Recurrent 3 He-rich solar energetic particle injections observed by Solar Orbiter at ∼ 0.5 au

We report Solar Orbiter observations of six recurrent solar energetic particle injections in 2022 March 3 − 6 at ∼ 0.5au. All but one were associated with jets emanating from a plage near a large sunspot in active region 12957. We saw large jets in injections with high 3 He and Fe enrichments and minor jets in injections with no or lower enrichments. Furthermore, the event with the highest enrichment showed a more compact conﬁguration of the underlying photospheric magnetic ﬁeld. The higher ﬂuences as well as harder spectra were seen in the event with a simultaneous jet and wider eruption. However, in this case, the energy buildup time in the source might be required to produce such spectra. Extreme-ultraviolet images from Solar Orbiter revealed a number of intersecting loops at the base of jets not seen from 1au that might be a precondition for the recurrent events.


Introduction
Impulsive or 3 He-rich solar energetic particle (SEP) events, characterized by 3 He and ultra-heavy ion abundances enhanced up to a factor of 10 4 above the coronal values (e.g., Mason 2007;Reames 2013), are the most unusual particle population in the Solar System. The events show high association with type III radio bursts (e.g., Nitta et al. 2006) and are accompanied by minor (B-or C-class) GOES soft X-ray flares (e.g., Nitta et al. 2006Nitta et al. , 2015. Solar sources of 3 He-rich SEPs are coronal jets, but sometimes a wider eruption or just a brightening is observed (e.g., Wang et al. 2006;Nitta et al. 2015;Bučík 2020).
There are reports on 3 He-rich SEP events measured in clusters from subflares in single active regions (Reames et al. 1985(Reames et al. , 1988Mazur et al. 1996;Mason et al. 1999Mason et al. , 2000Bučík et al. 2021;Ho et al. 2022) where abundance showed significant variations (Reames et al. 1988;Ho et al. 2022). Imaging observations revealed that sources of these recurrent 3 He-rich injections are jets from solar plages (patches of scattered magnetic fields) or coronal hole edges (Wang et al. 2006;Bučík et al. 2014; Movies associated to Fig. 4 are available at https://www.aanda.org Chen et al. 2015). The works on recurrent events were from both single and angularly separated spacecraft at ∼1 au.
Driving mechanisms of repeated injections from a single source and their spectral and abundance variations on short timescales is paramount for understanding the particle acceleration in 3 He-rich SEP events. Measurements at close distances to the Sun, where ions would be less affected by interplanetary propagation effects, are the most appropriate to explore the variability in recurrent events. In this Letter, we report a period of recurrent 3 He-rich events observed by Solar Orbiter (Müller et al. 2020) at ∼0.5 au on 2022 March 3−6. We examine the activity in the solar source to understand vastly different abundances in observed ion injections. For the first time, we can explore imaging observations of a 3 He-rich SEP source from an unprecedented close distance to the Sun at ∼0.5 au.   Maksimovic et al. 2020), which showed an enhanced level of interference at higher frequencies (>2.5 MHz), we employed data from the STEREO-A Waves instrument (Bougeret et al. 2008) that covers a frequency range of <16 MHz. clearly recognized in the inverted ion-speed plot (bottom panel of Fig. 1). The injections are approximately marked by slanted dashed red lines. They were obtained using the fitting method described in Hart et al. (in prep.) for injections 2−5. This method bins pulse-height analysis data into 5-min bins and varies the energy bin sizes and centers. Each energy bin is fit to an analytical Heaviside step function multiplied by an exponential decay. The maximum of the second derivative of the resulting fit determines the onset time of the particles within the specified energy range. The energy bins are varied, and the fitted injection that minimizes chi-squared is chosen. Vertical dashed black lines mark onsets of type III radio bursts that correspond to these ion injections. Time-intensity profiles at the 0.23−0.32 MeV nucleon −1 (top panel of Fig. 1) and the mass spectrogram at the 0.4−10 MeV nucleon −1 (middle panel of Fig. 1) show no 3 He during