Dependence of poleward auroral and equatorward motion on substorm current wedge

Abstract:

Flux pileup from fast flows and dipolarization, physical processes in the magnetotail, cause auroral evolution (brightening, poleward expansion, and equatorward motion) in the ionosphere during substorms. Although such flows have been shown to produce auroral brightening, the causes of auroral poleward expansion and equatorward motion remain unclear. Two mechanisms, tailward movement of the pileup region and dipolarization of the substorm current wedge (SCW), are thought to contribute to auroral poleward expansion, but no study has addressed which mechanism makes the dominant contribution. The hypothesis that auroral poleward expansion is caused by the tailward-moving pileup region is based on the assumption of a steady magnetosphere. This assumption is not necessarily true during substorms, however, because dipolarization of the SCW changes magnetospheric configuration and thus ionospheric footprints (and mapping) of the flows. Because they lack a dynamic SCW, previous magnetospheric models are statistical and static.

We evaluated the dynamic effect of the SCW using a dynamic magnetospheric model in which the SCW is superimposed on Tsyganenko model. The current wedge is obtained from a recently developed inversion model using only ground magnetic field data as input, and model parameters are updated every minute. Applying our dynamic magnetospheric model to data from an isolated substorm observed by THEMIS and GOES 10 spacecraft and ground ASIs on 13 February 2008, we found that 1) our model predicts dipolarization at GOES 10 (it can predict near-Earth magnetic variations with ground data alone); 2) there is a good temporal correlation between successive auroral brightenings and flows; 3) flow footprints from our model are collocated with auroral poleward expansion and equatorward motion. These results suggest that in this event, auroral poleward expansion and equatorward motion are mainly caused by mapping changes in the dynamic magnetosphere by the SCW.