Response of ionospheric field-aligned currents to a sudden increase in the solar wind dynamic pressure: high-resolution LFM simulations and AMPERE observations

Birkeland currents, i.e., magnetic field-aligned currents linking the magnetosphere and ionosphere, are one of the primary manifestations of solar wind-magnetosphere-ionosphere (SW-M-I) coupling. Until recently, recovering their global morphology and dynamics has only been possible indirectly: either from ground-based magnetometers or from radar observations of ionospheric convection. With the development of the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), global distributions of the magnetic perturbations associated with the Birkeland currents can be inferred from in-situ measurements by the >70 Iridium satelites located in six, approximately equally-spaced, polar orbit planes. At the same time, recent advances in global magnetohydrodynamic (MHD) modeling of the SW-M-I system have allowed simulations yielding realistic FAC distributions, which compare favorably with AMPERE observations. In this presentation we investigate the response of the magnetosphere-ionosphere system to a sudden increase in the solar wind dynamic pressure as seen in high-resolution Lyon-Fedder-Mobarry (LFM) simulations and AMPERE observations. Both simulations and observations indicate the formation of multiple longitudinally-extended field-aligned current sheets in response to the dynamic pressure jump (arriving at the bow shock at ~15:00 UT, January 24, 2012) and the subsequent propagation of fast-mode waves through the magnetosphere. We analyze the simulation to identify the primary MHD mechanisms of generation of these complex current patterns. By flying virtual satellites through the MHD simulation, we help interpret the observations of the transient currents by fast-moving spacecraft and test quantitatively the simulation predictions of these signatures.