Observations and Fokker-Planck Simulations of the L-Shell, Energy, and Pitch Angle Structure of Earth's Electron Radiation Belts During Quiet Times

Abstract

The evolution of the radiation belts in L-shell (L), energy (E), and equatorial pitch angle (α 0 ) is analyzed during the calm 11-day interval (4–15 March) following the 1 March 2013 storm. Magnetic Electron and Ion Spectrometer (MagEIS) observations from Van Allen Probes are interpreted alongside 1D and 3D Fokker-Planck simulations combined with consistent event-driven scattering modeling from whistler mode hiss waves. Three (L, E, α 0 ) regions persist through 11 days of hiss wave scattering; the pitch angle-dependent inner belt core (L ~ <2.2 and E < 700 keV), pitch angle homogeneous outer belt low-energy core (L > ~5 and E~ < 100 keV), and a distinct pocket of electrons (L ~ [4.5, 5.5] and E ~ [0.7, 2] MeV). The pitch angle homogeneous outer belt is explained by the diffusion coefficients that are roughly constant for α 0  ~ <60°, E > 100 keV, 3.5 < L < L pp  ~ 6. Thus, observed unidirectional flux decays can be used to estimate local pitch angle diffusion rates in that region. Top-hat distributions are computed and observed at L ~ 3–3.5 and E = 100–300 keV.