><td>D</td><td>AR (CH)</td><td>VD</td><td>4</td></tr><tr><td>12</td><td>2021-09-22</td><td>8</td><td>0.60</td><td>4.04</td><td>0.26</td><td>3.44</td><td>M</td><td>S (+)</td><td>AR</td><td>VD</td><td>3</td></tr><tr><td>13</td><td>2021-10-03</td><td>4</td><td>0.64</td><td>1.27</td><td>0.16</td><td>0.38</td><td>S</td><td>C</td><td>AR</td><td>VD</td><td>3</td></tr><tr><td>14</td><td>2021-12-03</td><td>8</td><td>1.00</td><td>3.95</td><td>0.14</td><td>2.4</td><td>M</td><td>D</td><td>AR</td><td>VD</td><td>3</td></tr><tr><td>15</td><td>2021-12-20</td><td>7</td><td>1.01</td><td>1.61</td><td>0.11</td><td>...</td><td>M</td><td>S</td><td>AR</td><td>VD</td><td>1</td></tr><tr><td>16</td><td>2021-12-30</td><td>10</td><td>1.00</td><td>17.77</td><td>1.12</td><td>1.33</td><td>S</td><td>S (+)</td><td>AR (CH)</td><td>VD</td><td>2</td></tr><tr><td>17</td><td>2022-01-13</td><td>6</td><td>0.96</td><td>25.42</td><td>0.09</td><td>0.71</td><td>M</td><td>S</td><td>AR (CH)</td><td>VD</td><td>1</td></tr><tr><td>18</td><td>2022-02-05</td><td>7</td><td>0.82</td><td>2.24</td><td>0.34</td><td>1.2</td><td>M</td><td>S (+)</td><td>AR (CH)</td><td>VD</td><td>2</td></tr><tr><td>19</td><td>2022-02-13</td><td>5</td><td>0.75</td><td>1.96</td><td>0.12</td><td>0.34</td><td>S</td><td>S</td><td>AR</td><td>VD</td><td>1</td></tr><tr><td>20</td><td>2022-03-02</td><td>7</td><td>0.56</td><td>72.05</td><td>0.47</td><td>2.91</td><td>M</td><td>S</td><td>AR</td><td>VD</td><td>1</td></tr><tr><td>21</td><td>2022-03-11</td><td>15</td><td>0.45</td><td>1848.21</td><td>0.15</td><td>1.13</td><td>M</td><td>D</td><td>AR</td><td>VD</td><td>3</td></tr><tr><td>22</td><td>2022-04-06</td><td>16</td><td>0.39</td><td>146.64</td><td>0.15</td><td>0.47</td><td>M</td><td>S (+)</td><td>AR<sup>a</sup></td><td>FS<sup>b</sup></td><td>4</td></tr><tr><td>23</td><td>2022-04-26</td><td>28</td><td>0.64</td><td>370.01</td><td>0.11</td><td>0.44</td><td>M</td><td>D (S)</td><td>AR<sup>a</sup> (CH)</td><td>FS<sup>b</sup></td><td>6</td></tr><tr><td>24</td><td>2022-06-30</td><td>7</td><td>1.01</td><td>35.55</td><td>0.1</td><td>0.42</td><td>C</td><td>S (+)</td><td>AR</td><td>FS<sup>b</sup></td><td>2</td></tr><tr><td>25</td><td>2022-07-15</td><td>9</td><td>1.01</td><td>119.55</td><td>0.08</td><td>0.86</td><td>S</td><td>S (+)</td><td>AR</td><td>FS<sup>b</sup></td><td>2</td></tr><tr><td>26</td><td>2022-08-07</td><td>4</td><td>0.93</td><td>0.34</td><td>1.33</td><td>2.58</td><td>S</td><td>D</td><td>AR (CH)</td><td>FS<sup>b</sup></td><td>2</td></tr><tr><td>27</td><td>2022-08-23</td><td>5</td><td>0.83</td><td>4.35</td><td>0.12</td><td>0.5</td><td>S</td><td>S (+)</td><td>AR</td><td>FS<sup>b</sup></td><td>2</td></tr><tr><td>28</td><td>2022-09-20</td><td>10</td><td>0.54</td><td>8.91</td><td>1.09</td><td>1.3</td><td>S</td><td>D</td><td>AR<sup>a</sup></td><td>FS<sup>b</sup></td><td>3</td></tr><tr><td>29</td><td>2022-10-06</td><td>3</td><td>0.33</td><td>110.11</td><td>0.18</td><td>4.12</td><td>M</td><td>S</td><td>AR</td><td>FS</td><td>1</td></tr><tr><td>30</td><td>2022-10-27</td><td>4</td><td>0.42</td><td>807.10</td><td>0.81</td><td>1.33</td><td>M</td><td>S</td><td>AR (CH)</td><td>VD</td><td>1</td></tr><tr><td>31</td><td>2022-11-09</td><td>8</td><td>0.59</td><td>29.15</td><td>0.22</td><td>0.87</td><td>M</td><td>D (S)</td><td>AR</td><td>VD</td><td>2</td></tr><tr><td>32</td><td>2022-11-23</td><td>3</td><td>0.74</td><td>0.20</td><td>1.34</td><td>...</td><td>C</td><td>S</td><td>AR</td><td>VD</td><td>1</td></tr><tr><td>33</td><td>2022-12-01</td><td>4</td><td>0.81</td><td>5.54</td><td>0.17</td><td>1.79</td><td>S</td><td>S</td><td>AR</td><td>VD</td><td>1</td></tr></tbody></table>

**Notes.**

<sup>a</sup> Uncertain Connectivity Characterization.

<sup>b</sup> Imaging data from EUI on Solar Orbiter.

single injection or a continuum. The same result was also apparent for the total duration. As a final step in our analysis, we calculated the ³He:⁴He and Fe:O ratio in the range of 0.2–10 MeV nucleon⁻¹, at the time when the ³He flux is maximum. These results are presented in column 4b of Table 1 for ³He:⁴He, and in column 4c for the Fe:O ratio. The maximum ³He:⁴He was observed for the time period of 2021 February 13, with a ratio of 1.65, while the minimum ratio was observed for 2021 May 8, with a ratio of 0.05. We also find a median value of 0.16 for the ³He:⁴He ratio. We did not find any correlation (cc: −0.24, *p*-value: 1.84 × 10⁻¹) between the ³He:⁴He ratio and the duration of the time periods. The Fe:O ratio was greater than 0.8 in 15 cases. Overall, our sample exhibited a median value of 0.78 for the Fe:O ratio.

Last, for each time period, we examined the energetic proton and electron characteristics. From Solar Orbiter's EPD observations, we find that there are associated energetic electrons and protons for every time period. This analysis of the energetic electron observations reveals that, similarly to ³He, there are multiple injections during the prolonged time periods. While some of the electron injections appear to be associated with the ³He injections, there are numerous cases

where there is no one-to-one correspondence. In particular, some electron injections are observed without any ³He particles. One reason may be that ³He ions are not always accelerated at electron injections. Another reason may be that these energetic electrons come from regions with poor magnetic connectivity to Solar Orbiter. Electrons can be transported more easily in the corona than He and heavy ions. Additionally, we have found that in 17 time periods some of the ³He injections were also associated with high-energy electrons (>1 MeV), and in eight cases with high-energy protons (>50 MeV).

