Magnetospheric Source of a Transpolar Auroral Arc: Coupled SWMF Simulation results

We present the first coupled MHD-ring current simulation results that produce the global transpolar auroral arc phenomenon. We examine a unique observation of a midnight transpolar auroral arc that is produced during compressed magnetosphere conditions and persistent into an extended interval of southward IMF and substorm onset. The IMAGE satellite FUV-WIC camera observed the transpolar auroral arc in the southern hemisphere on 15 May 2005. The IMAGE observations show that the transpolar auroral arc originates at 24 MLT and stretches sunward across the polar cap to form a theta aurora with dusk-dawn motion that does not correspond with IMF By sign reversal or continuous magnitude decrease. Even though the theta aurora is typically a northern IMF phenomenon, the IMAGE observations show that the theta aurora persisted for almost an hour under disturbed geomagnetic conditions with peak AL below -1500 nT and Dst around -100 nT. We use the University of Michigan Space Weather Modeling Framework (SWMF) global geospace simulation to study the ionospheric conditions and magnetotail configuration throughout the observation period. Our SWMF simulation results show good agreement with the observed SYM-H and AL indices during the event interval. In the simulation, we identify peaks in Joule heating, precipitation, and anti-sunward flows in the region where the theta aurora is observed. We also demonstrate the temporal evolution of the open-closed field line boundary with respect to the observed theta aurora location, which suggests that the theta aurora is a closed field line phenomenon. We analyze the open-closed field line boundary mapping into the magnetotail and search for causes of precipitation within the simulation as well as analyze the hemispheric conjugacy of the event.