Acceleration and Release of Solar Energetic Particles Associated with a Coronal Shock on 2021 September 28 Observed by Four Spacecraft

Abstract

The main driver of the acceleration of solar energetic particles (SEPs) is believed to be shocks driven by coronal mass ejections (CMEs). Extreme ultraviolet (EUV) waves are coronal disturbances that have been interpreted as the propagating footpoint of CME-driven shocks on the solar surface. One of the key questions in SEP research is the timing of the SEP release with respect to the time when the EUV wave magnetically connects with an observer. Taking advantage of the measurements by Parker Solar Probe (PSP) and Solar Orbiter (SolO) close to the Sun, we investigate a SEP event that occurred on 2021 September 28 and was observed at four different locations by SolO, PSP, STEREO-A, and the near-Earth spacecraft. During this time, SolO, PSP, and STEREO-A shared similar nominal magnetic footpoints but were at different heliocentric distances. We find that the SEP release times estimated at these four locations were delayed compared to the times when the EUV wave intercepted the footpoints of the nominal magnetic fields connecting to each spacecraft by around 30 to 60 minutes. Combining observations in multiple wavelengths from radio to EUV wavelengths passing by white light, with a geometrical shock model based on multi-viewpoint observations, we analyze the associated shock properties, and discuss the acceleration and delayed release processes of SEPs in this event as well as the accuracy and limitations of using EUV waves to determine the SEP acceleration and release times.