### 3.3. Magnetic Connectivity

#### 3.3.1. Models and Methods

For each time period, we calculate Solar Orbiter's magnetic connectivity to the solar surface through a two-step process. This process involves a ballistic back-mapping of the interplanetary (IP) magnetic field lines from the spacecraft location to a point in the low corona, followed by a tracing of magnetic field lines in the low corona to the solar surface. This method is similar to previous studies examining the source
<table><caption>**Table 1**
List of the Extended ³He-rich Time Periods</caption><thead><tr><th rowspan="2">No.</th><th rowspan="2">Date<br>(1)</th><th rowspan="2">Duration<br>(Days)<br>(2)</th><th rowspan="2">Dist.<br>(au)<br>(3)</th><th colspan="4">SIS</th><th colspan="4">Magnetic Connectivity</th></tr><tr><th><sup>3</sup>He<br>(4a)</th><th><sup>3</sup>He:<sup>4</sup>He<br>(4b)</th><th>Fe:O<br>(4c)</th><th>Inj.<br>(4d)</th><th>Type<br>(5a)</th><th>Source<br>(5b)</th><th>Vis.<br>(5c)</th><th>#ARs<br>(5d)</th></tr></thead><tbody><tr><td>1</td><td>2020-07-19</td><td>5</td><td>0.66</td><td>9.07</td><td>0.14</td><td>1.5</td><td>M</td><td>D</td><td>CH<sup>a</sup></td><td>FS</td><td>...</td></tr><tr><td>2</td><td>2020-11-17</td><td>10</td><td>0.93</td><td>12.10</td><td>0.07</td><td>0.18</td><td>M</td><td>D (C)</td><td>AR<sup>a</sup> (CH)</td><td>FS</td><td>2</td></tr><tr><td>3</td><td>2020-12-09</td><td>10</td><td>0.83</td><td>6.83</td><td>0.45</td><td>2.97</td><td>M</td><td>S (+)</td><td>AR</td><td>FS</td><td>2</td></tr><tr><td>4</td><td>2021-02-13</td><td>3</td><td>0.50</td><td>0.46</td><td>1.65</td><td>...</td><td>S</td><td>S (+)</td><td>AR<sup>a</sup> (CH)</td><td>FS</td><td>1</td></tr><tr><td>5</td><td>2021-03-04</td><td>4</td><td>0.57</td><td>0.41</td><td>0.2</td><td>...</td><td>S</td><td>D (C)</td><td>CH<sup>a</sup> (AR)</td><td>VD</td><td>...</td></tr><tr><td>6</td><td>2021-04-19</td><td>8</td><td>0.85</td><td>11.38</td><td>0.06</td><td>0.22</td><td>S</td><td>C</td><td>CH (AR)</td><td>VD</td><td>...</td></tr><tr><td>7</td><td>2021-05-08</td><td>5</td><td>0.92</td><td>8.47</td><td>0.05</td><td>0.33</td><td>C</td><td>D</td><td>AR (CH)</td><td>VD</td><td>2</td></tr><tr><td>8</td><td>2021-05-19</td><td>10</td><td>0.94</td><td>27.25</td><td>0.58</td><td>0.46</td><td>M</td><td>S (C)</td><td>AR (CH)</td><td>VD</td><td>1</td></tr><tr><td>9</td><td>2021-06-03</td><td>8</td><td>0.95</td><td>0.59</td><td>0.25</td><td>0.36</td><td>M</td><td>D (C)</td><td>CH<sup>a</sup></td><td>VD</td><td>...</td></tr><tr><td>10</td><td>2021-07-23</td><td>7</td><td>0.82</td><td>3.21</td><td>0.08</td><td>0.33</td><td>S</td><td>C</td><td>AR (CH)</td><td>VD</td><td>3</td></tr><tr><td>11</td><td>2021-08-23</td><td>11</td><td>0.64</td><td>36.75</td><td>0.06</td><td>0.68</td><td>M</td><td>D</td><td>AR (CH)</td><td>VD</td><td>4</td></tr><tr><td>12</td><td>2021-09-22</td><td>8</td><td>0.60</td><td>4.04</td><td>0.26</td><td>3.44</td><td>M</td><td>S (+)</td><td>AR</td><td>VD</td><td>3</td></tr><tr><td>13</td><td>2021-10-03</td><td>4</td><td>0.64</td><td>1.27</td><td>0.16</td><td>0.38</td><td>S</td><td>C</td><td>AR</td><td>VD</td><td>3</td></tr><tr><td>14</td><td>2021-12-03</td><td>8</td><td>1.00</td><td>3.95</td><td>0.14</td><td>2.4</td><td>M</td><td>D</td><td>AR</td><td>VD</td><td>3</td></tr><tr><td>15</td><td>2021-12-20</td><td>7</td><td>1.01</td><td>1.61</td><td>0.11</td><td>...</td><td>M</td><td>S</td><td>AR</td><td>VD</td><td>1</td></tr><tr><td>16</td><td>2021-12-30</td><td>10</td><td>1.00</td><td>17.77</td><td>1.12</td><td>1.33</td><td>S</td><td>S (+)</td><td>AR (CH)</td><td>VD</td><td>2</td></tr><tr><td>17</td><td>2022-01-13</td><td>6</td><td>0.96</td><td>25.42</td><td>0.09</td><td>0.71</td><td>M</td><td>S</td><td>AR (CH)</td><td>VD</td><td>1</td></tr><tr><td>18</td><td>2022-02-05</td><td>7</td><td>0.82</td><td>2.24</td><td>0.34</td><td>1.2</td><td>M</td><td>S (+)</td><td>AR (CH)</td><td>VD</td><td>2</td></tr><tr><td>19</td><td>2022-02-13</td><td>5</td><td>0.75</td><td>1.96</td><td>0.12</td><td>0.34</td><td>S</td><td>S</td><td>AR</td><td>VD</td><td>1</td></tr><tr><td>20</td><td>2022-03-02</td><td>7</td><td>0.56</td><td>72.05</td><td>0.47</td><td>2.91</td><td>M</td><td>S</td><td>AR</td><td>VD</td><td>1</td></tr><tr><td>21</td><td>2022-03-11</td><td>15</td><td>0.45</td><td>1848.21</td><td>0.15</td><td>1.13</td><td>M</td><td>D</td><td>AR</td><td>VD</td><td>3</td></tr><tr><td>22</td><td>2022-04-06</td><td>16</td><td>0.39</td><td>146.64</td><td>0.15</td><td>0.47</td><td>M</td><td>S (+)</td><td>AR<sup>a</sup></td><td>FS<sup>b</sup></td><td>4</td></tr><tr><td>23</td><td>2022-04-26</td><td>28</td><td>0.64</td><td>370.01</td><td>0.11</td><td>0.44</td><td>M</td><td>D (S)</td><td>AR<sup>a</sup> (CH)</td><td>FS<sup>b</sup></td><td>6</td></tr><tr><td>24</td><td>2022-06-30</td><td>7</td><td>1.01</td><td>35.55</td><td>0.1</td><td>0.42</td><td>C</td><td>S (+)</td><td>AR</td><td>FS<sup>b</sup></td><td>2</td></tr><tr><td>25</td><td>2022-07-15</td><td>9</td><td>1.01</td><td>119.55</td><td>0.08</td><td>0.86</td><td>S</td><td>S (+)</td><td>AR</td><td>FS<sup>b</sup></td><td>2</td></tr><tr><td>26</td><td>2022-08-07</td><td>4</td><td>0.93</td><td>0.34</td><td>1.33</td><td>2.58</td><td>S</td><td>D</td><td>AR (CH)</td><td>FS<sup>b</sup></td><td>2</td></tr><tr><td>27</td><td>2022-08-23</td><td>5</td><td>0.83</td><td>4.35</td><td>0.12</td><td>0.5</td><td>S</td><td>S (+)</td><td>AR</td><td>FS<sup>b</sup></td><td>2</td></tr><tr><td>28</td><td>2022-09-20</td><td>10</td><td>0.54</td><td>8.91</td><td>1.09</td><td>1.3</td><td>S</td><td>D</td><td>AR<sup>a</sup></td><td>FS<sup>b</sup></td><td>3</td></tr><tr><td>29</td><td>2022-10-06</td><td>3</td><td>0.33</td><td>110.11</td><td>0.18</td><td>4.12</td><td>M</td><td>S</td><td>AR</td><td>FS</td><td>1</td></tr><tr><td>30</td><td>2022-10-27</td><td>4</td><td>0.42</td><td>807.10</td><td>0.81</td><td>1.33</td><td>M</td><td>S</td><td>AR (CH)</td><td>VD</td><td>1</td></tr><tr><td>31</td><td>2022-11-09</td><td>8</td><td>0.59</td><td>29.15</td><td>0.22</td><td>0.87</td><td>M</td><td>D (S)</td><td>AR</td><td>VD</td><td>2</td></tr><tr><td>32</td><td>2022-11-23</td><td>3</td><td>0.74</td><td>0.20</td><td>1.34</td><td>...</td><td>C</td><td>S</td><td>AR</td><td>VD</td><td>1</td></tr><tr><td>33</td><td>2022-12-01</td><td>4</td><td>0.81</td><td>5.54</td><td>0.17</td><td>1.79</td><td>S</td><td>S</td><td>AR</td><td>VD</td><td>1</td></tr></tbody></table>

**Notes.**

<sup>a</sup> Uncertain Connectivity Characterization.

