PSP/IS$\odot$IS Observation of a Solar Energetic Particle Event Associated With a Streamer Blowout Coronal Mass Ejection During Encounter 6

In this paper we examine a low-energy SEP event observed by IS$\odot$IS's Energetic Particle Instrument-Low (EPI-Lo) inside 0.18 AU on September 30, 2020. This small SEP event has a very interesting time profile and ion composition. Our results show that the maximum energy and peak in intensity is observed mainly along the open radial magnetic field. The event shows velocity dispersion, and strong particle anisotropies are observed throughout the event showing that more particles are streaming outward from the Sun. We do not see a shock in the in-situ plasma or magnetic field data throughout the event. Heavy ions, such as O and Fe were detected in addition to protons and 4He, but without significant enhancements in 3He or energetic electrons. Our analysis shows that this event is associated with a slow streamer-blowout coronal mass ejection (SBO-CME) and the signatures of this small CME event are consistent with those typical of larger CME events. The time-intensity profile of this event shows that PSP encountered the western flank of the SBO-CME. The anisotropic and dispersive nature of this event in a shockless local plasma give indications that these particles are most likely accelerated remotely near the Sun by a weak shock or compression wave ahead of the SBO-CME. This event may represent direct observations of the source of low-energy SEP seed particle population.


INTRODUCTION
Solar energetic particles (SEPs) in the heliosphere are accelerated from a few keV up to GeV energies by at least two mechanisms, namely reconnection (e.g., associated with solar flares) and coronal mass ejection (CME)-driven shocks. Particle populations associated with flares are known as impulsive SEP events, while particle populations accelerated by near-Sun CME-shocks are termed as gradual SEP events, and those associated with local CME-shocks are known as energetic storm particle (ESP) events (for reviews, see e.g., Desai & Giacalone 2016;Vainio & Afanasiev 2018). The characteristics of impulsive and gradual SEP events at ∼1 AU can be found elsewhere (for reviews, see e.g., Reames 1999;Desai & Giacalone 2016;Vainio & Afanasiev 2018) but, to summarize, impul-The September 30, 2020 event as observed by PSP/IS IS 3 sive events are usually less intense, more prompt and shorter-lived SEP fluxes than gradual events. Impulsive SEP events are typically electron rich, show enhancements in 3He/4He up to about 1000 times greater than coronal values, are associated with type III radio bursts, and have high charge states of heavy ions (for reviews, see e.g., Reames 1999;Mason 2007;Desai & Giacalone 2016;Vainio & Afanasiev 2018;Bučík 2020). Gradual SEP events are associated with CMEs and CME-driven shocks and are often accompanied by type II radio bursts. They are more intense than impulsive events and last longer, and their composition is similar to that of the solar corona.
The most extensive and detailed observations of SEPs have been made from spacecraft near 1 AU.
Because of their low intensities and large observation distances (1 AU and beyond), the energetic particle environment of the quiet Sun, which is crucial for our understanding of the solar corona and solar wind, is not well understood. The quiet Sun is a solar region without sunspot-bearing active regions (Bellot Rubio & Orozco Suárez 2019) and includes solar features at granular (smallest so far resolvable magnetic features by current instruments, Ishikawa et al. 2008) and supergranular scales, which includes internetwork (Livingston & Harvey 1975), network field structures (Sheeley 1967), and other small-scale active regions, such as coronal bright points (CBPs; Harvey et al. 1975).
The connection between small-scale (quiet-Sun) and large-scale (active-region) magnetic structures as well as their generation mechanisms are topics of active research. Quiet-Sun magnetic field elements make a dominant contribution to the total magnetic field on the solar surface (Getachew et al. 2019a,b;Mursula et al. 2021), and may store and transfer huge amounts of energy to the upper atmosphere through different mechanisms. A CME can erupt from the quiet Sun, where the field is weak and no large filament or active region needs to be present in the pre-CME corona to initiate an eruption (e.g., Robbrecht et al. 2009;Podladchikova et al. 2010;Vourlidas & Webb 2018).
