Testing electric field models using ring current ion energy spectra from the Equator-S ion composition (ESIC) instrument

During the main and early recovery phase of a geomagnetic storm on February 18, 1998, the Equator-S ion composition instrument (ESIC) observed spectra l features which typically represent the differences in loss along the drif t path in the energy range (5-15 keV/e) where the drift changes from being E x B dominated to being gradient and curvature drift dominated. We compare the expected energy spectra modeled using a Volland-Stern electric field a nd a Weimer electric field, assuming charge exchange along the drift path, with the observed energy spectra for H+ and O+. We find that using the Weim er electric field gives much better agreement with the spectral features, a nd with the observed losses. Neither model, however, accurately predicts th e energies of the observed minima.