The evolution of the radiation belts in L‐shell (L), energy (E), and equatorial pitch angle (α0) 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, α0) 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 α0 ~ <60°, E > 100 keV, 3.5 < L < Lpp ~ 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.

Understanding the dynamic evolution of relativistic electrons in the Earth's radiation belts during both storm and nonstorm times is a challenging task. The U.S. National Science Foundation's Geospace Environment Modeling (GEM) focus group "Quantitative Assessment of Radiation Belt Modeling" has selected two storm time and two nonstorm time events that occurred during the second year of the Van Allen Probes mission for in-depth study. Here, we perform simulations for these GEM challenge events using the 3D Versatile Electron Radiation Belt code. We set up the outer L * boundary using data from the Geostationary Operational Environmental Satellites and validate the simulation results against satellite observations from both the Geostationary Operational Environmental Satellites and Van Allen Probe missions for 0.9-MeV electrons. Our results show that the position of the plasmapause plays a significant role in the dynamic evolution of relativistic electrons. The magnetopause shadowing effect is included by using last closed drift shell, and it is shown to significantly contribute to the dropouts of relativistic electrons at high L * . We perform simulations using four different empirical radial diffusion coefficient models for the GEM challenge events, and the results show that these simulations reproduce the general dynamic evolution of relativistic radiation belt electrons. However, in the events shown here, simulations using the radial diffusion coefficients from Brautigam and Albert (2000) produce the best agreement with satellite observations. Key Points:• We investigate the effect of different plasmapause positions and radial diffusion coefficients during the four GEM challenge events • Including the magnetopause shadowing effect by using the last closed drift shell helps to reproduce the dropouts • The three-dimensional Versatile Electron Radiation Belt code reproduces the general dynamics of relativistic electrons during GEM challenge events Supporting Information:• Supporting Information S1 , et al. (2020). The effect of plasma boundaries on the dynamic evolution of relativistic radiation belt electrons.

Discovered in 1958, the Earth's radiation belts are two torus shaped regions consisting of highly energetic protons and electrons that remain trapped by the Earth's magnetic field (Van Allen & Frank, 1959). The inner radiation belt is located between the 0.2-2 Earth radii (R e), McIlwain L shells = 1-3 (Roederer, 1970), and is composed of ∼100-700 keV electrons (Fennell et al., 2015) and protons with energies ranging from ∼10 to ∼100 MeV. The outer radiation belt extends from L = 2.8-8 (3-7 R e) and consists mostly of high-energy and relativistic (0.5 to ∼10 MeV) electrons. While the inner belt is generally stable, the outer belt is extremely dynamic, with variations of up to several orders of magnitude possible within hours (