Solar Energetic Electron Events with a Spectral Bump

Abstract

The energy spectrum of solar energetic electron (SEE) events carries crucial information on the origin/acceleration of energetic electrons at the Sun. Using  the Wind 3DP electron measurements at ~1 to 200 keV during 1995-2019, we select 11 good SEE events with a bump-like break in the peak flux vs. energy spectrum, different from the typical SEE events with a double-power-law spectrum. For the selected 11 events, the background-subtracted electron peak flux versus energy spectrum fits well to two functions: the sum of a single-power-law and a Gaussian function (spectral function #1) and the product of a single-power-law and the natural exponential form of a Gaussian function (spectral function #2). For the spectral function #1 (#2), on average, the fitted spectral index is 2.6±0.4 (2.7±0.6), significantly larger than the low-energy power-law index of typical SEE events, while the fitted center energy of spectral bump is 24±7 keV (75±38 keV) and the ratio of bump width and center is 2.0±0.7 (3.4±2.8). Among these 11 events, respectively, ~78%, ~89%, ~90%, 100% and ~55% are associated with GOES SXR flares, RHESSI HXR flares, west-limb CMEs, type III radio bursts and type II  radio bursts. Thus, these bump events have a stronger association with flares, coronal mass ejections (CMEs) and type II radio bursts, compared to the typical SEE events. In addition, we find a positive correlation between the center energy of bump and the CME speed. Therefore, we come up with an acceleration picture of these bump SEE events: the power-law portion is probably accelerated by flares with the acceleration efficiency larger at lower energies, while the bump portion is likely accelerated in CME-related processes with the acceleration efficiency increasing with the CME speed.