injections #1 and #3, but a high amount of 3 He in injections #2, #5, and #6. We note that event #1 starts at an energy below the ∼0.1 MeV nucleon −1 , and this is below the threshold for 3 He detection, so SIS does not show 3 He even if it was there earlier in the event (before Solar Orbiter connected to the source). The time-intensity profiles show a high abundance of 3 He relative to 4 He and Fe relative to O for injection #2, where very unusually the 3 He intensity curve lies on the H curve and Fe on 4 He. Furthermore, 3 He might even exceed H intensity that, unlike heavy ions, is enhanced before event #2. Injections #5 and #6 show similar 3 He abundances, which are higher than the 3 He abundance in injection #4. The Fe abundances are similar in injections #4 and #6, and they are smaller than the Fe abundance in injection #5 as indicated by the time-intensity profiles in Fig. 1. The highest 3 He and Fe relative abundances are in injection #2 (see also Table 1). Its 3 Heand Fe-rich enhancements are the most extreme of any event so far detected by Solar Orbiter (cf. Mason et al. 2021;Bučík et al. 2021;Ho et al. 2022). Figure 2 shows energy spectra for the most notable injections #2 and #5. To include particles from a given injection, the energy spectra are integrated into "smoosh" boxes. For injection #2, the smoosh box is between the second slanted dashed line and March 4 00:08 UT and for injection #5 between the fifth and sixth slanted dashed lines. The O and Fe spectra in injection #2 are markedly rounded toward the low (below the ∼200 keV nucleon −1 ) energies, which is probably related to a sudden "cut" near the end of March 3 (bottom panel of Fig. 1). The O and Fe spectra in injection #5 are similar to double power laws. The 3 He spectra are also curved. The 4 He and H spectra are similar to power laws, though the 4 He spectrum in injection #5 is somehow rounded at low energies. Such spectra have been previously reported from 1 au measurements of 3 He-rich SEPs (e.g., Mason et al. 2000Mason et al. , 2002. There are also measurements L5, page 2 of 7 The arrows indicate the longitudes of the solar source at onsets of type III radio bursts associated with injections #2 and #5. Solid and dotted Parker IMF lines are at onsets of type III radio bursts in injections #2 and #5, respectively. The Parker IMF lines were determined from measured 1 h averaged solar wind speeds. It is important to note, however, that for injection #2, the PSP solar wind speed is not available, and the IMF line is for a nominal solar wind speed of 350 km s −1 .

Energetic ion observations
from 1 au where spectra of all species are power laws or double power laws (e.g., Mason et al. 2000Mason et al. , 2002, which is not the case in these spectra. It has been discussed that similar powerlaw spectra for all species can be the result of the propagation of particles from the Sun to 1 au (Mason et al. 2002). To explore this further, energy spectra from other Solar Orbiter perihelia need to be obtained. We note that 3 He, 4 He, O, and Fe spectra in injection #2 fall more steeply with energy than in injection #5. Furthermore, 3 He fluences in injection #5 were higher than in the previous recurrent injection #2 over the measured energy range. Thus, the higher 3 He/ 4 He ratio in #2 compared to #5 was due to small 4 He fluences in injection #2. The spectra also show that 3 He/ 4 He increases with energy in injection #5 but remains constant over the energy range in injection #2. We note event #5 is relatively rich in Sulfur, though not as much as some rare cases reported previously (Mason et al. 2002). Figure 3 plots the locations of selected spacecraft in the heliocentric Earth ecliptic coordinates. Solar Orbiter and STEREO-A were well connected to the source along Parker spiral interplanetary magnetic field (IMF) lines in recurrent injections #2 and #5. Near-Earth spacecraft (e.g., ACE) were well connected only to source in injection #5. PSP was not connected to the source of these injections. For clarity, the connections for other injections are not plotted. Although STEREO-A was well connected to the source during injections #2 and #5, ions were almost not detected on the spacecraft. Nominally, SIT had a sunward viewing direction close to the average Parker IMF