Spectral Properties and the Influence of Coronal Mass Ejections in ³He-rich Solar Energetic Particle Events

Abstract We analyze the spectral properties of 3 He and 4 He as well as the heavy ions (oxygen, neon, magnesium, silicon, and iron) in 80 3 He-rich solar energetic particle (SEP) events observed by the Ultra-Low-Energy Ion Spectrometer on board the Advanced Composition Explorer spacecraft since its launch in 1997 until 2024. We split the spectral analysis into two criteria: events with fast and wide coronal mass ejections (CMEs; called “FW events”) and events with slow, narrow, or no observed CMEs (called “non-FW events”). Overall, we find that events with fast and wide CMEs exhibit more uniform spectra across all species, and their low-energy spectral indices are strongly correlated, suggesting a CME provides an additional reacceleration mechanism for the 3 He-rich SEPs. When comparing each species’ low-energy spectral index for events with no associated fast-and-wide CME, we find a primary peak in the spectral hardness of 3 He, and a secondary peak in Mg and Si. If we consider a plasma temperature of 1.0–1.3 MK, Mg and Si have a charge-to-mass ratio ( Q / M ) nearest to one-third (1/3), directly half that of 3 He. Thus, our results support the results of Roth & Temerin, which suggest heavy ions resonate with the second harmonic of the same ion cyclotron waves energizing 3 He. However, it is unclear why the Fe enhancement is not reflected in its spectral index, and we propose that additional acceleration and/or transport mechanisms are playing a role in the abundance enhancement of Fe and heavier ions.