<sup>b</sup> Imaging data from EUI on Solar Orbiter.

single injection or a continuum. The same result was also apparent for the total duration. As a final step in our analysis, we calculated the ³He:⁴He and Fe:O ratio in the range of 0.2–10 MeV nucleon⁻¹, at the time when the ³He flux is maximum. These results are presented in column 4b of Table 1 for ³He:⁴He, and in column 4c for the Fe:O ratio. The maximum ³He:⁴He was observed for the time period of 2021 February 13, with a ratio of 1.65, while the minimum ratio was observed for 2021 May 8, with a ratio of 0.05. We also find a median value of 0.16 for the ³He:⁴He ratio. We did not find any correlation (cc: −0.24, *p*-value: 1.84 × 10⁻¹) between the ³He:⁴He ratio and the duration of the time periods. The Fe:O ratio was greater than 0.8 in 15 cases. Overall, our sample exhibited a median value of 0.78 for the Fe:O ratio.

Last, for each time period, we examined the energetic proton and electron characteristics. From Solar Orbiter's EPD observations, we find that there are associated energetic electrons and protons for every time period. This analysis of the energetic electron observations reveals that, similarly to ³He, there are multiple injections during the prolonged time periods. While some of the electron injections appear to be associated with the ³He injections, there are numerous cases

where there is no one-to-one correspondence. In particular, some electron injections are observed without any ³He particles. One reason may be that ³He ions are not always accelerated at electron injections. Another reason may be that these energetic electrons come from regions with poor magnetic connectivity to Solar Orbiter. Electrons can be transported more easily in the corona than He and heavy ions. Additionally, we have found that in 17 time periods some of the ³He injections were also associated with high-energy electrons (>1 MeV), and in eight cases with high-energy protons (>50 MeV).

### 3.3. Magnetic Connectivity

#### 3.3.1. Models and Methods

For each time period, we calculate Solar Orbiter's magnetic connectivity to the solar surface through a two-step process. This process involves a ballistic back-mapping of the interplanetary (IP) magnetic field lines from the spacecraft location to a point in the low corona, followed by a tracing of magnetic field lines in the low corona to the solar surface. This method is similar to previous studies examining the source
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Statistical results from the characteristics of the extended $^3$He time periods. Panel (a) shows the relation between the total duration of the extended $^3$He-rich time periods with the spacecraft radial distance measured at the beginning of each time period. At the bottom and right axis, we show the distributions of the total duration and the radial distance, respectively, and we note the median values for each case. Panel (b) shows the relation between the total duration of the time periods and the peak flux of the $^3$He. On the right axis, we show the distribution of the peak flux of $^3$He and note the median value. Panel (c) shows the distributions of the peak flux of $^3$He (blue) and the total duration of the time periods (red) for the three injection classes using a box-plot format.</figcaption></figure>

regions of solar wind (Liewer et al. 2004). First, we calculate the interplanetary (IP) magnetic field lines, where we assume a Parker spiral derived using the average solar wind speed measured by Solar Orbiter during the investigated time period. For the majority of the considered time periods, Solar Orbiter was magnetically connected to regions of slow solar wind. Considering all the periods with Solar Orbiter solar wind speed measurements, we find an average solar wind speed of 401 km s⁻¹. When solar wind speed measurements are not available during full intervals, we assume a solar wind speed of 350 km s⁻¹. This is the case for 7 out of 33 time periods. We calculate the Parker spiral starting from the spacecraft location and ending at the source surface, which is a spherical surface where all the solar magnetic field lines become radial and open. We assume that the radius of the source surface is at $2.5 R_⊙$. It is important to note that the calculation of the IP magnetic field assumes that the solar wind flow is constant and radial, which is a rough approximation but widely used for its simplicity. Although this approach may be accurate for locations further from the Sun, where the acceleration of the solar wind becomes less significant, other effects that may come into play, such as the development of stream or corotation interaction regions, can impact the shape of the spiral (e.g., Schwadron et al. 2021) and hence affect the reliability of the back-mappings. While more sophisticated approaches are possible, this approach is adequate for the purpose of this analysis.

To determine the magnetic connectivity below the source surface, we employ the PFSS model, which represents the simplest case of a force-free coronal magnetic field model. This model extrapolates the photospheric magnetic field to the source surface. The PFSS coronal magnetic field solutions are derived using the pfsspy Python package (Stansby et al. 2020). To calculate the connected field lines to the observers, we perform a field line tracing to the PFSS coronal magnetic field solutions starting from the location of the Parker spiral footpoint at the source surface. The tracing is performed in a narrow region of 2.5° around the connection point of the Parker spiral at the source surface. For the input to the PFSS model, we utilize the Air Force Data Assimilative Photospheric Flux Transport (ADAPT; Arge et al. 2010) photospheric magnetic field maps based on the Global Oscillation Network Group magnetograms. The ADAPT model utilizes a flux-transport

model (Worden & Harvey 2000) to provide an update of the magnetic field for regions that are not visible from Earth. As ADAPT is an ensemble model, the provided maps consist of 12 different realizations, and each realization yields slightly different connectivity because the derived coronal magnetic field differs for each PFSS solution. For the connectivity analysis, we utilize the tenth ADAPT realization, which has been demonstrated in previous analyses to compare better with white-light observations of the solar corona (e.g., see Poirier et al. 2021 for further details). The ADAPT maps have a two-hour cadence, but the final connectivity estimates have a cadence of one hour. For the intermediate time steps, we select the magnetic map closest to the time that we determine the connectivity.

### 3.3.2. Characterization of Connectivity

In order to characterize the magnetic connectivity of the spacecraft, we construct Carrington maps of the solar corona utilizing EUV images of the solar corona obtained from SDO/AIA at 193 Å. We overlay on the Carrington maps the results of the PFSS model and the connected field lines to Solar Orbiter, which are derived from the magnetic connectivity analysis. For the visible part of the maps, we use images from AIA taken at the time of the magnetic connectivity analysis. The unobserved far-side part of the maps is filled by using past SDO/AIA images over a solar rotation. The heliospheric current sheet (neutral line) at the source surface, the field lines from the PFSS solution, the location of Solar Orbiter, and the connected field lines to the spacecraft are projected onto the Carrington map. The maps are produced with a cadence of one hour for the entire duration of the extended $^3$He time periods. In Figure 4, we present a few examples of the solar Carrington maps that we produced for our analysis.

