Determining radial boundary conditions of outer radiation belt electrons using THEMIS observations

Abstract

The external source for the radiation belt structure can be approximately reproduced or predicted by a diffusion equation, which takes into account the diffusion processes in pitch angle, energy, and radial coordinate L. The solution of such an equation depends on several factors: initial and boundary conditions, diffusion coefficients, and plasmapause location. A precise determination of these main factors is therefore important and requires appropriate observations. In this paper, we have attempted to determine the radial boundary conditions of the energetic electron fluxes near the outer edge of the outer radiation belt. Specifically, we have determined two main aspects: first, the energy spectrum of the electron fluxes at the boundary, and second, the solar wind conditions that have the greatest impact on the electron fluxes at the boundary. For this study, we used the electron flux data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites. We found that statistically the energy spectrum for the energy range from a few tens of keV to several hundreds of keV can be best described by a kappa function with kappa = 1.8 even though individual energy spectra can vary significantly. Regarding the solar wind dependence, we find that the boundary electron flux level correlates well with either the solar wind speed positively or the solar wind density negatively, and therefore a combination of the two variables, with a certain response time. The response time differs for the solar wind variables and among electron energies. Using the identified correlations, we further determined fit functions expressed in terms of the solar wind speed or density or their combination that best reproduced average observed boundary fluxes. This study is the first observational attempt to comprehensively determine the radial boundary conditions of the outer radiation belt. In addition, the results of this study can be practically used as inputs for radiation belt modeling and forecasting.