Guiding-Center Simulations of Stormtime Ring Current Electrons

We examine the stormtime injection of electrons to the stormtime ring current by simulating the drift and loss of electrons from the plasma sheet to the inner magnetosphere. We use a similar method and magnetic and electric field model that Chen et al. [JGR, 99, 5745-5759, 1994] used previously to account for the stormtime injection of ring current ions to determine the extent to which that model can also account for observed stormtime electron injections. The model traces the guiding-center motion of representative particles, having selected first adiabatic invariants μ. The magnetic field model is a dipole field plus constant southward IMF, and we impose corotation and a 25-kV quiescent Stern-Volland cross polar cap potential. We simulate two real storms (August 27, 1990, and October 10, 1990) by applying additional storm-associated enhancements in the convection electric field that are less well shielded than the Stern-Volland field. The enhancements in the cross-polar cap potential are obtained by linearly interpolating DMSP measurements of the cross polar cap potential minus an assumed 25-kV quiescent potential. We perform simulations for representative equatorially-mirroring electrons for μ = 1 MeV/G to 200 MeV/G. Using the simulation results, we map stormtime phase space distributions by invoking Liouville­_s Theorem modified by losses. We consider electron loss due to precipitation via a model in which there is strong diffusion far in the plasma sheet (L > ~8) and weak diffusion within the plasmasphere. The loss rates in the region L ~8 to the plasmapause boundary are based on diffusion rates calculated by Lyons [JGR, 79, 575-580, 1974]. We apply a boundary spectrum at geosynchronous orbit that is based on averaging 12 years of geosynchronous LANL/MPA electron data and is parameterized by Kp and binned in 0.5 hr MLT increments. The initial quiescent electron distribution for trapped electrons is taken from the steady-state balance between radial diffusion and weak-pitch-angle-scattering losses. From the simulation results, we find significant stormtime enhancements of ring current electrons at equatorial radial distance r = 2.6 to 6.6 R_E for energies from tens of keV up to 180 keV. We compare features of our stormtime electron flux distribution with CRRES/LEPA and CRRES/MEA data. We find good agreement between simulated and observed electron flux profiles at low energies (<= 20 keV). Measurements are not available from ~20 keV to 150 keV. However, our model does not reproduce stormtime enhancements of electron flux at high energies (>= 150 keV) at L ~3 to 4, suggesting that a local energization may be responsible for these enhancements.