Spectral distributions of electron phase space density in the Earth's radiation belt

The energy spectra of the Earth's radiation belt electrons provide easy access to analyze the dynamics of the inner magnetosphere. The present study quantitatively investigates the spectral distributions of the radiation belt electron phase space density (PSD) at μ = 10–104 MeV/G (μ is the first adiabatic invariant) within L* = 2.5–6.0 (L* denotes the third adiabatic invariant) under TS04 geomagnetic coordinate, based on Van Allen Probe observations from 2012 to 2019. Statistical results reveal that most of the observed PSD spectra can also be classified into three types, including exponential, power law, and bump-on-tail (BOT). Power law distributions are mainly located outside the plasmapause location (Lpp) and can penetrate to L*<3 after intense geomagnetic storms. By contrast, BOT distributions dominate from the inner edge of the outer belt to the slot region, i.e., L* = 2.5–4.0, and become pronounced during long-term geomagnetic quiet times. Moreover, the PSD peaks are located at μ = 102–103 MeV/G and the PSD valleys range from μ = 101.5 to 102.5 MeV/G, with the ratio between them increasing at higher L*. Besides, exponential distributions are likely the intermediate ones between power law and BOT distributions inside and close to the plasmapause location (Lpp), which become intense immediately after the inward movement of Lpp and then gradually decay during the recovery phase. The high dynamic pressure, geomagnetic storms and plasma waves may play critical roles in triggering the transformation between the three types of electron spectra.