SIMULATIONS OF RING CURRENT PROTON PITCH-ANGLE DISTRIBUTIONS

We employ a three-dimensional ring current model to trace the bounce-a veraged drift of singly charged ions during storm-associated enhanceme nts in the convection electric field. Using the simulation results, we map proton phase space density during the main and recovery phases of a storm in accordance with conservation of phase space density f. We map from an initial quiescent phase space distribution that is obtaine d by solving the steady state transport equation (bounce-averaged char ge exchange balancing bounce-averaged radial diffusion) with observed plasma sheet proton spectra as outer boundary conditions. We obtain pr oton pitch angle distributions at L similar to 3-4.5 by evaluating f a t representative ring current energies (similar to 20-170 keV). We fin d that the prestorm and stormtime proton pitch angle anisotropy at any given L between 3 and 4.5 increases with particle energy in agreement with observations. The actual anisotropy at specific energies depends strongly on the shape of the plasma sheet source spectrum at similar to 0.5-3 keV. Relatively large enhancements in the stormtime phase spa ce density from the quiescent distribution occurs at all pitch angles for low energies (less than or similar to 80 keV) except where the qui escent distribution lies on open drift shells. These increases result primarily from stormtime access to L less than or similar to 4.5 along open drift trajectories from the plasma sheet. For higher-energy (gre ater than or similar to 150 keV) protons, which are transported via ra dial diffusion, there is little change in the anisotropy over a 3-hour storm. At intermediate energies (E similar to 80-150 keV) the stormti me enhancements in the phase space density can vary quite strongly wit h equatorial pitch angle. For such particles the stormtime transport i s intermediate between directly convective and quasi-diffusive, which can complicate the analysis of stormtime pitch angle anisotropies at i ntermediate energies. Decay lifetimes obtained from CRRES data during the recovery phase of a moderate storm are found to be considerably sh orter than charge-exchange lifetimes. It thus appears that charge exch ange alone is not enough to explain the observed rapid decay of the pr oton ring current.