Guiding-Center Simulations of Stormtime Ring Current Electrons in a More Realistic Magnetic Field Model

We examine the consequences of using a more realistic magnetic field for simulating stormtime electron ring current formation. In the past, we have simulated the guiding-center drift of electrons from the plasma sheet to the inner magnetosphere and their loss as they drift in a Dungey magnetic field model consisting of a dipole plus uniform southward field. We improve upon this in the present study by including realistic day-night asymmetry and time variations in the magnetic field by varying the magnitude of the added unidirectional southward field with time (UT) and magnetospheric longitude (MLT) so as to match the modeled polar cap boundary to the auroral poleward boundary provided by the empirically-based OVATION model [Newell et al.}, JGR, 2002]. Our model electric field consists of corotation, quiescent Stern-Volland convection, and storm-associated enhancements in the convection electric field that are less well shielded than the Stern-Volland field. Our enhancements in the cross-polar-cap potential are based on DMSP measurements. We trace the guiding-center drifts of representative equatorially-mirroring electrons with first adiabatic invariants μ = 1 -- 200 MeV/G for the 27 August 1990 storm. Using these simulation results, we map stormtime phase space distributions by invoking Liouville's Theorem modified by losses. Our boundary spectrum at geosynchronous orbit and our initial quiescent distribution are taken from CRRES observations. With both the static Dungey and the more realistic magnetic field model, there are significant stormtime enhancements of ring-current electron fluxes at equatorial radial distance r₀ = 2.6 to 6.6 R_E for energies from tens of keV up to 180 keV. However, the electron drift speed is slower on the dayside than on the nightside in the more realistic asymmetric magnetic field model because the magnetic field intensity is stronger on the dayside than the nightside at a given r₀. This makes the stormtime electron ring current more strongly asymmetric. With the more realistic time-dependent magnetic field model there are also partially adiabatic fluctuations in the flux profiles of ∼ 90 to 150 keV electrons at r₀ = 2.5 to 5 R_E due to time variation of the magnetic field.