Simulations of ring current proton pitch angle distributions

Details Simulations of ring current proton pitch angle distributions Simulations of ring current proton pitch angle distributions

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998jgr...103..165c&link_type=abstract

Journal of Geophysical Research, Volume 103, Issue A1, p. 165-178

Physics

Plasma Physics

18

Magnetospheric Physics: Numerical Modeling, Magnetospheric Physics: Ring Current, Space Plasma Physics: Kinetic And Mhd Theory, Space Plasma Physics: Transport Processes

Scientific paper

We employ a three-dimensional ring current model to trace the bounce-averaged drift of singly charged ions during storm-associated enhancements 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 obtained by solving the steady state transport equation (bounce-averaged charge exchange balancing bounce-averaged radial diffusion) with observed plasma sheet proton spectra as outer boundary conditions. We obtain proton pitch angle distributions at L~3-4.5 by evaluating f at representative ring current energies (~20-170keV). We find 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 ~0.5-3keV. Relatively large enhancements in the stormtime phase space density from the quiescent distribution occurs at all pitch angles for low energies (<~80keV) except where the quiescent distribution lies on open drift shells. These increases result primarily from stormtime access to L<~4.5 along open drift trajectories from the plasma sheet. For higher-energy (>~150keV) protons, which are transported via radial diffusion, there is little change in the anisotropy over a 3-hour storm. At intermediate energies (E~80-150keV) the stormtime enhancements in the phase space density can vary quite strongly with equatorial pitch angle. For such particles the stormtime transport is intermediate between directly convective and quasi-diffusive, which can complicate the analysis of stormtime pitch angle anisotropies at intermediate energies. Decay lifetimes obtained from CRRES data during the recovery phase of a moderate storm are found to be considerably shorter than charge-exchange lifetimes. It thus appears that charge exchange alone is not enough to explain the observed rapid decay of the proton ring current.

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