Utilizing the connectivity maps, we examine the magnetic connectivity of the spacecraft for each time period. First, we analyze the evolution of connectivity and separate it into three distinct categories, which are presented in Table 1 (see column 5a) for each time period. In Table 2, we give the definitions of the four categories of magnetic connectivity that we used in this study. The first group in Table 2 is referred to as “stable” (S), where the spacecraft remains connected to the same region(s) for several days and minimal changes in the footpoints’
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Statistical results from the characteristics of the extended $^3$He time periods. Panel (a) shows the relation between the total duration of the extended $^3$He-rich time periods with the spacecraft radial distance measured at the beginning of each time period. At the bottom and right axis, we show the distributions of the total duration and the radial distance, respectively, and we note the median values for each case. Panel (b) shows the relation between the total duration of the time periods and the peak flux of the $^3$He. On the right axis, we show the distribution of the peak flux of $^3$He and note the median value. Panel (c) shows the distributions of the peak flux of $^3$He (blue) and the total duration of the time periods (red) for the three injection classes using a box-plot format.</figcaption></figure>

regions of solar wind (Liewer et al. 2004). First, we calculate the interplanetary (IP) magnetic field lines, where we assume a Parker spiral derived using the average solar wind speed measured by Solar Orbiter during the investigated time period. For the majority of the considered time periods, Solar Orbiter was magnetically connected to regions of slow solar wind. Considering all the periods with Solar Orbiter solar wind speed measurements, we find an average solar wind speed of 401 km s⁻¹. When solar wind speed measurements are not available during full intervals, we assume a solar wind speed of 350 km s⁻¹. This is the case for 7 out of 33 time periods. We calculate the Parker spiral starting from the spacecraft location and ending at the source surface, which is a spherical surface where all the solar magnetic field lines become radial and open. We assume that the radius of the source surface is at 2.5 $R_⊙$. It is important to note that the calculation of the IP magnetic field assumes that the solar wind flow is constant and radial, which is a rough approximation but widely used for its simplicity. Although this approach may be accurate for locations further from the Sun, where the acceleration of the solar wind becomes less significant, other effects that may come into play, such as the development of stream or corotation interaction regions, can impact the shape of the spiral (e.g., Schwadron et al. 2021) and hence affect the reliability of the back-mappings. While more sophisticated approaches are possible, this approach is adequate for the purpose of this analysis.

To determine the magnetic connectivity below the source surface, we employ the PFSS model, which represents the simplest case of a force-free coronal magnetic field model. This model extrapolates the photospheric magnetic field to the source surface. The PFSS coronal magnetic field solutions are derived using the pfsspy Python package (Stansby et al. 2020). To calculate the connected field lines to the observers, we perform a field line tracing to the PFSS coronal magnetic field solutions starting from the location of the Parker spiral footpoint at the source surface. The tracing is performed in a narrow region of 2.5° around the connection point of the Parker spiral at the source surface. For the input to the PFSS model, we utilize the Air Force Data Assimilative Photospheric Flux Transport (ADAPT; Arge et al. 2010) photospheric magnetic field maps based on the Global Oscillation Network Group magnetograms. The ADAPT model utilizes a flux-transport

model (Worden & Harvey 2000) to provide an update of the magnetic field for regions that are not visible from Earth. As ADAPT is an ensemble model, the provided maps consist of 12 different realizations, and each realization yields slightly different connectivity because the derived coronal magnetic field differs for each PFSS solution. For the connectivity analysis, we utilize the tenth ADAPT realization, which has been demonstrated in previous analyses to compare better with white-light observations of the solar corona (e.g., see Poirier et al. 2021 for further details). The ADAPT maps have a two-hour cadence, but the final connectivity estimates have a cadence of one hour. For the intermediate time steps, we select the magnetic map closest to the time that we determine the connectivity.

### 3.3.2. Characterization of Connectivity

In order to characterize the magnetic connectivity of the spacecraft, we construct Carrington maps of the solar corona utilizing EUV images of the solar corona obtained from SDO/AIA at 193 Å. We overlay on the Carrington maps the results of the PFSS model and the connected field lines to Solar Orbiter, which are derived from the magnetic connectivity analysis. For the visible part of the maps, we use images from AIA taken at the time of the magnetic connectivity analysis. The unobserved far-side part of the maps is filled by using past SDO/AIA images over a solar rotation. The heliospheric current sheet (neutral line) at the source surface, the field lines from the PFSS solution, the location of Solar Orbiter, and the connected field lines to the spacecraft are projected onto the Carrington map. The maps are produced with a cadence of one hour for the entire duration of the extended $^3$He time periods. In Figure 4, we present a few examples of the solar Carrington maps that we produced for our analysis.

Utilizing the connectivity maps, we examine the magnetic connectivity of the spacecraft for each time period. First, we analyze the evolution of connectivity and separate it into three distinct categories, which are presented in Table 1 (see column 5a) for each time period. In Table 2, we give the definitions of the four categories of magnetic connectivity that we used in this study. The first group in Table 2 is referred to as “stable” (S), where the spacecraft remains connected to the same region(s) for several days and minimal changes in the footpoints’
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Connectivity maps that are produced for four extended periods of ³He, with each map being constructed at the beginning of its respective time period. Panel (a) displays the map for 2022 January 13, panel (b) for 2022 February 05, panel (c) for 2022 July 19, and panel (d) for the time period of 2021 July 23. Each map shows the projected location of the heliospheric current sheet (white line), as well as the open (red and green lines) and closed (black) field lines from the PFSS model. Furthermore, the location of Solar Orbiter is indicated by the blue circle. From that location, the Parker spiral is displayed with a dashed blue line, and the footpoint location of the Parker spiral to the source surface is depicted with a blue cross. From that point, the field lines are traced below the source surface using the PFSS model, and they are depicted with blue lines. The locations of the footpoints of the field lines connecting to the spacecraft during subsequent times and within the ³He time periods are displayed using colored circles. The color coding of these points is given by the color bar on the inner left of each panel. Green (red) footpoints are used to represent times near the beginnings (ends) of the periods.</figcaption></figure>

location are observed. An example of this type of connectivity is shown in Figure 4(a), where the footpoints are concentrated around the periphery of the Coronal Hole (CH) and AR. If the connectivity switches to another stable region during the time interval, we characterize it as stable and denote it with $S^+$ in Table 1 (column 5a). Figure 4(b) shows an example of this second category, where the Solar Orbiter spacecraft maintains a stable connection with two ARs during the time period. The third category is the “dynamic” (D) category, where magnetic connectivity continuously changes from one nearby region to another. The changes in connectivity occur gradually and last for several hours. This dynamic change in connectivity is primarily due to the natural drift of the footpoints on the solar surface caused by the spacecraft’s motion when the field lines are not firmly anchored to an AR below the source surface. An example of this case is seen in Figure 4(c) for the presented time interval. The final category is the “complex” (C) category, where the connectivity changes rapidly between multiple regions over a few hours. This occurs mainly when the field lines connected to Solar Orbiter are near the heliospheric current sheet, and the magnetic connectivity can be quite ambiguous. An example is displayed in Figure 4(d). Typically, one category dominates throughout the entire period, indicating that the connectivity evolution can be categorized using only one of the three categories for a prolonged duration.

In Figure 5(a), we show the histogram of the registered categories from the classification of the evolution of connectivity. We find that in 18 cases the connectivity is either stable or stable+, in 12 cases it is dynamic, and in the remaining 3 cases it is complex. From the 18 cases with stable

or stable+ connectivity, we find that in 9 cases the connectivity remains stable to only one region. Therefore, in the remaining 9 cases the connectivity is stable to one region for some time but then it changes to another stable region during the time interval. Additionally, in six (out of 33) cases we registered an additional minor classification because another class appears for a very short time period, usually near the edges of the time intervals. Most of the cases that we had to use this additional minor classification belong to the “dynamic connectivity” class. We present this minor classification in Table 1 (column 5a), next to the main classification. Last, we find that the time periods associated with the S & $S^+$ category exhibit lower total duration (mean duration 6.9 ± 3.3 days) compared to these associated with the C or D categories (mean duration 9.0 ± 6.0 days).

