Comparison of Iridium Determined Field-Aligned Current Patterns With High-Resolution MHD Simulations

The engineering magnetometers aboard the 70+ Iridium satellites arranged in six equally spaced polar orbital planes provide a unique database for determination of global field-aligned currents [Waters et al., 2001]. A previous study compared these field-aligned currents with MHD simulation results to quantitatively evaluate the MHD results in a global way [Korth et al., 2002]. The analysis of three events of prolonged steady interplanetary magnetic field orientation, stable to within 25 degrees of the average direction, revealed considerable differences between observed field-aligned current densities and the MHD simulations. The field-aligned current densities in the Lyon-Fedder MHD simulations were evaluated at an inner simulation boundary of 3 Re and mapped on dipole field lines to ionospheric altitudes. In the present study we expand on the previous work by moving the inner simulation boundary inward to 2 Re. The achieved increase in spatial resolution of the simulation grid results in a dramatically improved agreement of the simulated Region-1 currents with the observations. Moreover, the high-resolution MHD simulations lead to a better representation of the observed dayside Region-2 current system. However, the Region-2 currents show expectedly larger differences since ring current drift physics necessary to drive these currents in the magnetosphere is not implemented in the MHD evaluations. DMSP particle source identifications are used to compare source regions in the observed FAC maps with those in the MHD simulations.