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

Electrons of similar to 10(2) 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 mu and K dependent, and azimuthally asymmetric. In the nightside, as L* increases, PSD at lower mu always increases, while PSD at higher mu first increases and then decreases. In contrast, in the noonside, PSD first increases and then decreases at all covered mu. Crossover mu, defined as mu at which PSD radial gradient is equal to 0, is identified from the statistics. It relatively stays a constant of similar to 300 MeV/G inside L* similar to 6, but decreases as L* increases outside L* similar to 6. Crossover.. 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.