Simulation of off-equatorial ring current ion spectra measured by Polar for a moderate storm at solar minimum

Abstract

We use our kinetic drift-loss model to study ring current dynamics during moderate geomagnetic activity at the minimum phase of the current solar cycle, on March 20-22, 1996. Interplanetary conditions monitored by Wind during this period show fluctuations about zero in the (GSM) Bz component, which are due to Alfvén waves in a stream-stream interaction region. The corresponding Dst index shows general activity punctuated by 2 moderate storms (min Dst ≈-50 nT and ≈-70 nT, respectively) of ∼15 hours duration each. We calculate the off-equatorial ring current proton spectra in the energy range 100 eV to 300 keV and compare them with measurements provided by the HYDRA, TIMAS, and CEPPAD instruments on the Polar spacecraft at MLT≈2 and MLT≈14. We find reasonable overall agreement. An eastward offset of 3 hours in the Kp-dependent Volland-Stern convection electric field with γ=2 is necessary in order to match modeled with observed dips in the energy spectra. Collisional losses have larger effect on the postnoon spectra and reduce significantly the low-energy proton distribution at low L shells and at higher magnetic latitudes. These losses reach maximum values for slowly drifting (∼2-10 keV) particles. Our study suggests that a more realistic magnetospheric electric field and additional diffusion processes should be considered to reduce the overestimated width and depth of the dip in the modeled dayside spectra. Copyright 1999 by the American Geophysical Union.