The Golden Age of 3He-rich Solar Energetic Particle Events – Improved Insights from the Suprathermal Ion Spectrograph on Solar Orbiter

(Solar Orbiter Nugget #42 by Samuel Hart¹, George C. Ho¹, Athanasios Kouloumvakos², Glenn M. Mason², Radoslav Bučík¹, Robert C. Allen¹)

Introduction

Energetic ions and electrons accelerated at the Sun can be directly injected into interplanetary space through magnetic “interchange” reconnection between a closed coronal loop and an open magnetic field line. Half a century ago [1], these transient events, most often associated with impulsive X-ray flares, were observed to produce extreme enrichments in energetic 3He resulting in their namesake – 3He-rich solar energetic particle (SEP) events. However, greatly enriched 3He is only one of many properties of 3He-rich SEP events, and each property still has numerous outstanding questions. 

The primary in situ observational constraints to improving our understanding of ³He-rich SEP events are single-point measurements and limited statistics. While 1 au is close enough to the Sun such that extreme SEP events pose serious space weather hazards, it is undoubtedly too far to study the less intense 3He-rich solar flares in necessary detail. In many events, the C – Fe abundances above 1 MeV/nuc have large uncertainties or, in smaller events, cannot be measured altogether. Additionally, much like SEP events associated with coronal mass ejections (CMEs) and interplanetary shocks, 3He-rich SEPs are subject to turbulent scattering, cross-field diffusion, and other transport processes as they propagate through the corona and heliosphere, and single-point measurements are unable to constrain their longitudinal and radial evolution. As a result, deconvolving acceleration from transport processes becomes increasingly difficult as SEPs travel greater distances.

This nugget is organized to describe four key results exploiting Solar Orbiter instrumentation to target the problem of SEP acceleration and transport. As we will show in each section, the Suprathermal Ion Spectrograph (SIS) on board Solar Orbiter is uniquely poised to investigate the peculiar properties of ³He-rich SEP events by addressing the aforementioned observational constraints, thereby causing a flurry of new and incredible insights into their origins, acceleration, and journey through the heliosphere.

Ultra-Heavy Ion Observations
 

In addition to the 10,000-fold abundance enhancement of ³He associated with impulsive X-ray flares, the relative abundance of ultra-heavy ions (mass > 78 amu, e.g., Kr, Ag, Xe, Au) relative to O exceeds 100 to 1,000 times that seen in shock-accelerated SEP events [2]. Despite the great enrichment of ultra-heavy (UH) nuclei, it is uncommon that trans-iron elements (Z > 26) are seen in any given ³He-rich event observed at 1 AU due to their extreme rarity. However, with Solar Orbiter observing at radial distances much closer than 1 AU, the flux of heavy and UH nuclei are significantly increased. Figure 1 shows UH observations from a single ³He-rich event observed on November 9, 2022 by Solar Orbiter at 0.59 AU [3] compared with observations from the Advanced Composition Explorer (ACE) consisting of multiple ³He-rich events integrated over 295 days in a 5.5-year period [2]. This single ³He-rich event observed by Solar Orbiter has, with some deviations, reproduced the relative abundances of trans-iron elements that required extensive integration times at 1 AU. As Solar Orbiter continues to observe UH enhancements in 3He-rich events, we can begin to understand how they fit into our current picture of heavy ion acceleration in small SEP events.
 

Figure 1. Mass histogram of mass >= 40 amu ions above 100 keV/nuc for the November 9, 2022 ³He-rich SEP event observed by Solar Orbiter (red) compared with an ACE survey of many ³He-rich events integrated over a  5.5-year time period (blue). With increased trans-iron fluxes seen by Solar Orbiter, we can begin to investigate how their acceleration relates to that of ³He and the other heavy ions (C – Fe) [3].

Longitudinal Extent  
From the perspective of the observing spacecraft, ³He-rich SEP events are commonly associated with bright extreme ultraviolet (EUV) jets associated with emerging magnetic flux in sunspot regions in the western hemisphere [4, 5, 6, 7]. Since ³He-rich SEPs are thought to be accelerated within the reconnection site (i.e., nearly a point source) and only gain access to open magnetic field lines spanning a few 10s of degrees, observations showing ³He-rich SEPs from a single event spanning nearly 180^o in heliospheric longitude [8] is puzzling and remains an open science question. Are these events exceptionally rare and only occur under highly specific conditions, or is there some common process that allows 3He-rich SEPs from point-source flares to be dispersed over a wide array of heliospheric longitudes (reminiscent of the old solar flare paradigm where it was believed all SEPs originated from solar flare processes and the large longitudinal spread was due to unknown SEP transport physics [9])?

Solar Orbiter provides an excellent opportunity to investigate the transport of ³He-rich SEP events through multi-spacecraft analyses and constrain not only their longitudinal spread, but also their solar sources. Using Solar Orbiter, ACE, and the Solar Terrestrial Relations Observatory (STEREO-A), [10] surveys 15 ³He-rich SEP events observed by one or multiple spacecraft and characterize each event’s longitudinal spread. The source of each event was identified and compared with the magnetic connectivity of each individual spacecraft using instantaneous solar wind conditions and the Potential-field Source-surface (PFSS) model [11]. The results of the analysis show multi-spacecraft observations of 3He-rich events is likely to occur when the spacecraft have a central separation angle less than 23^o (see Figure 2). Importantly, beyond a central separation angle of 32^o, no multi-spacecraft ³He-rich events are observed. These results suggest that widespread ³He-rich events, such as those in [8], do not happen under typical solar and heliospheric conditions. 
 

