The 3He-rich SEP on event 2024 April 1 at 27 Solar Radii-Indications of acceleration by waves and implications of coronal source location

Abstract

Parker Solar Probe (PSP) observations of a small dispersive event on 2024 April 1 reveal highly enriched 3He abundance in the suprathermal energy range (40–250 keV nucleon−1). Time dispersion observed in this impulsive solar energetic particle (SEP) event is consistent with an estimated onset time of 22:50 UT on 2024 March 31. The event occurred during an orbital segment as PSP moved from about 27 Rs to about 29 Rs while outbound in orbit 19. For this segment of the orbit, PSP’s motion in Carrington coordinates remains within about 1° of the same Carrington longitude. The 3He content is much larger than the usual SEP abundances, especially in this energy range. Additionally, the 3He energy spectrum is not power law in shape, unlike the H and 4He spectra. Heavy ions (O, Fe) are also somewhat enhanced. These compositional signatures suggest that this is an example of resonant wave–particle acceleration, as hypothesized for generating the suprathermal seed particles required to account for the enrichment of various species above solar abundances in shock (or compression) acceleration models of SEP events. Like many previously reported impulsive 3He-rich events, this event is associated with an energetic electron beam, type III radio emission, and a hard X-ray flare. However, associating the hard X-ray event with this SEP event requires significant (∼60°) prompt longitudinal propagation of energetic particles in the low corona before they access open magnetic flux, complicating its interpretation as part of the source process for this particular event.