Variability of the Proton Radiation Belt

Significant steady but slow variability of radiation belt proton intensity, in the energy range \~19\textendash200 MeV and for L<2.4, has been observed in an empirical model derived from data taken by Van Allen Probes during 2013\textendash2019. It is compared to predictions of a theoretical model based on measured initial and boundary conditions. Two aspects of the variability are considered in detail and require adjustments to model parameters. Observed inward transport of proton intensity maxima near L=1.9 and associated increasing intensity are caused in the model by inward radial diffusion from an external source while conserving the first two adiabatic invariants. The diffusion coefficient is constrained by these observations and is required to have increased near the start of 2015 by a factor \~2. Observed decay of proton intensity at L<1.6 can be caused only in part by energy loss to free and bound electrons in the local plasma and neutral atmosphere. Another, unknown loss mechanism is required to match observed proton decay rates as a function of energy. Accounting for the expected influence of slow radial diffusion at low L, the additional loss should have a mean lifetime near 22 years, independent of L and energy in the range \~19\textendash70 MeV. Several candidate loss mechanisms are considered\textemdashadded plasma or neutral density, elastic Coulomb scattering, plasma wave scattering, field-line curvature scattering, and collision with orbital debris\textemdashbut none are found viable.