Spatial Distribution and Geomagnetic Dependence of Radiation Belt Electron Reversed Energy Spectrum

Abstract

Using high-quality electron measurements from Van Allen Probes during October 2013 and March 2019, this study investigates the spatial distribution and geomagnetic dependence of the electron reversed energy spectrum in the Earth's radiation belts. The reversed energy spectrum is primarily observed within the L-shell range of ∼2.6–5.2, with peak occurrence rates reaching ∼50% at L = ∼4. Occurrence rates are higher in the post-noon to midnight sectors and lower on the pre-dawn side. In terms of magnetic latitude (MLAT), the spectrum spans ∼−20°–20°, exhibiting south-north asymmetry, particularly in the noon and night regions. The characteristic energies defining the spectrum correspond to the flux minimum (Ev) and maximum (Ep), which typically range from ∼100 keV to ∼1 MeV and hundreds of keV–∼2 MeV, respectively, with both Ev and Ep decreasing as L increases. The spectrum is more frequently observed during geomagnetically quiet periods, with maximum occurrence rates exceeding 50%. However, as geomagnetic activity intensifies, the occurrence rates decrease significantly, and the favorable region contracts toward lower L-shells. Analysis of geomagnetic indices shows that the reversed energy spectrum is more strongly affected by the Dst index than the auroral electrojet (AE) index. This could suggest a more substantial influence of geomagnetic storms than the substorm activity on suppressing the electron reversed energy spectrum. These results improve our understanding of how radiation belt electron dynamics respond to geomagnetic disturbances, emphasizing the interplay between storms, substorms, and wave-particle interactions in shaping the evolution of the reversed electron energy spectrum.