Comparison Between Observed High Latitude Field-Aligned Current Systems and Predictions from Global Simulations

We compare predictions of high latitude field-aligned current (FAC) systems and the associated convection patterns obtained with global magnetohydrodynamic (MHD) simulations with observations at ~4000 km altitude with the Fast Auroral Snapshot Explorer (FAST). Field line tracing within the MHD simulations is used to define the magnetic field topology, and the open-closed field line boundary. By comparing the model results with the particle and field observations we are able to determine if the high latitude field-aligned currents are on open or closed field lines. This cannot be determined from FAC observations alone, and particle observations, while strongly indicative, can be ambiguous. (An absence of plasmasheet precipitation could be an indicator of open field lines, or simply flux tubes that have lost particles as they convect from the nightside to the dayside.) At the same time, the simulations can also be ambiguous, as the FAC morphology depends on the magnetic reconnection at the magnetopause, which in turn depends on the assumed resistivity. Thus the observations and simulations should be viewed as a whole, with an eye towards self consistency. For example, comparisons between data and simulations for the September 24/25, 1998, storm time interval indicate that while the Interplanetary Magnetic Field (IMF) was weakly northward, polar cap convection was dominated by the effects of a strong IMF y-component. The FACs observed by FAST on open field lines were found to be an extension of the duskside Region-1 current into the polar cap.