### 3.3.3. Sources of Magnetic Connection

We continue our analysis by examining the solar regions that Solar Orbiter is magnetically connected to during each extended ³He-rich time period. For this analysis, we rely again on the produced connectivity maps (Figure 4). First, we examine the location of the footpoint with respect to Earth’s viewpoint. The results are presented as a histogram in Figure 5(b1) and Table 1 (column 5b). We observe that, in most cases (21), the footpoints of the magnetically connected field lines are located at the visible solar disk (noted as VD in Figure 5(b1)) as viewed from Earth. In contrast, for the remaining 12 cases, the footpoints are on the far side (noted as FS & $FS^*$ in Figure 5(b1)). In the cases where the footpoints are
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Connectivity maps that are produced for four extended periods of ³He, with each map being constructed at the beginning of its respective time period. Panel (a) displays the map for 2022 January 13, panel (b) for 2022 February 05, panel (c) for 2022 July 19, and panel (d) for the time period of 2021 July 23. Each map shows the projected location of the heliospheric current sheet (white line), as well as the open (red and green lines) and closed (black) field lines from the PFSS model. Furthermore, the location of Solar Orbiter is indicated by the blue circle. From that location, the Parker spiral is displayed with a dashed blue line, and the footpoint location of the Parker spiral to the source surface is depicted with a blue cross. From that point, the field lines are traced below the source surface using the PFSS model, and they are depicted with blue lines. The locations of the footpoints of the field lines connecting to the spacecraft during subsequent times and within the ³He time periods are displayed using colored circles. The color coding of these points is given by the color bar on the inner left of each panel. Green (red) footpoints are used to represent times near the beginnings (ends) of the periods.</figcaption></figure>

location are observed. An example of this type of connectivity is shown in Figure 4(a), where the footpoints are concentrated around the periphery of the Coronal Hole (CH) and AR. If the connectivity switches to another stable region during the time interval, we characterize it as stable and denote it with $S^+$ in Table 1 (column 5a). Figure 4(b) shows an example of this second category, where the Solar Orbiter spacecraft maintains a stable connection with two ARs during the time period. The third category is the “dynamic” (D) category, where magnetic connectivity continuously changes from one nearby region to another. The changes in connectivity occur gradually and last for several hours. This dynamic change in connectivity is primarily due to the natural drift of the footpoints on the solar surface caused by the spacecraft’s motion when the field lines are not firmly anchored to an AR below the source surface. An example of this case is seen in Figure 4(c) for the presented time interval. The final category is the “complex” (C) category, where the connectivity changes rapidly between multiple regions over a few hours. This occurs mainly when the field lines connected to Solar Orbiter are near the heliospheric current sheet, and the magnetic connectivity can be quite ambiguous. An example is displayed in Figure 4(d). Typically, one category dominates throughout the entire period, indicating that the connectivity evolution can be categorized using only one of the three categories for a prolonged duration.

In Figure 5(a), we show the histogram of the registered categories from the classification of the evolution of connectivity. We find that in 18 cases the connectivity is either stable or stable+, in 12 cases it is dynamic, and in the remaining 3 cases it is complex. From the 18 cases with stable

or stable+ connectivity, we find that in 9 cases the connectivity remains stable to only one region. Therefore, in the remaining 9 cases the connectivity is stable to one region for some time but then it changes to another stable region during the time interval. Additionally, in six (out of 33) cases we registered an additional minor classification because another class appears for a very short time period, usually near the edges of the time intervals. Most of the cases that we had to use this additional minor classification belong to the “dynamic connectivity” class. We present this minor classification in Table 1 (column 5a), next to the main classification. Last, we find that the time periods associated with the S & $S^+$ category exhibit lower total duration (mean duration 6.9 ± 3.3 days) compared to these associated with the C or D categories (mean duration 9.0 ± 6.0 days).

### 3.3.3. Sources of Magnetic Connection

We continue our analysis by examining the solar regions that Solar Orbiter is magnetically connected to during each extended ³He-rich time period. For this analysis, we rely again on the produced connectivity maps (Figure 4). First, we examine the location of the footpoint with respect to Earth’s viewpoint. The results are presented as a histogram in Figure 5(b1) and Table 1 (column 5b). We observe that, in most cases (21), the footpoints of the magnetically connected field lines are located at the visible solar disk (noted as VD in Figure 5(b1)) as viewed from Earth. In contrast, for the remaining 12 cases, the footpoints are on the far side (noted as FS & $FS^*$ in Figure 5(b1)). In the cases where the footpoints are
<table><caption>**Table 2**
Definitions of Magnetic Connectivity</caption><thead><tr><th>Connectivity type</th><th>Symbol</th><th>Description</th><th>Example</th></tr></thead><tbody><tr><td>stable</td><td>S</td><td>Connected to same AR for several days.</td><td>Figure 4(a)</td></tr><tr><td>stable</td><td>S<sup>+</sup></td><td>Switches between first and second AR.</td><td>Figure 4(b)</td></tr><tr><td>dynamic</td><td>D</td><td>Continuous change from one AR to another nearby.</td><td>Figure 4(c)</td></tr><tr><td>complex</td><td>C</td><td>Rapid connectivity changes between multiple ARs over a few hours.</td><td>Figure 4(d)</td></tr></tbody></table>

located on the visible disk, remote-sensing observations are available. In these cases, the magnetic maps are updated with the observations; therefore, the connectivity analysis is more reliable. Additionally, it is less ambiguous to register the source regions. On the other hand, when the footpoints are on the far side, the magnetic maps at the points of connection are not updated with observations. Determining the sources of magnetic connection is also a difficult task; however, for seven of the far-side periods, remote-sensing EUV observations from STEREO-A and Solar Orbiter assisted our analysis for the registration of the source regions. We note these cases as FS* in Figure 5(b1) and Table 1 (column 5c).

We investigate the regions of magnetic connectivity by examining the locations of the footpoints on the solar disk. Two distinct cases are observed: (1) a magnetic connection to an active region, and (2) a magnetic connection to a coronal hole. We note that, where feasible, we supplement the analysis with STEREO/EUVI or Solar Orbiter/EUI images (if available). The resulting classification is presented in Figure 5(b2) and Table 1. In Table 1 (column 5b), we label the cases where we find that the characterization of connectivity is ambiguous, and we also parenthetically add minor characterizations for the cases where there are brief connections to other regions. We also note that, when the footpoints are not near the core of the AR (e.g., the AR's bright loops) but instead they are located far in the periphery of the AR, these cases have been registered in the AR category if no CH was visible around the connected footpoints. We find that in 29 out of 33 cases Solar Orbiter is magnetically connected to an active region, whereas in the other four cases it is connected to a coronal hole. For 11 cases of the first category and one case of the second, the footpoints of the field lines are located on the far side. These cases are shown using a different color in the stacked histogram of Figure 5(b2). We expect that the connectivity for these periods has some degree of uncertainty. Finally, for the cases where we have found a connection to an AR, we registered the number of ARs connected to the spacecraft during the extended time periods (see column 5d in Table 1). There are 19 cases where Solar Orbiter is connected to at least two ARs during the time intervals, whereas there are a few cases (9) where there is a connection to more than two ARs. On average, we find that the spacecraft is connected to at least two ARs during the intervals. Using also the total duration of the time periods, we calculate that the average connection time to an AR in this study is $4.1 \pm 1.8$ days.

We also examine the connectivity of Solar Orbiter near the boundaries of the extended $^3$He-rich time periods, to identify any noticeable changes. To accomplish this, we evaluate the connectivity maps one day before and one day after the time periods and register if there is a notable change in magnetic connectivity. Typically, these changes in connectivity correspond to shifts of the field lines from one solar region to another, active region or coronal hole. We find that at the

beginning of a time period there are 25 cases where the connectivity changes from one region to another, while in seven other cases we do not observe any change in connectivity. As expected, similar results are obtained for the end of each time period, where 25 cases exhibit a change in connectivity, and for the remaining eight cases no change is observed near the end of the time intervals. The resulting histograms of this analysis are shown in 5(c).

## 4. Discussion and Conclusion

By utilizing observations obtained from the Solar Orbiter spacecraft's SIS instrument, we have compiled a catalog of extended periods rich in $^3$He that were observed during the first five orbits of the spacecraft around the Sun. We have identified 33 periods with durations greater than one day that exhibit a high abundance of $^3$He. Over half of the recorded periods have a duration exceeding seven days, suggesting that there may be a continuous production of $^3$He in certain regions at the solar corona and subsequent injection of the $^3$He-rich SEPs into interplanetary space (Mason 2007).

Our analysis has also demonstrated an increase on the occurrence of the extended $^3$He-rich time periods (and $^3$He flux) with time (see Figure 1), as expected during the rising phase of the solar cycle (e.g., Wang et al. 2012). Similar correlation has recently been shown by Mason et al. (2023a) for the daily intensities of O measured by SIS. Indicative of this correlation with the solar activity is the fact that during 2022 we registered 141 $^3$He-rich days, compared to 2021 where we registered 93 days. Our analysis did not show any correlation between the characteristics of the observed periods and the orbital characteristics of the Solar Orbiter spacecraft. For instance, the radial distance of the spacecraft does not appear to play a role in determining the overall duration of these periods, indicating that the presence and activity levels of the ARs dominates over radial effects in this data set.

