This proposal is a request for the continuation of a 1 year 1991 award. We propose to use data obtained from the coordinated GEM data campaigns to test and refine the open-magnetosphere model of Toffoletto and Hill ?1989! with respect to the mapping of the magnetospheric cusp to low altitude. The model provides quantitative predictions of the local time extent and location of the equatorward boundary of the cusp and the direction and strength of ionospheric convection as functions of the cross- polar-cap potential, IMF direction and solar wind dynamic pressure. The equatorward boundary of the cusp is provisionally defined as the ionospheric footprint of that portion of the dayside merging line that lies sunward of the magnetosheath sonic line. We plan to do model-data comparisons as a means of testing the model's assumptions and boundary conditions; this test will provide a guide for setting an agenda for model improvements, some of which are currently under development. Ultimately, we expect to develop a model of cusp ion precipitation, calculated by tracing a distribution of particles from the dayside merging line to ionospheric altitude within the large-scale magnetic and electric fields specified by the model.