Streamer-blowout coronal mass ejections (SBO-CMEs) are one of the manifestations of the quiet-Sun magnetic field. SBO-CMEs are usually slow (with an average speed of about 390 km/s) and originate in the solar streamer belt. They are commonly characterized by a gradual swelling of the overlying streamer over a period of a few hours to a few days, followed by emergence of a bright and well-structured flux rope, and a generally slow CME from the streamer that leaves behind a depleted corona (Illing & Hundhausen 1986;Sheeley et al. 1982). Although SBO-CMEs show some variation with solar cycle, they do not follow the sunspot cycle, implying that they are not associated with active regions but originate in the quiet Sun. Their average duration, from the start of the streamer swelling to the release of the CME, is about 40 hours (Vourlidas & Webb 2018). SBO-CMEs are observed only in streamer belts, and their locations follow the global dipolar field (the tilt of the global heliospheric current sheet). SBO-CMEs are typically stealth CMEs (see Robbrecht et al. 2009, who first reported the eruption of a CME that left "no trace behind"), but in-situ signatures during their passage over a spacecraft do not differ significantly from those typical of interplanetary CMEs (see Möstl et al. 2009;Lynch et al. 2010;Nieves-Chinchilla et al. 2011, who analyzed the Robbrecht et al. event at 1 AU). Lynch et al. (2016) suggested that SBO-CMEs are formed along the polarity inversion line below the streamer belt when magnetic energy accumulated by solar differential rotation is released via reconnection.
As the perihelion of the orbit of the Parker Solar Probe (PSP; Fox et al. 2016) spacecraft approaches closer and closer to the Sun (perihelion from 35 solar radii (R ) for the first orbit to <10 R for the final three orbits), we are able to obtain in-situ measurements of solar output of plasma, energetic particles, and electromagnetic fields of the near-Sun environment that had not been previously explored Kasper et al. 2019;McComas et al. 2019;Howard et al. 2019 Giacalone et al. 2020;Hill et al. 2020;Schwadron et al. 2020;Joyce et al. 2021a,b; The September 30, 2020 event as observed by PSP/IS IS 5 2020a,b). Recently, Joyce et al. (2021b) studied the radial evolution of energetic particles using PSP/IS IS and Solar Terrestrial Relations Observatory Ahead (STEREO-A; Kaiser et al. 2008) spacecraft data, and showed that the properties of energetic particles observed at PSP's orbit and 1 AU are quite different, indicating that transport effects acted on the energetic particle populations and affected their properties in transit between the two spacecraft.
In this paper, we study the weak, low-energy SEP event of September 30, 2020 using PSP data from its sixth solar encounter around perihelion, and investigate the possible sources associated with this event. Our results show that the September 30, 2020 SEP event is weak, dispersive, and anisotropic and is associated with a slow SBO-CME observed by the coronagraph onboard STEREO-A off the solar east limb. The paper is organized as follows: Section 2 presents observations based on remote solar data. Section 3 focuses on the PSP/IS IS in-situ observations. Finally, we discuss the results and present our conclusions in Section 4.

REMOTE SOLAR OBSERVATIONS
STEREO-A observed a slow and narrow CME on September 29, 2020 ejected from the eastern limb of the Sun, i.e., in close proximity to PSP's longitudinal location. Figure 1 shows the position of the planets and spacecraft within ∼1 AU from the Sun together with the weak CME as it passes PSP on September 30, 2020 at 16:00 UT. The figure is generated using simulation results from the WSA-ENLIL+Cone model (Odstrcil 2003;Arge et al. 2004). The different panels in the figure show results for the solar wind density in the ecliptic (out to 1 AU and zoomed-in to 0.3 AU), meridional, and radial planes. CME input parameters for the simulation were obtained via application of the  Domingo et al. 1995, located near Earth). Figure 1 shows that PSP is magnetically connected to the CME, which grazes the spacecraft from its western 6 Getachew et al.

flank.
Figure 2 shows STEREO-A COR2 (upper panels) and the corresponding running-difference (COR2/RD, bottom panels) images taken on September 29, 2020 at 05:06, 10:06, 15:06, and 20:06 UT (from left to right). As can be seen from Figure 2, at the start of the event, a streamer off the solar eastern limb is seen to brighten and swell prior to the CME eruption (better seen in the associated movie). Subsequently, the CME is released slowly into the outer corona, followed by plasma outflows that also last for many hours. The CME leaves behind a depleted streamer, which is consistent with the properties of a typical streamer-blowout type CME (Vourlidas & Webb 2018;Korreck et al. 2020;Liewer et al. 2021). Figure 3 shows remote-sensing observations in extreme ultra-violet (EUV) of the September 29, 2020 SBO-CME taken by the SECCHI Extreme UltraViolet Imager (EUVI) instrument on board the STEREO-A spacecraft in the 171Å (upper panels) and 195Å (bottom panels) channels. As can be seen from Figure 3, a flux rope-like structure (marked by the white arrows) slowly lifts off the northeastern limb (better seen in 171Å) starting on September 27, 2020 around 18:00 UT, and is seen to deflect gradually towards the equator and the heliospheric current sheet. This structure moves in a "rolling" fashion (most evident in the associated movie), which is a common characteristics for slow, quiet-Sun eruptions during solar minimum (e.g., Panasenco et al. 2013). The CME started leaving the Sun already on late September 27, 2020, and completely left the EUVI field of view about 2 days later, which is a typical time-frame for SBO-CMEs (e.g., Vourlidas & Webb 2018;Liewer et al. 2021;Palmerio et al. 2021). We note that the strong southward deflection observed in EUVI imagery is not reflected in COR2 data (Figure 2), where the CME is seen to propagate almost radially along the direction of the overlying coronal streamer. This is consistent with previous findings, which showed that the most dramatic CME deflections and rotations tend to occur below ∼5 R due to magnetic forces in the low corona (e.g., Kay & Opher 2015).
