Energy Spectrum of Solar Energetic Electron Events over 25 Years

We investigate the peak flux energy spectrum of 458 solar energetic electron (SEE) events with a clear velocity dispersion detected at energies from ≤4.2 to ≥108 keV by Wind/3DP from 1994 December through 2019 December, utilizing a pan-spectrum fitting method. According to the fitted spectral parameters, these 458 events are self-consistently classified into five spectral types: 304 downward double-power-law (DDPL) events, 32 upward double-power-law (UDPL) events, 23 single-power-law (SPL) events, 44 Ellison–Ramaty (ER) events, and 55 logarithmic–parabola (LP) events. The DDPL events can be further divided into two types: 231 ${\mathrm{DDPL}}_{{E}_{B}\geqslant 20\mathrm{keV}}$ events and 73 ${\mathrm{DDPL}}_{{E}_{B}\lt 20\mathrm{keV}}$ events, since their break energy E _B exhibits a double-peak distribution separated by a dip at ∼20 keV. The ${\mathrm{DDPL}}_{{E}_{B}\lt 20\mathrm{keV}}$ ( ${\mathrm{DDPL}}_{{E}_{B}\geqslant 20\mathrm{keV}}$ ) events show a power-law spectral index of ${2.0}_{-0.2}^{+0.2}$ ( ${2.1}_{-0.3}^{+0.3}$ ) at energies below ${E}_{B}={5.6}_{-2.4}^{+2.3}$ ( ${61}_{-12}^{+23}$ ) keV and an index of ${3.3}_{-0.3}^{+0.5}$ ( ${3.9}_{-0.7}^{+0.6}$ ) at energies above. The UDPL events have a spectral index of ${3.0}_{-0.3}^{+0.3}$ at energies below ${E}_{B}={5.1}_{-1.8}^{+4.2}$ keV and an index of ${2.2}_{-0.3}^{+0.2}$ at energies above. The SPL events exhibit a spectral index of ${2.8}_{-0.2}^{+0.5}$ . The ER events show a spectral index of ${1.9}_{-0.3}^{+0.3}$ at energies below ${E}_{c}={31}_{-11}^{+19}$ keV. The LP events are characterized by a spectral slope of ${1.8}_{-0.3}^{+0.4}$ ( ${3.6}_{-0.5}^{+0.7}$ ) at 2.8 keV (108 keV). The six spectral types also behave differently in the relationship between spectral parameters and in solar cycle variations. The spectral shape of most SEE events appears to be unrelated to the estimated electron path lengths. These results suggest that the formation of SEE events can involve complex processes/sources.