Multi-spacecraft Observations of Gradual Solar Energetic Particle Events
with Enhanced 3He Abundance
Abstract
Flare suprathermal ions with enhanced 3He and heavy-ion abundances are
an essential component of the seed population accelerated by CME-driven
shocks in gradual solar energetic particle (GSEP) events. However, the
mechanisms through which CME-driven shocks gain access to flare
suprathermals and produce spectral and abundance variations in GSEP
events remain largely unexplored. We report two recent GSEP events: one
observed by Solar Orbiter on 2020 Nov 24 (the first GSEP event on Solar
Orbiter) and the other by ACE on 2021 May 29 (the most intense GOES
proton event in the present solar cycle). The events were preceded by
impulsive SEP (ISEP) events. Abundances and energy spectra are markedly
different in the examined events at < 1 MeV/nucleon. For
example, in the May event, Fe/O is typical of ISEP events, a factor of
100 to 10 higher than Fe/O in the November event. 3He abundance in the
November event is high, typical of ISEP events, while in the May event,
it is much lower, though finite. The May event shows a hard 4He spectrum
with a power-law index of −1.6, and the November event a soft spectrum
with an index of −3.5. The events were associated with halo CMEs with
speeds around 900 km/s. The November event was also measured by Parker
Solar Probe and the May event by STEREO-A and Solar Orbiter. This paper
discusses the origin of vastly different abundances and spectral shapes
in terms of variable remnant population from preceding ISEP events.
Furthermore, we discuss a possible direct contribution from parent
flares.