Ionospheric flows relating to transpolar arc formation

Abstract

Transpolar arcs are large‐scale auroral features which are observed within the polar cap when the IMF has a northward component. One leading candidate formation mechanism proposes that they are formed by reconnection in the magnetotail some time after a period of dayside reconnection with a non‐zero IMF BY component which introduces a twist into the magnetotail. As a result of the twist, the mechanism predicts that the return flows of the newly closed magnetic field lines are asymmetric about midnight; their direction should depend upon the IMF BY component in the hours beforehand and should be opposite in the northern and southern hemispheres. In this paper, we use data from the SuperDARN network of high‐latitude ionospheric radars to examine whether such ionospheric flows are present before the formation of 33 transpolar arcs. We find that the flows are present and in a manner that is consistent with the reconnection mechanism for 76% of the events; in the remaining few, the discrepancy can be attributed either to an uncertainty in the formation time determined for the arc (due to previous polar cap activity in the same local time sector) or due to the geometry of the radars which observe the backscatter.