Figure 2. Central separation angle of single (circle) and multi-spacecraft (x) detections of ³He-rich SEP events seen by some combination of Solar Orbiter, STEREO-A, or ACE. The red line shows the probability of a multi-spacecraft detection, with the 50% probability intersecting at 23^o [10]. 

Continuous Emission of 3He-rich SEPs
Solar rotation causes the solar magnetic footpoint location of the Earth to change by about 13 degrees per day. As a result, the quasi-stationary observer at 1 AU can only monitor the same magnetic footpoint for a short time period (roughly two days or less) before connecting to flux tubes emerging from elsewhere on the solar surface. In some cases, the observer can rapidly alternate between multiple flux ropes [12, 13]. However, Solar Orbiter’s increased orbital speed in the inner heliosphere enables it to maintain connection with the same magnetic footpoint for longer periods. In orbits 1 – 5, SIS on board Solar Orbiter observed 33 ³He-rich time periods lasting over 24 hours [13]. Surprisingly, many of these periods persisted for seven days with continuous 3He-rich emissions often from recurrent injections [14, 15, 16]. The number of extended durations appears to increase as solar cycle 25 progresses (see Figure 3). Such time periods are ripe for additional research investigating their respective solar source characteristics in order to understand what source conditions favor repeated injections.
 

Figure 3. ³He and ⁴He intensities during the first five (5) orbits of Solar Orbiter. ³He and ⁴He intensities are shown on the outside and inside of the orbital trajectory, respectively. Orbits 4 and 5 show increased ³He-rich time periods as a result of the increase in solar activity. In some cases, magnetic connectivity is unstable, and Solar Orbiter will rapidly change between multiple footpoints throughout the ³He-rich period [13].

Acceleration of ³He and Heavy Ions
 

One of the most challenging observations to resolve regarding ³He-rich SEP events is the relation between ³He enrichment and heavy ion (Ne – Fe) enrichment. While ³He/⁴He is observed to be enhanced relative to shock-associated SEP events by anywhere between 100 – 10,000 times, Fe/O is also enhanced by approximately 10 times with much less variation [17]. Of course, this led to the belief that the mechanisms enhancing ³He and heavy ions are one in the same. However, the lack of correlation between ³He/⁴He and Fe/O cast doubt on such theories. How can ³He and heavy ions be enhanced by the same mechanisms when their relative enhancements are independent? As a result, numerous models seek to separate the two process [ex., 18, 19]. 
Recent works using SIS on board Solar Orbiter have shed light on this topic. In a survey of 34 isolated ³He-rich injections (i.e., minimal contamination from other SEP sources), it was shown that the energy spectra of ³He and Fe are remarkably consistent with one another when shifted in energy per nucleon by a relatively stable factor of 3.0 +- 1.3 indicating the same primary acceleration process [20]. Figure 4 shows the ³He and Fe energy spectra from all events overplotted to highlight the consistent spectral index and rollover shape. This result provides a major new constraint on future models, as the relative abundances can vary by several orders of magnitude from event-to-event, but the energy scaling factor of approximately 3.0 is far less variable. Continued research expounding on this result and investigating the outliers (a small fraction of events required an energy scaling factor >10) will surely yield more interesting results.
 

Figure 4. Differential energy spectra of ³He (red) and Fe (green) for all 34 surveyed events. The ³He energy has been shifted by a factor of 4 (see top axis) to align the starting point. Both ³He and Fe exhibit consistent spectral shapes across all events suggesting the primary acceleration and release mechanisms are the same. A minority of Fe spectra do exhibit a continued power-law above 1 MeV/nuc, suggesting some other acceleration or transport processes can alter the spectra. However, the majority of events show a clear rollover consistent in both 3He and Fe suggesting the same initial acceleration mechanism [20].

Conclusions
 

³He-rich SEP events are an excellent tool to understand how ions and electrons can be accelerated to >MeV energies in some turbulent reconnection regions. Due to its inner heliospheric orbit, SIS on board Solar Orbiter enables us to observe ³He-rich events with better statistics most notably for heavy and UH ions, and with a broad energy range [3]. From this improvement, a pattern between the energy spectra of ³He and Fe has emerged suggesting a common acceleration mechanism [20]. We have also shown that Solar Orbiter can maintain magnetic connectivity with active regions longer, and some active regions are observed to continually produce ³He enrichments [13]. Finally, multi-spacecraft analysis shows that 3He-rich SEP events are typically spread over 23^o in heliospheric longitude [10]. These four major results highlight the pivotal contributions of Solar Orbiter thus far, and as solar cycle 25 progresses, future observations from within 1 AU will continue to reveal new information about the underlying processes governing the origins, acceleration, and transport of ³He-rich SEP events.

Acknowledgments
Solar Orbiter data analysis at JHU/APL and SwRI is supported by the NASA contract 80MSFC19F0002. We also thank NASA headquarters and Goddard Space Flight Center for their support.
 

Affiliations
[1] Southwest Research Institute, San Antonio, TX, 78238, USA

[2] Johns Hopkins University Applied Physics Laboratory, Laurel, MD, 20723, USA

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