Based on our analysis of the SEP characteristics during the observed time periods, we have found that, in the majority of cases (63%), there are multiple $^3$He injections within the time intervals. In the remaining cases, we have recorded either single injections or a continuum close to background levels. Similar findings have been previously reported by Chen et al. (2015) and Bučík et al. (2021) for other cases of long-duration $^3$He-rich periods (see also Ho et al. 2022; Mason et al. 2023b). In the case of the extended $^3$He-rich period of 2020 November, Bučík et al. (2021) identified at least five ion injections. We have found that one-fourth of the cases in our list have at least four injections of $^3$He during the time periods. Additionally, we found that the cases with multiple injections exhibit larger total duration and higher peak values for the $^3$He flux. Furthermore, the time periods with multiple injections exhibit longer duration and higher peak values for $^3$He flux. These findings suggest that, during the extended $^3$He-rich time periods, there is
<table><caption>**Table 2**
Definitions of Magnetic Connectivity</caption><thead><tr><th>Connectivity type</th><th>Symbol</th><th>Description</th><th>Example</th></tr></thead><tbody><tr><td>stable</td><td>S</td><td>Connected to same AR for several days.</td><td>Figure 4(a)</td></tr><tr><td>stable</td><td>S<sup>+</sup></td><td>Switches between first and second AR.</td><td>Figure 4(b)</td></tr><tr><td>dynamic</td><td>D</td><td>Continuous change from one AR to another nearby.</td><td>Figure 4(c)</td></tr><tr><td>complex</td><td>C</td><td>Rapid connectivity changes between multiple ARs over a few hours.</td><td>Figure 4(d)</td></tr></tbody></table>

located on the visible disk, remote-sensing observations are available. In these cases, the magnetic maps are updated with the observations; therefore, the connectivity analysis is more reliable. Additionally, it is less ambiguous to register the source regions. On the other hand, when the footpoints are on the far side, the magnetic maps at the points of connection are not updated with observations. Determining the sources of magnetic connection is also a difficult task; however, for seven of the far-side periods, remote-sensing EUV observations from STEREO-A and Solar Orbiter assisted our analysis for the registration of the source regions. We note these cases as FS* in Figure 5(b1) and Table 1 (column 5c).

We investigate the regions of magnetic connectivity by examining the locations of the footpoints on the solar disk. Two distinct cases are observed: (1) a magnetic connection to an active region, and (2) a magnetic connection to a coronal hole. We note that, where feasible, we supplement the analysis with STEREO/EUVI or Solar Orbiter/EUI images (if available). The resulting classification is presented in Figure 5(b2) and Table 1. In Table 1 (column 5b), we label the cases where we find that the characterization of connectivity is ambiguous, and we also parenthetically add minor characterizations for the cases where there are brief connections to other regions. We also note that, when the footpoints are not near the core of the AR (e.g., the AR's bright loops) but instead they are located far in the periphery of the AR, these cases have been registered in the AR category if no CH was visible around the connected footpoints. We find that in 29 out of 33 cases Solar Orbiter is magnetically connected to an active region, whereas in the other four cases it is connected to a coronal hole. For 11 cases of the first category and one case of the second, the footpoints of the field lines are located on the far side. These cases are shown using a different color in the stacked histogram of Figure 5(b2). We expect that the connectivity for these periods has some degree of uncertainty. Finally, for the cases where we have found a connection to an AR, we registered the number of ARs connected to the spacecraft during the extended time periods (see column 5d in Table 1). There are 19 cases where Solar Orbiter is connected to at least two ARs during the time intervals, whereas there are a few cases (9) where there is a connection to more than two ARs. On average, we find that the spacecraft is connected to at least two ARs during the intervals. Using also the total duration of the time periods, we calculate that the average connection time to an AR in this study is $4.1 \pm 1.8$ days.

We also examine the connectivity of Solar Orbiter near the boundaries of the extended $^3$He-rich time periods, to identify any noticeable changes. To accomplish this, we evaluate the connectivity maps one day before and one day after the time periods and register if there is a notable change in magnetic connectivity. Typically, these changes in connectivity correspond to shifts of the field lines from one solar region to another, active region or coronal hole. We find that at the

beginning of a time period there are 25 cases where the connectivity changes from one region to another, while in seven other cases we do not observe any change in connectivity. As expected, similar results are obtained for the end of each time period, where 25 cases exhibit a change in connectivity, and for the remaining eight cases no change is observed near the end of the time intervals. The resulting histograms of this analysis are shown in 5(c).

## 4. Discussion and Conclusion

By utilizing observations obtained from the Solar Orbiter spacecraft's SIS instrument, we have compiled a catalog of extended periods rich in $^3$He that were observed during the first five orbits of the spacecraft around the Sun. We have identified 33 periods with durations greater than one day that exhibit a high abundance of $^3$He. Over half of the recorded periods have a duration exceeding seven days, suggesting that there may be a continuous production of $^3$He in certain regions at the solar corona and subsequent injection of the $^3$He-rich SEPs into interplanetary space (Mason 2007).

Our analysis has also demonstrated an increase on the occurrence of the extended $^3$He-rich time periods (and $^3$He flux) with time (see Figure 1), as expected during the rising phase of the solar cycle (e.g., Wang et al. 2012). Similar correlation has recently been shown by Mason et al. (2023a) for the daily intensities of O measured by SIS. Indicative of this correlation with the solar activity is the fact that during 2022 we registered 141 $^3$He-rich days, compared to 2021 where we registered 93 days. Our analysis did not show any correlation between the characteristics of the observed periods and the orbital characteristics of the Solar Orbiter spacecraft. For instance, the radial distance of the spacecraft does not appear to play a role in determining the overall duration of these periods, indicating that the presence and activity levels of the ARs dominates over radial effects in this data set.

Based on our analysis of the SEP characteristics during the observed time periods, we have found that, in the majority of cases (63%), there are multiple $^3$He injections within the time intervals. In the remaining cases, we have recorded either single injections or a continuum close to background levels. Similar findings have been previously reported by Chen et al. (2015) and Bučík et al. (2021) for other cases of long-duration $^3$He-rich periods (see also Ho et al. 2022; Mason et al. 2023b). In the case of the extended $^3$He-rich period of 2020 November, Bučík et al. (2021) identified at least five ion injections. We have found that one-fourth of the cases in our list have at least four injections of $^3$He during the time periods. Additionally, we found that the cases with multiple injections exhibit larger total duration and higher peak values for the $^3$He flux. Furthermore, the time periods with multiple injections exhibit longer duration and higher peak values for $^3$He flux. These findings suggest that, during the extended $^3$He-rich time periods, there is
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Histograms of the characterization and analysis of ³He-rich event magnetic connectivity. Panel (a) displays the resulting histogram from the classification of the magnetic connectivity to four different classes ("stable," "stable+," "dynamic," and "complex"). Panel (b) displays the histogram resulting from the analysis of the regions of connectivity. The histogram located on the left illustrates the number of instances in which the magnetic connectivity is observed either on the visible disk (VD) or on the far side (FS & FS*; see text for details). On the right-hand side, the histogram exhibits the cases in which the connectivity is located in an active region or a coronal hole. Panel (c) displays the histogram from the analysis of the connectivity changes at the boundaries. The histogram on the left shows connectivity changes at the beginnings of the periods, and that on the right shows changes at the ends of the periods.</figcaption></figure>

a recurring production and release of ³He SEPs in the regions magnetically connected to Solar Orbiter.

