Solar energetic electron events with a spectral bump break

¹ School of Earth and Space Sciences, Peking University, Beijing, 100871, China
² College of Meteorology and Oceanography, National University of Defense Technology, Changsha, 410015, China
³ Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China
⁴ Institute of 4D Technologies, University of Applied Sciences Northwestern Switzerland, 5210 Windisch, Switzerland
⁵ Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
⁶ Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723 USA

^⋆ Corresponding author: wanglhwang@pku.edu.cn, wanglhwang@gmail.com

Received: 11 December 2024
Accepted: 24 April 2025

Abstract

Aims. We present ten solar energetic electron (SEE) events measured by Wind/3DP at ∼1 to 200 keV with a bump break in the electron peak flux versus energy spectrum. We examined their acceleration sources and/or processes at the Sun.

Methods. We assumed that these bump SEE events consist of two electron populations: a primary population (described by the pan-spectrum (PS) function), and a bump population (described by the Gaussian function), which dominate at low and high energies, respectively. We constructed two formulae to fit the SEE energy spectrum by multiplying a PS function with a natural exponential form of the Gaussian function (i.e., the MUL formula) and by adding a PS function with a Gaussian function (i.e., the ADD formula).

Results. The fitting results suggest that the MUL fitting can reflect the physical nature in the formation of these bump events. For the primary electron population, the MUL fitting obtains an upward-bending double power-law spectrum for event 10 with a spectral index of 3.58 (1.74) at energies below (above) ∼4.6 keV, and a single power-law spectrum for the other nine events with a median spectral index of 2.52^+0.29_−0.25. For the bump electron population, the fitted center energy has a median value of 59.1_−3.2^+18.1 keV. For the events associated with soft X-ray flares (west limb coronal mass ejections), the flare class (angular width of the coronal mass ejection) is positively correlated with the estimated electron number of the power-law population N_pl and of the bump population N_bp (the number ratio N_bp/N_pl at 10–400 keV).

Conclusions. These results indicate that for these bump SEE events, the power-law electron population can be produced by some flare-related processes that occur high in the corona, while the bump population can be accelerated by some processes related to coronal mass ejections that act on the power-law population. The bump-like spectrum might also be the intermediate spectrum during the evolution from single power-law to downward-bending double power-law.