ION-TRANSPORT AND LOSS IN THE EARTH QUIET RING CURRENT .1. DATA AND STANDARD MODEL

We have studied the transport and loss of ions in the Earth's quiet time ring current, comparing the standard radial diffusion model developed for the higher-energy radiation belt particles with measurements of the lower-energy ring current ions. We compiled a data set with full local time coverage from the quietest days seen by the AMPTE/CCE/CHEM instrument in near-equatorial orbit at L=2-9 R(E). This data set provides, for the first time, ionic composition information in an energy range that includes the bulk of the ring current energy density, 1-300 keV/epsilon. Protons were found to dominate the quiet time energy density at all altitudes, peaking near L approximately 4 at 60 keV cm-3, with much smaller contributions from O+(1 - 10%), He+(1 - 5%), and He++ (< 1%). The proton densities were azimuthally symmetric excepting a small dawn-dusk distortion caused by the cross-tail electric field, and a plasma sheet contribution for L > 6 near midnight. Thus the standard radial diffusion model, which incorporates an outer source boundary at 7.5 R(E) from the Earth, and diffuses ions earthward while undergoing charge exchange and Coulomb energy loss, should fit the data. We improved on previously used model loss processes by incorporating the latest atomic physics cross sections from the literature, updating the last survey done 15 years ago. We also included the effects of finite electron temperature on Coulomb drag. A chi2 minimization procedure was used to fit the amplitudes of the standard electric radial diffusion coefficient, giving D(LL)E = 5.8 x 10(-11) R(E)2/s. Yet the model was unable to fit the data (to within a factor of 10) over 50% of the energy and radial ranges of the data set, particularly at L <4 or E< 30 keV. Assuming that the loss terms in the model are correct, the data can be inverted to extract a radial diffusion coefficient that had nearly constant amplitude from 2-7 R(E). This suggests that another transport mechanism is operating in the ring current region, which is strongest at smaller radial distances. We speculate that fluctuating ionospheric electric fields may be the source of this additional diffusion.