line; however, after solar conjunction in 2015, STEREO spacecraft were rolled 180 • about the spacecraft-Sun line. Consequently, the SIT field of view (FOV) is perpendicular to the Parker IMF line. Since SIS measured ∼10:1 forward to backward anisotropies (not shown), the pointing of STEREO-A probably is the reason the intensities there are so low. ULEIS aboard ACE points to the sunward hemisphere at an angle of 60 • to the spacecraft-Sun line. It measured the 3 He event during injection #5 (see Fig. A.1) when ACE was well connected to the source. We found that SIS ion fluences in injection #5 are about 20× higher than ULEIS. The Solar Orbiter distance of 0.51 au accounts only roughly for a factor of 4, implying that other effects, such as details of magnetic connection, must play a significant role. The SIS abundance ratios match the ULEIS ratios closer (see Table 1). IS IS EPI-Lo PSP data show no intensity enhancement during injection #5. The data are not available for injection #2. Table 1 summarizes the characteristics of the recurrent injections. Type III radio bursts for injections #1 and #2 were observed by San Vito Solar Observatory (SVI) at 25−180 MHz (reported in the US National Oceanic and Atmospheric Administration (NOAA) Edited Events catalog). Type III radio bursts with an onset on March 5 at 19:32 UT (injection #4) and March 6 at 08:25 UT (injection #6) were not observed above ∼1−2 MHz (see right panels in Figs. B.2 and B.3). Injection #6 is probably associated with two type III radio bursts separated by ∼25 min (see Fig. B.3). The NOAA catalog lists type VI bursts on March 6 at 25−139 MHz from SVI from 08:21 to 09:04 UT. The event-associated jets and brightening occurred in the NOAA active region (AR) 12957. Injections #1, #2, and #4 show no evidence of X-ray flux enhancement; the values in Table 1 indicate levels of the background. The footpoint longitude of Solar Orbiter was obtained from a simple Parker IMF approximation using a measured solar wind speed. Employing a potential field source surface (PFSS) model, we found that the open coronal field lines (negative polarity) emanating from the AR 12957 connect to the Solar Orbiter footpoint for all injections (see also magnetic connection tool 2 , Rouillard et al. 2020). Figure 4 displays radio, soft X-ray, and EUV imaging observations of the solar source for recurrent injections #2 and #5. The figures for the remaining injections are shown in Appendix B. The running difference images in Fig. 4 (middle panels) show that injection #2 was associated with a straight long jet and injection #5 with a complex emission. The emission in injection #5 contains two simultaneous eruptions, one jet-like in the south and the other broader in the north. When approximated by ellipses, the source area of the jet in event #2 is ∼280 Mm 2 , and the eruption in event #5 is ∼490 Mm 2 . The images for injection #2 are shown at the time of 211 Å intensity maximum, and for injection #5 the images are shown three minutes after the intensity maximum when the jet-like ejection becomes clearer. Injection #1 was associated with small brightening and the remaining injections were with tiny jets. The composite images of AIA 211 Å and HMI line-of-sight magnetic field in Fig. 4 (bottom panels) and Figs. B.2 and B.3 indicate that jets in all injections originated in the plage, located westward from the two big sunspots of opposite polarities. The jets' ejection started in the region between positive (white) polarity (marked by P) and negative (black) polarity that is westward (marked by N1) of the positive polarity P. There is also negative polarity eastward (marked by N2) of the positive polarity P. Interestingly, these negative polarity areas are compact in injection #2 and more dispersed in the remaining injections. The brightening associated with injection #1 arose from a positive polarity fleck within the negative polarity site near the periphery of the negative polarity sunspot.  