Energy Dependence of SEP Electron and Proton Onset Times

Abstract:

We study the large solar energetic particle (SEP) events that were detected by GOES in the > 10 MeV energy channel during December 2006 to March 2014. Using multi-spacecraft observations from STEREO A, B and SOHO, we are able to determine accurately the solar particle release (SPR) time of SEP electrons and protons. We first compute connection angles (CA) between the solar events and magnetic foot-points connecting to each spacecraft. By choosing the smallest CA, we derive the electron and proton SPRs using electron fluxes from the SOHO Electron Proton and Helium Instrument (EPHIN), proton fluxes from the SOHO Energetic and Relativistic Nuclei and Electron instrument (ERNE), and from the High Energy Telescope (HET) on STEREO. It is found that: 1) the 0.25 MeV-0.7 MeV electron SPRs are ~10 min earlier than 2.64 MeV - 10.4 Mev electron SPRs; 2) the proton SPRs inferred from high-energy channels (> 50 MeV) are similar to electron SPRs; 3) the proton SPRs inferred from lower energy channel (10 - 16.9 MeV) can be either ~ 7 min earlier than or delayed from the electron SPRs for tens of minutes to hours, especially for SEPs with large pre-event background flux levels. In this study, we evaluated the effects of large scattering and high background levels on SPRs and made suggested corrections for the background effect on SPR times. We also find that for some large SEP events, the observed EPHIN electron and ERNE proton intensity profiles show a double-peak feature. The onset of the first peak corresponds well to the associated Type III and metric Type II onset and tends to be nearly scattering-free.