Multi-Spacecraft Observations of Possible Bow Shock Current Closure through Birkeland Currents

Abstract:

On the bow shock in front of Earth’s magnetosphere flows a large-scale current due to the curl of the interplanetary magnetic field (IMF) across the shock. The closure of this current remains uncertain; it is unknown whether the bow shock current closes with the Chapman-Ferraro current on the magnetopause, along magnetic field lines into the ionosphere, or through some combination of both. We present simultaneous observations from MMS, AMPERE, and DMSP during a period of strong IMF By, weakly negative IMF Bz, and very small IMF Bx. This IMF orientation should lead to a bow shock current flowing mostly from south to north on the shock. AMPERE observations show a current poleward of the Region 1 and Region 2 Birkeland currents flowing into the northern polar cap and out of the south, the correct polarity for bow shock current to be closing along open field lines. A southern hemisphere DMSP F18 flyover confirms that this current is poleward of the convection reversal boundary. Additionally, we investigate the bow shock current closure for the above-mentioned solar wind conditions using an MHD simulation of the event. We conclude that the evidence points to at least a partial closure of the bow shock current through the ionosphere.