The source in event #2 shows two small bright loops and the corresponding jet spires (see images and animation in Fig. 4, left), resembling standard interchange reconnection jet (Shibata et al. 1992). In line with the reconnection model, these loops, connecting the positive-polarity area (P) and the negativepolarity area in the west (N1), would reconnect with seemingly open field lines emanating from the negative-polarity area in the east (N2). The eruptions in event #5 contain a minifilament that formed between positive polarity region P and negative polarity region N1. It is seen in the animation of Fig. 4 (right) as a dark S-shaped form on 2022 March 5 at 23:51−23:56 UT. The eruption of the minifilament as a trigger of coronal jets in active regions has been reported in previous studies (e.g., Sterling et al. 2016Sterling et al. , 2017. Figure 5 shows SDO AIA (top panel) and Solar Orbiter EUI HRI_EUV (bottom panel) EUV images of the area where recur-rent injections originated. The high-resolution EUI HRI_EUV image is shown at the time closest to the recurrent injection #2 (∼3.5 h before the injection). We lack high-resolution EUI observations at times of the injections' associated type III radio bursts. To account for light travel time (220 s) between Solar Orbiter at 0.55 au and SDO at 0.99 au, the AIA image is shown at a later time to compare with the EUI image. The Solar Orbiter image appears more structured than the SDO image which is mainly due to its closer heliocentric distance to the Sun. We note that the angular pixel size of EUI HRI_EUV and AIA are comparable (see Sect. 2). At 0.55 au, Solar Orbiter provides a factor of 2.2 better resolution than SDO. Although both the EUI 174 Å and AIA 171 Å wavelength bands observe low corona, they peak at slightly different temperatures: 174 Å near 1 MK and 171 Å near 0.8 MK. The source of ion injections is different from the neighboring region in the east where tall, closed loops connect with L5, page 4 of 7 sunspots of opposite polarities. EUI on Solar Orbiter observed a substantial number of intersecting loops in the source of 3 Herich SEPs. Such a fine structure is not seen in the SDO image.

Summary
We examined repeated ion injections from a single source, measured by Solar Orbiter in 2022 March 3−6, when the spacecraft was at 0.51−0.55 au from the Sun. Six injections were identified within a ∼3-day period.
SDO EUV imaging observations revealed a straight, large jet in the injection (#2) with the highest 3 He and Fe enrichments. Smaller or no 3 He and Fe enrichments were measured in injections (#1, #3, #4, and #6) with brightening and minor jets. The most intense injection (#5) that still shows relatively high 3 He enrichment was associated with the complex eruption with the source size almost twice the source of the jet in injection #2. It is consistent with the suggestion of Ho et al. (2005) that observed fluences depend on the size of the acceleration region. The energy spectrum in injection #5 was harder than the spectrum in #2. It may indicate that a time (∼2.5 days in this case) is required for the magnetic energy buildup process to produce more energetic ions in the follow-up injection.
SDO HMI line-of-sight magnetograms indicated that the source of the jets in 3 He recurrent injections was a plage region. Injection #2, with the highest enrichments, shows compact areas of negative polarities; whereas, in other recurrent injections, the areas are more dispersed. This suggests that magnetic flux density might be related to high enrichment.
There is likely no single factor that affects the observed abundance and spectral variability in recurrent injections. We saw that these properties vary with the size and shape of the associated jets (i.e., acceleration regions) and the distribution of the photospheric field. Solar Orbiter EUI, providing high-resolution EUV imaging observations from a closer distance (∼0.55 au) to the Sun, disclosed features that were not seen from 1 au. Specifically, the 3 He-rich SEPs' source showed a network of crisscrossing loops that might be prone to repeated reconnection. This network may also correspond to the simulated turbulent magnetic structures arising intrinsically from magnetic reconnection in the corona (Daughton et al. 2011) that enhance particle acceleration in impulsive flares (Dahlin et al. 2015). Reames et al. (1985) suggested favorable solar escape conditions for such events. We will explore characteristics of recurrent sources further when imaging observations from Solar Orbiter perihelia capturing 3 He-rich jet eruptions become available.  L5, page 7 of 7