Helium-3 Enrichment in Gradual Solar Energetic Particle Events: Evidence for Jet-Supplied Seed Population

TL;DR

This study addresses why $^{3}$He is enriched in gradual solar energetic particle (GSEP) events by analyzing high-energy ($25$--$50$ MeV) protons observed by SOHO that coincide with $^{3}$He-rich periods from ACE. Using a timing-based refinement and EUV imaging of solar sources, it demonstrates that jet-like activity in active regions supplies a fresh $^{3}$He-rich seed population, which is then re-accelerated by CME-driven shocks, producing the observed enrichments in a cleaned sample of 15 events. The work finds a strong link between jet activity and extreme $^{3}$He/$^{4}$He values, and a positive correlation between $^{3}$He/$^{4}$He and Fe/O that is most pronounced for jet-associated events, supporting a jet-seed plus shock-reacceleration scenario. It also identifies that a substantial fraction (~40%) of apparent $^{3}$He enhancements arise from overlap with impulsive SEP activity, underscoring the mixture of seed sources and the importance of carefully separating ISEP and GSEP contributions for interpretation and modeling.

Abstract

Enhancements in 3He abundance, a characteristic feature of impulsive solar energetic particle (ISEP) events, are also frequently observed in gradual SEP (GSEP) events. Understanding the origin of this enrichment is crucial for identifying the mechanisms behind SEP generation. We investigate the origin of 3He enrichment in high-energy (25-50 MeV) solar proton events observed by SOHO, selecting events that coincide with <1 MeV/nucleon 3He-rich periods detected by ACE during 1997-2021. The identified 3He enhancements include cases where material from independent impulsive (3He-rich) SEP events is mixed with GSEP proton populations. Two high-energy proton events exhibit elemental composition and solar source characteristics consistent with ISEPs. Extreme-ultraviolet imaging from SDO and STEREO reveals narrow, jet-like eruptions in the parent active regions of about 60% of the remaining events. Notably, the highest 3He/4He ratios occur when coronal jets are present, consistent with fresh, jet-driven injection of suprathermal 3He that is subsequently re-accelerated during the event. Correspondingly, jet-associated events show fewer pre-event (residual) 3He counts, indicating that enrichment in these cases does not primarily come from remnant material. We find a positive correlation between 3He/4He and Fe/O, strongest in jet-associated events, consistent with a common jet-supplied seed population re-accelerated by the CME shock.

Figures (13)

• Figure 1: Multi-spacecraft measurements of event #7. (a) Wind/WAVES dynamic radio spectrogram. Color-coding indicates electric field intensity (dB above background). The vertical black arrow marks the type III radio burst associated with the event; the subsequent slow-drifting feature corresponds to a type II burst. The locations of STEREO-A (A), STEREO-B (B) and near-Earth (E) spacecraft (Wind, SOHO) at the time of the type III burst are shown in Heliocentric Earth Ecliptic (HEE) X-Y coordinates next to panel (a). The tilted arrow indicates the flare longitude (W47). (b) Two-hour proton intensities from STEREO-B/HET, STEREO-A/HET, and SOHO/ERNE in the indicated energy ranges. (c) One-hour ion intensities from ACE/ULEIS in 0.23--0.32 MeV nucleon$^{-1}$ energy range. (d) Mass vs. time spectrogram from ACE/ULEIS covering 0.4--10 MeV nucleon$^{-1}$. (e) Inverse ion speed vs. arrival time for ions with mass 10--70 AMU. Slanted lines indicate expected arrival times for scatter-free propagation along a nominal Parker spiral path of 1.2 au. The magenta polygon marks the region, defined in inverse speed-time space over which elemental abundance ratios were calculated.
• Figure 2: ACE/ULEIS 3-hr ${}^3\mathrm{He}$ (2.8--3.1 AMU) and 1-hr ${}^4\mathrm{He}$ (3.5--6.0 AMU) 0.4--10 MeV nucleon$^{-1}$ count rates for events #1 (left) and #22 (right). Error bars with arrows denote ULEIS single-count data points. SOHO/ERNE 2-hr proton fluxes (in arbitrary units) in two energy ranges are overploted as black dotted and solid curves.
• Figure 3: Helium mass histograms for events #8 (left) and #19 (right). The vertical red lines mark the ${}^3\mathrm{He}$ mass range. The ${}^3\mathrm{He}/{}^4\mathrm{He}$ ratios are shown in the upper right corner. Left: The two tilted black lines are the least square fits of the background and ${}^4\mathrm{He}$ spillover. The extrapolations to the ${}^3\mathrm{He}$ range are marked by blue.
• Figure 4: Left: Element abundances relative to O for events #1, 2, 4, 12 in this study (blue-shaded triangles). Right: Survey-averaged elemental abundances relative to O (blue triangles). For comparison, GSEP and ISEP reference abundances are shown as red circles and orange squares, respectively.
• Figure 5: Left: Distributions of 0.5--2.0 MeV nucleon$^{-1}$${}^3\mathrm{He}/{}^4\mathrm{He}$ (red solid line) and 386 keV nucleon$^{-1}$ Fe/O (blue dashed line). The two vertical dotted lines indicate the slow solar wind ${}^3\mathrm{He}/{}^4\mathrm{He}$ ratio 1998SSRv...84..275G and Fe/O ratio 2000JGR...10527217V. Right: Scatter plot of ${}^3\mathrm{He}/{}^4\mathrm{He}$ versus Fe/O. The three numbers indicate the Spearman correlation coefficient, the $p$-value, and the sample size. Note that outlier event #4 (${}^3\mathrm{He}/{}^4\mathrm{He} \sim4 \times 10^{-1}$) and event #12 (${}^3\mathrm{He}/{}^4\mathrm{He} \sim 2 \times 10^{-1}$) are not considered. Blue squares mark events associated with jets at the source; red circles denote events without observed jets. White crosses represent events with Fe and O time profiles that differ from the remaining events (see text). The blue-shaded histogram (left panel) excludes these events. Dotted horizontal and vertical lines indicate slow solar wind values.