Proton ring current pitch angle distributions: Comparison of simulations with CRRES observations

In this study we compare the proton pitch angle distributions (PADs) in the ring current region (L similar to 3-4) obtained from Combined Release and Radiation Effects Satellite (CRRES) observations during the large magnetic storm (minimum Dst = -170 nT) on August 19, 1991, with results of phase-spa ce mapping simulations in which we trace the bounce-averaged drift of proto ns during storm-associated enhancements in a model of the convection electr ic field. We map the phase-space density f according to Liouville's theorem except for attenuation by charge exchange, which we compute for both an em pirical model [Rairden et al., 1986] and a theory-based model [Hedges, 1994 ] of the neutral H density distribution. We compare simulated pitch angle d istributions at 48 keV, 81 keV, and 140 keV at L = 3 and L = 4 directly wit h the CRRES distributions at the same energies and L values before and duri ng the storm. A steady-state application of our transport model, using the empirical neutral H density model of Rairden et al. [1986], reproduces the absolute intensities well except for E = 140 keV at L = 3 (M = 10 MeV/G) an d E = 48 keV at L = 4 (M = 13 MeV/G). The anisotropies (A similar to 0.2-0. 8) of the CRRES and modeled pre-storm pitch angle distributions agree withi n factors less than or similar to 2. Time-dependent application of our tran sport model reproduces measured recovery phase anisotropies (t = 10-12 h af ter storm onset; A similar to 0.4-1.2) similarly well at the selected energ ies and L values, but agreement between modeled and measured absolute inten sities is energy-dependent and not consistently good. Our model underpredic ts the proton intensities found by CRRES for E > 80 keV at L = 4 in early r ecovery phase (t = 10-12 h). Perhaps the impulsive stormtime convection ele ctric field was stronger during the main phase than we have assumed here. C omparisons were more difficult in late recovery phase (t = 20 h) because CR RES was too far off the magnetic equator. Proton life times inferred from t he CRRES data during the recovery phase of this storm are considerably shor ter than charge-exchange lifetimes for either model, but the empirical neut ral H density model of Rairden et al. [1986] leads to smaller discrepancies with the CRRES data at all the selected energies and L values than the the ory-based neutral H density model of Hedges [1994] for parameters that most closely represent the seasonal and solar maximum conditions of the August 19, 1991, storm. It appears that charge exchange alone is not enough to exp lain the observed rapid decay of the ring current proton intensities during the recovery phase of this storm.