On Phase Space Density and Its Radial Gradient of Outer Radiation Belt Seed Electrons: MMS/FEEPS Observations

Abstract

AbstractElectrons of ∼102 keV in the inner magnetosphere are suggested potentially to be the “seeds” of outer radiation belt electrons. Investigations into them could shed light on the origination of the outer radiation belt. Here we conduct statistics on seed electron phase space density (PSD) and its radial gradient, using 2‐year data obtained by Fly's Eye Energetic Particle Spectrometer (FEEPS) onboard Magnetospheric Multiscale (MMS). The radial distributions of the PSD are μ and K dependent, and azimuthally asymmetric. In the nightside, as L* increases, PSD at lower μ always increases, while PSD at higher μ first increases and then decreases. In contrast, in the noonside, PSD first increases and then decreases at all covered μ. Crossover μ, defined as μ at which PSD radial gradient is equal to 0, is identified from the statistics. It relatively stays a constant of ∼300 MeV/G inside L*∼6, but decreases as L* increases outside L*∼6. Crossover μ is larger in the nightside than in the noonside and is larger at smaller K. The radial structures of seed electron PSD should be taken into account when considering the dynamics of the outer radiation belt.