The September 30, 2020 event as observed by PSP/IS IS   corresponding energetic particle measurements. The EPI-Hi LET1 A count rates (counts/sec) of protons and EPI-Lo count rates (counts/sec) of ions are shown on the outside and inside of the orbit, respectively. Color intensifications and taller bars indicate the occurrence of energetic particle events.
The event analyzed in this paper is highlighted with an orange circle. As can be seen in Figure 4, this event is detected by the EPI-Lo instrument, but is too small to extend into the EPI-Hi energy range ( 1 MeV). The orbit 6 period is within the solar minimum phase and therefore populated with The September 30, 2020 event as observed by PSP/IS IS 11 rather quiet energetic particle conditions. This gives a good opportunity to study the near-quiet-Sun energetic particle populations.
3.2. The September 30, 2020 Event  A dispersive event indicates that the source of the energetic particles is remote, instead of being local. Figure 5 shows that the energy of ions starts to increase up to a couple hundred keV at about 02:00 on September 30, 2020 (DOY 274-2020) when solar wind speed, density, and magnetic field (particularly in the T-component) properties start to change, indicating the passage of a new field structure.
The strongest intensity for this event is observed in the time interval between 02:00 and 08:50 UT on September 30 (bounded by two green vertical lines) and shows a flat (constant) intensity-time profile throughout the interval, which is slightly different from the rest of the event. The flat intensity-time profile that persisted for about 8 hours suggests a constant acceleration of particles of that energy (Reames 1999) and prolonged magnetic connectivity. The plasma and magnetic field data show that this enhancement (peak) in energetic particles seem to be associated with extremely slow solar wind speed and open magnetic field structure (indicated by the unidirectional strahl electron flow) and that the enhancement is observed before the predicted arrival time of the CME (dashed vertical  (Gosling et al. 1987).
A dropout in energetic particles is observed in the time interval between 11:00 and 16:00 UT. The dropout is clearly seen in both the spectrogram and intensity plots shown in panels (b) and (c) of Figure 5. This dropout coincides with the CME arrival time as predicted by WSA-Enlil (vertical dashed red line), a bidirectional flow of strahl electrons, and a somewhat smooth rotation of the magnetic field vectors. The solar wind speed features an increasing profile and the plasma density is rather high, indicating a structure that is being compressed from behind by a faster solar wind flow (Pizzo 1978;Gosling 1996). Note that a bidirectional flow of strahl electrons (Gosling et al. 1987) and a smooth rotation of the magnetic field components (Burlaga et al. 1981) are commonly used as indicators of a CME magnetic structure, which we identify to encounter PSP between ∼11 and 16 UT on September 30, 2020 (solid vertical red lines in Figure 5). We note that the magnetic field strength (panel (f)) of the identified CME is not significantly higher than its surroundings, as is common in CME ejecta (e.g., Kilpua et al. 2017). However, we consider that (1) the weak, slow CME observed in remote-sensing imagery likely featured no strong, intrinsic magnetic fields, and (2) the trend of the magnetic field components (B T and B N rotate in the first half of the ejecta and stay around zero during the second half, while B R rotates from negative to positive) are suggestive of a flank/leg encounter, in agreement with the WSA-Enlil simulation results (Figure 1). The ion dropout during the CME arrival time is not surprising since energetic particles are often suppressed within CME magnetic clouds (Forbush 1937;Cane 2000).
