Statistical Analysis of Distributions of Electron Energy Spectra in the Earth’s Radiation Belts Based on Van Allen Probes Observations

The characteristics of energy spectra distributions of radiation belt electrons can help reveal the dominant physical mechanism behind the dynamics of the radiation belt. In this study, the temporal and spatial distribution characteristics of energy spectra of energetic electrons are statistically analyzed using the measurements from Van Allen probes during 2014–2018. The results show that most of the electron spectra in the radiation belt can be classified into three types: exponential, power-law and reversed spectra. The exponential spectra dominate in the outer radiation belt outside the plasmapause. The power-law spectra usually occur at higher L values and move to lower L values during geomagnetic storms. During quiet geomagnetic activity periods, the power-law energy spectra stay at the high L with longer time and higher proportion. The reversed energy spectra dominate at L>2.5 inside the plasmasphere. As the increase of plasmapause locations, more reversed energy spectra occur while the proportion of the exponential energy spectra decreases. During long-term quiet periods, electron fluxes near the peak of the reversed energy spectra (about 2 MeV) are relatively low to form obvious reversed spectral. The results also show that the peak location of reversed energy spectra is about 2 L inside the plasmapause while the peak location of exponential energy spectra is about 1.5 L outside the plasmapause. Appearances of power-law energy spectra are related to substorm injections of electrons. Stronger magnetic storms produce wider L ranges where reversed energy spectra are replaced by exponential spectra. During quiet times, exponential energy spectra inside the plasmapause are replaced by reversed energy spectra due to pitch angle scattering produced by plasmaspheric hiss waves.