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

Enhancements in ³He abundance, a characteristic feature of impulsive solar energetic particle events, are also frequently observed in gradual solar energetic particle events, but the origin of the ³He-rich contribution to the seed population (remnant material versus fresh injection from the parent active region) remains unresolved. We investigate the origin of ³He enrichment in high-energy (25–50 MeV) solar proton events observed by the Solar and Heliospheric Observatory, selecting events that coincide with <1 MeV/nuc ³He-rich periods detected by the Advanced Composition Explorer from 1997 to 2021. Extreme-ultraviolet imaging from the Solar Dynamics Observatory and STEREO reveals narrow, jetlike eruptions in the parent active regions of about 60% of the 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 reaccelerated 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 reaccelerated by the coronal mass ejection shock. We also find that a substantial fraction of apparent ³He enrichments arise from overlap with independent impulsive SEP activity, highlighting the need to separate superpositions from true reacceleration signatures.