Simulation of Radiation Belt Electrons in a Magnetic Storm in the Declining Phase of the Solar Cycle

Recurring, declining-phase storms are typically weak to moderate storms associated with long-lasting, high-velocity solar wind and long-duration substorm activity. These storms are of particular interest for the radiation belts because they produce some of the largest sustained enhancements of MeV electrons. In this work, we study the variation of relativistic electron fluxes for a model storm that is typical of the declining phase of the solar cycle. The storm parameters are based on the observations by Tsurutani et al. [1995]. We use a radial diffusion model with a time-dependent radial diffusion coefficient parameterized by Kp from Brautigam and Albert [2000], a time-dependent geosynchronous orbit boundary condition from Li et al. [2001],and a Kp-dependent loss term. The Hilmer-Voigt magnetic field model [Hilmer and Voigt, 1995] is used to map between equatorial phase space density and particle fluxes. The results show that radial diffusion propagates outer boundary variations into the heart of the outer radiation belt, resulting in phase space density increases during the recovery phase. The results are only qualitatively consistent with the observations reported by Hilmer et al. [2000] since the model fluxes at R = 4 Re increase more rapidly than observed fluxes, but if we divide the Brautigam-Albert radial diffusion coefficient by a factor of five, the simulation results reproduce the measured flux variations very well. We also explore the sensitivity of the simulation results to the underlying magnetic field model by recalculating the results using the T01 magnetic field model [Tsyganenko 2002] and a dipolar magnetic field. The main differences in the simulation results for different magnetic field models occur during the main phase and early recovery phase of the model storm, but in the late recovery phase the simulation results are quite similar.