Probing the magnetic structure of a pair of transpolar arcs with a solar wind pressure step

We present observations of the northern hemisphere auroras taken with the Far UV cameras onboard the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft during a compression of the magnetosphere by a solar wind pressure step on 30 December 2001. The compression occurs during a period of northward IMF which has given rise to the presence of a pair of transpolar arcs (TPAs) near the dawnside oval. The compression causes a brightening of the oval, from dayside to nightside over the course of 10 mins, followed by a brightening of the midnight sector oval and TPAs from nightside to dayside, again over 10 mins. We suggest that the brightening is caused by pitch angle scattering of particles trapped on closed magnetic field lines, and that the sequence of the brightening tracks the solar wind pressure step as it progresses along the length of the magnetotail. Travelling at 600 km s$^{-1}$, the step reaches up to 90 $R_E$ down-tail over the period of brightening, suggesting that the magnetic f...