Time-dependent magnetospheric configuration and breakup mapping during a substorm

摘要

We analyze an isolated substorm on 29 March 2009 observed by the Thermal Emission Imaging System (THEMIS) and well monitored by ground-based observatories at and near station Gillam. The event provides a rare opportunity for monitoring 'the substorm magnetic topology thanks to fortuitous clustering of the THEMIS probes, complemented by the GOES 12 spacecraft. The neutral sheet position was found to be displaced by ～0.5 RE northward from its average location. The peak cross-tail current density was estimated to be ～20 nA/m~2 at the end of the growth phase, revealing the formation of a thin current sheet during the last 15 min prior to the expansion onset. The fortuitous spacecraft conjunction allowed us to construct an adjusted time-varying model based on magnetic field and pressure observations during the substorm. We then used the adjusted model to map the location of the spacecraft to the ionosphere and the breakup from the ionosphere to the equatorial region. Significant time-dependent differences between this and the standard models (e.g., T96) do exist, resulting in breakup mapping to ～22 R_E, compared to 12 R_E if classical models are used. Moreover, we find that spacecraft footprints in the ionosphere move significantly equatorward (2°) over tens of minutes during the growth phase but jump poleward (2°-4°) after expansion onset. Since such motions are also typical for auroral arcs during substorms, we infer that magnetic field reconfiguration during various substorm phases, rather than plasma motion in the equatorial magnetosphere, is largely responsible for the observed motion of the aurora.