Magnetic mapping effects of substorm currents leading to auroral poleward expansion and equatorward retreat

Abstract

Magnetotail fast flows, magnetic field dipolarization, and its relaxation are linked to auroral brightening, poleward expansion, and equatorward motion during substorm onset, expansion, and recovery, respectively. While auroral brightening is often attributed to the field-aligned currents produced by flow vorticity and pressure redistribution, the physical causes of auroral poleward expansion and equatorward retreat are not fully understood. Simplistically, such latitudinal changes can be directly associated to the tailward motion of the flux pileup region and the earthward flux transport toward the dayside that depletes the near-Earth plasma sheet. However, because the equatorial magnetic field profile and the magnetospheric field-aligned current system change significantly, mapping is severely distorted. To investigate this distortion, we superimpose a substorm current wedge model (dynamically driven by ground-based observations) on the global Tsyganenko model T96 during an isolated substorm on 13 February 2008, observed by the Time History of Events and Macroscale Interactions during Substorms and GOES 10 spacecraft and by ground all-sky imagers. We validate our model by showing that the timing and ionospheric projection of the flux pileup region and flow bursts observed at the spacecraft match auroral activations. We then use the improved mapping enabled by the model to demonstrate that in this event, auroral poleward expansion and equatorward retreat are mainly caused by substorm-current-wedge-induced mapping changes.