We do not find signatures of a local shock in the plasma or magnetic field data shown in Figures   5 and 6. The absence of a local shock may indicate that these particles were remotely accelerated somewhere near the Sun. 150 kev/nuc protons can travel the spiral-distance between the solar low corona and PSP (∼0.14 AU) in shortly over a hour, hence they would have departed around 18:30 UT on September 30, 2020, when the CME was already well into the outer corona (see its location in the corona a day earlier in Figure 2). As discussed in Section 2, PSP was well connected to the CME since it encountered the eruption in situ, but such encounter was determined to be a flank one through both WAS-Enlil simulation results ( Figure 1) and in-situ observations ( Figure 6).
Hence, it is possible that particles traveled toward PSP once the western flank of the CME became magnetically connected to it as a consequence of expansion. Hence, it is possible that the CME initially drove a (weak) shock near the Sun. The profile of the SEP event studied here is consistent with that of magnetic connectivity achieved from the west of the observer (see Figure 3.4 in Reames 1999), in agreement with our interpretation mentioned above. In this case, the strongest acceleration is assumed to occur near the central region of the shock, where the shock is presumably strongest and the speed is likely to be highest, and tends to decline around on the flanks (see, e.g., Reames The September 30, 2020 event as observed by PSP/IS IS 1999). It is worth noting that the strongest acceleration typically reflects the geometry of the shock and magnetic field, which can also make the flanks important for accelerating particles (see, e.g., Tylka et al. 2005;Zank et al. 2006;Hu et al. 2017Hu et al. , 2018. As can be seen from panel (c) of Figure   5, there is a clear anisotropy of energetic particles, indicating that the energetic particles of this event are propagating predominantly outward from the Sun, which gives additional evidence that the energetic particles are likely accelerated remotely. et al. (2020)) is reproduced and shown in blue line. As can be seen in Figure 7, the shape of the spectrum of this event and the November 11, 2018 event are somewhat similar except that the spectrum of this event is softer than the November 11, 2018 event. Note that the November 11, 2018 event is a CME-related event observed by PSP/IS IS during its first orbit when it was about 0.25 AU from the Sun, where PSP encountered the central region of the CME.

Ion Composition
The September 30, 2020 event as observed by PSP/IS IS 17 SEP ion composition has been used as a good indicator of the origin, acceleration, and transport of SEPs (Reames 2021). Figure 8 shows the ion composition of the September 30, 2020 event.
For reference, panels (a) and (b) reproduce panels (a) and (b) of Figure 5, respectively. Panels (c), (d), (e), and (f) show the spectrograms for protons, helium (4He), oxygen (O), and iron (Fe), respectively, and panel (g) reproduces panel (f) of Figure 5. As can be seen from Figure 8, heavy ions are observed in this event. It is populated with protons, helium, oxygen, and iron. Heavy ions are observed especially in the leading edge of the event (well before the arrival of the SBO-CME).
The dispersive characteristic during the onset of this event (vertical purple line) is also clearly seen in Figure 8. This event has similar ion composition as the CME event of November 11, 2018 studied by Giacalone et al. (2020) and Mitchell et al. (2020a), which was associated with an SBO-CME  with no shock-sheath system identified during the eruption of the CME or in the in-situ plasma data at PSP. We note that during the September 30, 2020 event, 3He and energetic electrons are not observed to be significantly above the EPI-Lo detector background, and also the FIELDS instrument did not detect radio bursts above the FIELDS instrument background.

DISCUSSION AND CONCLUSIONS
In this paper we have studied in detail a low-energy SEP event observed by IS IS/EPI-Lo on September 30, 2020 during the encounter 6 period inside 0.18 AU. The September 30, 2020 event is interesting in that there was no sunspot-bearing active region (at least around the longitudinal position of PSP), which gives an excellent opportunity to study the energetic particle environment of the near-quiet Sun. The event onset occurs at approximately 19:40 UT on September 29, 2020.
The event shows clear velocity dispersion during its onset, with the fastest particles arriving first.
The STEREO/COR2-A coronagraph observed a CME being ejected from the solar eastern limb on September 29, 2020 around 07:00 UT, 12 hrs before the onset of the SEP event at PSP. The CME arrived at PSP (0.17 AU) about a day after the onset of the SEP event and and PSP encountered the CME's western flank. Throughout this SEP event, strong particle anisotropies were observed, showing that more particles are streaming outward from the Sun. This small SEP event has a very interesting time profile. The energetic particle intensity rises gradually from a few to several hours and reaches a plateau later on. The strongest intensity for this event is observed in the time interval between 02:00 and 08:50 UT on September 30, 2020, which is well before the CME passage. It is quite interesting that at the start of the plateau (first green vertical line in Figures 5), the magnetic field and plasma properties start to change, indicating that PSP crossed a new field structure. It is possible that this new field structure is channeling energetic particles from a remote acceleration site, which most likely happened at the front of the CME. The in-situ magnetic field and strahl electron PAD indicate that these strongest energetic particle intensities are observed in the open magnetic field structure prior to the CME arrival, and there is a dropout both in intensity and number of relatively high-energy particles inside the CME structure as shown in Figures   5 and 6. As noted earlier, PSP crosses the western flank of the CME during its passage, and the time profile of this event is consistent with a western flank encounter (see Figure 3.4 in Reames 1999).
