Insights from the spectra of solar energetic electron events

We study the energy spectra of near-relativistic (50-400keV) solar electron events observed with the Solar Electron and Proton Telescope (SEPT) aboard the two STEREO spacecraft. For a large statistical event sample from 2007 to 2018, the electron peak intensity spectra are analyzed regardless of the event impulsiveness, the onset timing with respect to the solar counterpart, or the presence of accompanying phenomena such as Coronal Mass Ejections (CMEs) or type II radio bursts. We present the distributions and mean values of the spectral indices, which illustrate the spectral variety of near-relativistic solar electron events during solar cycle 24. We further analyze the spectral index in terms of the rise times of the events and find evidence for the presence of acceleration contributions additional to solar flares, which must be involved for events characterized by hard-spectra and long-rise times.