From the detailed analysis of the magnetic connectivity of the spacecraft we find that, in more that one-third of the cases, there is a stable magnetic connection to one or more regions. There is, however, a significant number of cases where the connectivity changes dynamically over multiple regions or it is complex. Additionally, from the connectivity analysis, we find that the sources of magnetic connection during the time periods are mostly ARs. This result is consistent with previous studies (e.g., Wang et al. 2006) that showed that impulsive ³He-rich events typically originate at the interface between ARs and regions of open field line regions (e.g., nearby coronal holes). Kocharov et al. (2008) also showed that about half of their long-duration ³He-rich periods were associated with ARs and leading and(or) trailing coronal holes. From the connectivity analysis, we also find that, for most of the cases, there is a change in the magnetic connection of the spacecraft before and after the extended time periods. This result suggests that changes in the magnetic connection of the spacecraft to regions significant activity could play an important role in the observability of these ³He-rich periods. Finally, assessing the results from the connectivity analysis, we find that the differences in connectivity between the different realizations are less than ten degrees. For regions that are on the far side, the uncertainty of the

connectivity estimates can be much greater, especially when there is a significant emergence(submergence) of magnetic flux that is not included in the magnetic maps.

To summarize the results of our analysis, we have found that extended ³He-rich time periods have been commonly observed by Solar Orbiter, especially since the solar activity started to elevate. We showed that most of these periods are associated with multiple injections of ³He that seem to play some role into the production of ³He-rich periods for a long time. In many cases, the magnetic connectivity is stable and the spacecraft is mainly connected to ARs during the extended periods. In the future, further observations from Solar Orbiter close to the Sun will give more information about the recurrent production of ³He that produces these interesting time intervals. The source regions of the recurring injections, as well as their characteristics in relation to other features of the solar activity (e.g., SXRs, HXRs, Type III radio burst, etc.), are interesting aspects that deserve further analysis. These properties will be explored in a future study.

## Acknowledgments

Solar Orbiter is a mission of international cooperation between ESA and NASA, operated by ESA. The Suprathermal Ion Spectrograph (SIS) is a European facility instrument
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Histograms of the characterization and analysis of ³He-rich event magnetic connectivity. Panel (a) displays the resulting histogram from the classification of the magnetic connectivity to four different classes ("stable," "stable+," "dynamic," and "complex"). Panel (b) displays the histogram resulting from the analysis of the regions of connectivity. The histogram located on the left illustrates the number of instances in which the magnetic connectivity is observed either on the visible disk (VD) or on the far side (FS & FS*; see text for details). On the right-hand side, the histogram exhibits the cases in which the connectivity is located in an active region or a coronal hole. Panel (c) displays the histogram from the analysis of the connectivity changes at the boundaries. The histogram on the left shows connectivity changes at the beginnings of the periods, and that on the right shows changes at the ends of the periods.</figcaption></figure>

a recurring production and release of ³He SEPs in the regions magnetically connected to Solar Orbiter.

From the detailed analysis of the magnetic connectivity of the spacecraft we find that, in more that one-third of the cases, there is a stable magnetic connection to one or more regions. There is, however, a significant number of cases where the connectivity changes dynamically over multiple regions or it is complex. Additionally, from the connectivity analysis, we find that the sources of magnetic connection during the time periods are mostly ARs. This result is consistent with previous studies (e.g., Wang et al. 2006) that showed that impulsive ³He-rich events typically originate at the interface between ARs and regions of open field line regions (e.g., nearby coronal holes). Kocharov et al. (2008) also showed that about half of their long-duration ³He-rich periods were associated with ARs and leading and(or) trailing coronal holes. From the connectivity analysis, we also find that, for most of the cases, there is a change in the magnetic connection of the spacecraft before and after the extended time periods. This result suggests that changes in the magnetic connection of the spacecraft to regions significant activity could play an important role in the observability of these ³He-rich periods. Finally, assessing the results from the connectivity analysis, we find that the differences in connectivity between the different realizations are less than ten degrees. For regions that are on the far side, the uncertainty of the

connectivity estimates can be much greater, especially when there is a significant emergence(submergence) of magnetic flux that is not included in the magnetic maps.

To summarize the results of our analysis, we have found that extended ³He-rich time periods have been commonly observed by Solar Orbiter, especially since the solar activity started to elevate. We showed that most of these periods are associated with multiple injections of ³He that seem to play some role into the production of ³He-rich periods for a long time. In many cases, the magnetic connectivity is stable and the spacecraft is mainly connected to ARs during the extended periods. In the future, further observations from Solar Orbiter close to the Sun will give more information about the recurrent production of ³He that produces these interesting time intervals. The source regions of the recurring injections, as well as their characteristics in relation to other features of the solar activity (e.g., SXRs, HXRs, Type III radio burst, etc.), are interesting aspects that deserve further analysis. These properties will be explored in a future study.

## Acknowledgments

Solar Orbiter is a mission of international cooperation between ESA and NASA, operated by ESA. The Suprathermal Ion Spectrograph (SIS) is a European facility instrument
funded by ESA. The SIS instrument was constructed by the JHU/Applied Physics Lab, with assistance from CAU Kiel. A.K., G.M.M., G.C.H., and R.C.A acknowledge financial support from NASA NNN06AA01C (SO-SIS Phase-E) contract. R.F.W.-S. acknowledges support by the German Space Agency (DLR) under contract 50OT2002. For the production of the bottom panel of Figure 1, we used a modification of the ept_orbit_plot.py script provided by Freiherr von Forstner in github. We are thankful for this script being made publicly available to the community under the GNU General Public License v3.0. This research used version 4.1 of the SunPy (The SunPy Community et al. 2020) and version 1.1.2 of the pfsspy (Stansby et al. 2020) open-source software packages.

## ORCID iDs

A. Kouloumvakos https://orcid.org/0000-0001-6589-4509

G. M. Mason https://orcid.org/0000-0003-2169-9618

G. C. Ho https://orcid.org/0000-0003-1093-2066

R. C. Allen https://orcid.org/0000-0003-2079-5683

Robert F. Wimmer-Schweingruber https://orcid.org/0000-0002-7388-173X

## References

Arge, C. N., Henney, C. J., Koller, J., et al. 2010, in AIP Conf. Proc. 1216, Twelfth International Solar Wind Conference, ed. M. Maksimovic et al. (Melville, NY: AIP), 343

Bučík, R., Innes, D. E., Chen, N. H., et al. 2015, JPhCS, 642, 012002

Bučík, R., Innes, D. E., Mall, U., et al. 2014, ApJ, 786, 71

Bučík, R., Innes, D. E., Mason, G. M., et al. 2018, ApJ, 852, 76

Bučík, R., Innes, D. E., Mason, G. M., & Wiedenbeck, M. E. 2016, ApJ, 833, 63

Bučík, R., Mason, G. M., Gómez-Herrero, R., et al. 2021, A&A, 656, L11

Chen, N.-h., Bučík, R., Innes, D. E., & Mason, G. M. 2015, A&A, 580, A16

Ho, G. C., Mason, G. M., Allen, R. C., et al. 2022, FrASS, 9, 939799

Ho, G. C., Roelof, E. C., Hawkins, S. E. I., et al. 2001, ApJ, 552, 863

Howard, R. A., Moses, J. D., Vourlidas, A., et al. 2008, SSRv, 136, 67

Jebaraj, I. C., Kouloumvakos, A., Dresing, N., et al. 2023, A&A, 675, A27

Kaiser, M. L., Kucera, T. A., Davila, J. M., et al. 2008, SSRv, 136, 5

Kocharov, L., Laivola, J., Mason, G. M., Didkovsky, L., & Judge, D. L. 2008, ApJS, 176, 497

Kollhoff, A., Kouloumvakos, A., Lario, D., et al. 2021, A&A, 656, A20

Kouloumvakos, A., Kwon, R. Y., Rodríguez-García, L., et al. 2022, A&A, 660, A84

Lario, D., Wijsen, N., Kwon, R. Y., et al. 2022, ApJ, 934, 55

Lemen, J. R., Title, A. M., Akin, D. J., et al. 2012, SoPh, 275, 17

Liewer, P. C., Neugebauer, M., & Zurbuchen, T. 2004, SoPh, 223, 209

Mason, G. M. 2007, SSRv, 130, 231

Mason, G. M., Gold, R. E., Krimigis, S. M., et al. 1998, SSRv, 86, 409

Mason, G. M., Ho, G. C., Allen, R. C., et al. 2023a, A&A, 673, L12

Mason, G. M., Nitta, N. V., Bučík, R., et al. 2023b, A&A, 669, L16

Mazur, J. E., Mason, G. M., Dwyer, J. R., et al. 2000, ApJL, 532, L79

Müller, D., St. Cyr, O. C., Zouganelis, I., et al. 2020, A&A, 642, A1

Nitta, N. V., Reames, D. V., De Rosa, M. L., et al. 2006, ApJ, 650, 438

Pesnell, W. D., Thompson, B. J., & Chamberlin, P. C. 2012, SoPh, 275, 3

Poirier, N., Rouillard, A. P., Kouloumvakos, A., et al. 2021, FrASS, 8, 84

Reames, D. V. 2021, FrASS, 8, 164

Rochus, P., Auchère, F., Berghmans, D., et al. 2020, A&A, 642, A8

Rodríguez-Pacheco, J., Wimmer-Schweingruber, R. F., Mason, G. M., et al. 2020, A&A, 642, A7

