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

Abstract Enhancements in 3 He abundance, a characteristic feature of impulsive solar energetic particle (ISEP) events, are also frequently observed in gradual solar energetic particle (GSEP) events. Understanding the origin of this enrichment is crucial for identifying the mechanisms behind solar energetic particle (SEP) generation. We investigate the origin of 3 He enrichment in high-energy (25–50 MeV) solar proton events observed by the Solar and Heliospheric Observatory, selecting events that coincide with ≲1 MeV nucleon −1 3 He-rich periods detected by the Advanced Composition Explorer during 1997–2021. The identified 3 He enhancements include cases where material from independent impulsive ( 3 He-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 the Solar Dynamics Observatory and STEREO reveals narrow, jetlike eruptions in the parent active regions of about 60% of the remaining events. Notably, the highest 3 He/ 4 He ratios occur when coronal jets are present, consistent with fresh, jet-driven injection of suprathermal 3 He that is subsequently reaccelerated during the event. Correspondingly, jet-associated events show fewer preevent (residual) 3 He counts, indicating that enrichment in these cases does not primarily come from remnant material. We find a positive correlation between 3 He/ 4 He and Fe/O, strongest in jet-associated events, consistent with a common jet-supplied seed population reaccelerated by the coronal mass ejection shock.