Energy Spectrum of Solar Energetic Electron Events Over 25 Years

Solar Energetic electron (SEE) events are the most common solar particle acceleration phenomenon detected in situ in the interplanetary medium and the energy spectrum of SEE events carries crucial information on the acceleration and/or transport processes of SEEs. In our research, we investigate the peak flux energy spectrum of 458 SEE events with a clear velocity dispersion detected at energies from ≤ 4.2 keV 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 shapes: 304 DDPL events, 32 UDPL events, 23 SPL events, 44 ER events and 55 LP events. The DDPL events can be further divided in to two types: 231 E_B≥20 keV DDPL events and 73 E_B<20 keV DDPL events, since the distribution of break energy E_B exhibits a primary peak around 60 keV and a secondary peak around 7 keV, separated by a dip at ~20 keV. The E_B≥20 keV (E_B<20 keV) DDPL events exhibit a power-law spectral index of 2.0^+0.2_-0.2(2.1^+0.3_-0.3) (values shown in a form of A^+B_-C means the median value with the first and the third quartiles) at energies below E_B=5.6^+2.3_-2.4 keV (60⁺²³_-12 keV) and index of 3.3^+0.5_-0.3 (3.9^+0.6_-0.7) at energies above.The UDPL events have a spectral index of 3.0^+0.3_-0.3 at energies below E_B=5.1^+4.2_-1.8 keV and index of 2.2^+0.2_-0.3 at energies above. The SPL events shows a spectral index of 2.8^+0.5_-0.2. The ER events exhibit a spectral index 1.9^+0.3_-0.3 at energies below E_c=30⁺¹⁹_-10 keV. The six spectrum types also behave differently in the relationship between spectral parameters and in solar cycle variations. Furthermore, propagation effects in the IPM from the Sun to 1 AU appear to have no obvious influence on the spectral shape of most SEE events. These results suggest that the formation of SEE events can involve complex processes/sources.