Simulation of the geospace response to a sudden change in IMF orientation.

Abstract:

We have conducted simulations of the response of the geospace system to a sudden change in the orientation of the interplanetary magnetic field (IMF) using the Lyon-Fedder-Mobarry (LFM) global magnetohydrodynamic code. Specifically, we have explored a sudden change in the IMF orientation from an extended period during which it was steady northward, to a steady southward IMF. The change in the IMF orientation first causes a reversal in the direction of the bow shock current, which launches a fast mode wave that propagates through the system, causing changes in the overall current pattern even before the southward IMF arrives at the dayside magnetopause to initiate low latitude merging. When the southward IMF does arrive at the magnetopause, the preceding northward IMF in the solar wind flow is still driving high latitude merging poleward of the cusp. Even when the two-cell convection pattern in the ionosphere becomes the dominant convection, the LFM results show that there is still some remnant of the high latitude reverse cell convection that is associated with northward IMF that had previously merged with the geomagnetic field and which takes some time to be cleared out of the system. We will present a detailed account of the timescales and changes in the magnetic topologies, current systems, magnetospheric plasma flows, and ionospheric potential patterns associated with the transition from northward to southward IMF. We will also discuss the implications of these findings for understanding the effect of transient changes in IMF orientations.