Statistical results for solar energetic electron spectra observed over 12 years with STEREO/SEPT

TL;DR

This study analyzes 12 years of solar energetic electron spectra observed by STEREO, revealing relationships between spectral features, event impulsiveness, and particle acceleration mechanisms.

Contribution

It provides a comprehensive statistical analysis of electron spectra, identifying spectral break energies and their relation to event characteristics, suggesting complex acceleration processes.

Findings

Broken power law spectra are common, with a mean break energy around 120 keV.

Softer spectra are associated with impulsive events and shorter rise times.

Longer rise-time events show harder spectra and higher energy electrons, indicating additional acceleration mechanisms.

Abstract

We present a statistical analysis of near-relativistic (NR) solar energetic electron event spectra near 1au. We use measurements of the STEREO Solar Electron and Proton Telescope (SEPT) in the energyrange of 45-425 keV and utilize the SEPT electron event list containing all electron events observed bySTEREO A and STEREO B from 2007 through 2018. We select 781 events with significant signal tonoise ratios for our analysis and fit the spectra with single or broken power law functions of energy.We find 437 (344) events showing broken (single) power laws in the energy range of SEPT. The eventswith broken power laws show a mean break energy of about 120 keV. We analyze the dependence ofthe spectral index on the rise times and peak intensities of the events as well as on the presence ofrelativistic electrons. The results show a relation between the power law spectral index and the risetimes of the events with softer spectra belonging to rather impulsive events. Long rise-time events areassociated with hard spectra as well as with the presence of higher energy (>0.7 MeV) electrons. Thisgroup of events cannot be explained by a pure flare scenario but suggests an additional accelerationmechanism, involving a prolonged acceleration and/or injection of the particles. A dependence of thespectral index on the longitudinal separation from the parent solar source region was not found. Astatistical analysis of the spectral indices during impulsively rising events (rise times<20 minutes) isalso shown.