Rouillard, A. P., Pinto, R. F., Vourlidas, A., et al. 2020, A&A, 642, A2

Schatten, K. H., Wilcox, J. M., & Ness, N. F. 1969, SoPh, 6, 442

Schwadron, N. A., Joyce, C. J., Aly, A., et al. 2021, A&A, 650, A24

Stansby, D., Yeates, A., & Badman, S. 2020, JOSS, 5, 2732

The SunPy Community, Barnes, W. T., Bobra, M. G., et al. 2020, ApJ, 890, 68

Wang, L., Lin, R. P., Krucker, S., & Mason, G. M. 2012, ApJ, 759, 69

Wang, Y. M., Pick, M., & Mason, G. M. 2006, ApJ, 639, 495

Wang, Y. M., & Sheeley, N. R. J 1992, ApJ, 392, 310

Wiedenbeck, M. E., Mason, G. M., Cohen, C. M. S., et al. 2013, ApJ, 762, 54

Wimmer-Schweingruber, R. F., Berger, L., Kollhoff, A., et al. 2023, A&A, in press

Wimmer-Schweingruber, R. F., Janitzek, N. P., Pacheco, D., et al. 2021, A&A, 656, A22

Worden, J., & Harvey, J. 2000, SoPh, 195, 247

Wuelser, J.-P., Lemen, J. R., Tarbell, T. D., et al. 2004, Proc. SPIE, 5171, 111
funded by ESA. The SIS instrument was constructed by the JHU/Applied Physics Lab, with assistance from CAU Kiel. A.K., G.M.M., G.C.H., and R.C.A acknowledge financial support from NASA NNN06AA01C (SO-SIS Phase-E) contract. R.F.W.-S. acknowledges support by the German Space Agency (DLR) under contract 50OT2002. For the production of the bottom panel of Figure 1, we used a modification of the ept_orbit plot.py script provided by Freiherr von Forstner in github. We are thankful for this script being made publicly available to the community under the GNU General Public License v3.0. This research used version 4.1 of the SunPy (The SunPy Community et al. 2020) and version 1.1.2 of the pfsspy (Stansby et al. 2020) open-source software packages.

## ORCID iDs

A. Kouloumvakos https://orcid.org/0000-0001-6589-4509

G. M. Mason https://orcid.org/0000-0003-2169-9618

G. C. Ho https://orcid.org/0000-0003-1093-2066

R. C. Allen https://orcid.org/0000-0003-2079-5683

Robert F. Wimmer-Schweingruber https://orcid.org/0000-0002-7388-173X

## References

Arge, C. N., Henney, C. J., Koller, J., et al. 2010, in AIP Conf. Proc. 1216, Twelfth International Solar Wind Conference, ed. M. Maksimovic et al. (Melville, NY: AIP), 343

Bučík, R., Innes, D. E., Chen, N. H., et al. 2015, JPhCS, 642, 012002

Bučík, R., Innes, D. E., Mall, U., et al. 2014, ApJ, 786, 71

Bučík, R., Innes, D. E., Mason, G. M., et al. 2018, ApJ, 852, 76

Bučík, R., Innes, D. E., Mason, G. M., & Wiedenbeck, M. E. 2016, ApJ, 833, 63

Bučík, R., Mason, G. M., Gómez-Herrero, R., et al. 2021, A&A, 656, L11

Chen, N.-h., Bučík, R., Innes, D. E., & Mason, G. M. 2015, A&A, 580, A16

Ho, G. C., Mason, G. M., Allen, R. C., et al. 2022, FrASS, 9, 939799

Ho, G. C., Roelof, E. C., Hawkins, S. E. I., et al. 2001, ApJ, 552, 863

Howard, R. A., Moses, J. D., Vourlidas, A., et al. 2008, SSRv, 136, 67

Jebaraj, I. C., Kouloumvakos, A., Dresing, N., et al. 2023, A&A, 675, A27

Kaiser, M. L., Kucera, T. A., Davila, J. M., et al. 2008, SSRv, 136, 5

Kocharov, L., Laivola, J., Mason, G. M., Didkovsky, L., & Judge, D. L. 2008, ApJS, 176, 497

Kollhoff, A., Kouloumvakos, A., Lario, D., et al. 2021, A&A, 656, A20

Kouloumvakos, A., Kwon, R. Y., Rodríguez-García, L., et al. 2022, A&A, 660, A84

Lario, D., Wijsen, N., Kwon, R. Y., et al. 2022, ApJ, 934, 55

Lemen, J. R., Title, A. M., Akin, D. J., et al. 2012, SoPh, 275, 17

Liewer, P. C., Neugebauer, M., & Zurbuchen, T. 2004, SoPh, 223, 209

Mason, G. M. 2007, SSRv, 130, 231

Mason, G. M., Gold, R. E., Krimigis, S. M., et al. 1998, SSRv, 86, 409

Mason, G. M., Ho, G. C., Allen, R. C., et al. 2023a, A&A, 673, L12

Mason, G. M., Nitta, N. V., Bučík, R., et al. 2023b, A&A, 669, L16

Mazur, J. E., Mason, G. M., Dwyer, J. R., et al. 2000, ApJL, 532, L79

Müller, D., St. Cyr, O. C., Zouganelis, I., et al. 2020, A&A, 642, A1

Nitta, N. V., Reames, D. V., De Rosa, M. L., et al. 2006, ApJ, 650, 438

Pesnell, W. D., Thompson, B. J., & Chamberlin, P. C. 2012, SoPh, 275, 3

Poirier, N., Rouillard, A. P., Kouloumvakos, A., et al. 2021, FrASS, 8, 84

Reames, D. V. 2021, FrASS, 8, 164

Rochus, P., Auchère, F., Berghmans, D., et al. 2020, A&A, 642, A8

Rodríguez-Pacheco, J., Wimmer-Schweingruber, R. F., Mason, G. M., et al. 2020, A&A, 642, A7

Rouillard, A. P., Pinto, R. F., Vourlidas, A., et al. 2020, A&A, 642, A2

Schatten, K. H., Wilcox, J. M., & Ness, N. F. 1969, SoPh, 6, 442

Schwadron, N. A., Joyce, C. J., Aly, A., et al. 2021, A&A, 650, A24

Stansby, D., Yeates, A., & Badman, S. 2020, JOSS, 5, 2732

The SunPy Community, Barnes, W. T., Bobra, M. G., et al. 2020, ApJ, 890, 68

Wang, L., Lin, R. P., Krucker, S., & Mason, G. M. 2012, ApJ, 759, 69

Wang, Y. M., Pick, M., & Mason, G. M. 2006, ApJ, 639, 495

Wang, Y. M., & Sheeley, N. R. J 1992, ApJ, 392, 310

Wiedenbeck, M. E., Mason, G. M., Cohen, C. M. S., et al. 2013, ApJ, 762, 54

Wimmer-Schweingruber, R. F., Berger, L., Kollhoff, A., et al. 2023, A&A, in press

Wimmer-Schweingruber, R. F., Janitzek, N. P., Pacheco, D., et al. 2021, A&A, 656, A22

Worden, J., & Harvey, J. 2000, SoPh, 195, 247

Wuelser, J.-P., Lemen, J. R., Tarbell, T. D., et al. 2004, Proc. SPIE, 5171, 111