The ion composition plot shown in Figure 8 depicts that some heavy ions such as oxygen and iron are observed, in addition to protons and helium-4, but no significant enhancement of helium-3 and energetic electrons, giving evidence that the source of this event is unlikely to be a solar flare.
Therefore, this small SEP event is not a typical small impulsive event, rather, it seems to be a small gradual SEP event. In fact, both remote-sensing (as discussed in Section 2) and in-situ (as discussed in Section 3.2) data show that this event is associated with a slow CME. Remote solar data observed using STEREO/COR2-A show that the source of this event is most likely an SBO-CME on September 29, 2020. The in-situ magnetic field and strahl electron PAD data show that the magnetic field is open and mostly radial prior to the arrival of the SBO-CME but closed upon arrival. As shown in Figures 5 and 6, throughout the event, the strahl electron flow is unidirectional except in the time interval between 11:00 and 16:00 on 274-2020 (DOY-year), where the flow is bidirectional indicating a closed magnetic field structure. This closed structure observed during the period of bidirectional electrons coincides with the SBO-CME arrival time. During its earlier orbits, PSP encountered SBO-CMEs Lario et al. 2020;Nieves-Chinchilla et al. 2020) and SEP enhancements associated with SBO-CMEs were also observed (Giacalone et al. 2020;Lario et al. 2020;Mitchell et al. 2020a), consistent with our observation.
We do not see a local shock in the in-situ plasma or magnetic field data throughout the event. An anisotropic and dispersive SEP event in a shockless local plasma give evidence that the dominant sources of particles were remote rather than local, consistent with earlier observations (see, e.g., Giacalone et al. 2020;Mitchell et al. 2020a;Joyce et al. 2021b). There are several candidates as a possible source of remote acceleration mechanism of these energetic particles. One possible mechanism may be that particle acceleration occurring either at plasma compressions formed in front of the propagating CME or at a weak shock initially driven by the CME that was not detected at its arrival at PSP (Giacalone et al. 2020). Another candidate as a possible source of remote acceleration mechanism of these energetic particles is the one proposed by Mitchell et al. (2020a), which is associated with strong field-aligned current system that run in the solar atmosphere (Janvier et al. 2014). These energetic particles could be also accelerated due to magnetic-island reconnection-related processes (Zank et al. 2014(Zank et al. , 2015Zhao et al. 2018Zhao et al. , 2019a. In this scenario, the magnetic islands will be located closer to the Sun than the location where the energetic particles are observed. However, the intensity-time profile of this event gives indications that the particles are likely accelerated by a weak shock or compression driven by the CME near the Sun in agreement with earlier observations (Giacalone et al. 2020). However, it should be noted here that the SEP profile of this event is consistent with a western flank connectivity, as noted above, and the event-averaged spectrum is an indicative of a weaker event, while the event studied by (Giacalone et al. 2020) was consistent with central connectivity. During this event, PSP was not radially aligned with other spacecraft and thus we could not compare this event with other energetic particle observations. It is quite interesting to see that the in-situ signatures of this small gradual SEP event (associated with a weak CME) show similar properties to the well-established signatures of larger CME events, which are often associated with active regions containing sunspots, giving evidence that small SEP events may be generated by similar mechanisms as in large SEP events regardless of the size and strength of the CME. Small gradual SEP events are not totally absent at 1 AU ( see, e.g., Reames 2020; Joyce et al. 2021b), but they are rare (particularly, those with energies below 1 MeV) due The September 30, 2020 event as observed by PSP/IS IS 21 to a variety of reasons. One reason may be that they are just too small and spread out. Another possible reason may be that these are a feature of SBO-CMEs in agreement with our analysis. This event may represent direct observations of the source of low-energy SEP seed particle populations and provides a unique opportunity to show how particles evolve in the near quiet-Sun environment and also to further investigate the connection between small gradual SEP events and SBO-CMEs.
This work highlights the importance of small SEP events in understanding the solar activity of the near-Sun environment that had not been previously explored.