IMF control of cusp aurora location during multiple transpolar arcs

It is well-known that the dayside tip of a transpolar arc (TPAs) typically merges with the auroral cusp. Studying the location of the auroral cusp during TPA events allows us to better understand how the evolution of transpolar arcs is coupled to processes along the dayside magnetopause.

This work is based on 12 months DMSP SSUSI images. Only those images are taken into account, where near-simultaneous SSUSI images exist from both hemispheres. We identified several tens of cases where the auroral cusp is clearly visible while multiple TPAs appear simultaneously in at least one hemisphere. The results show that the cusp location during TPAs depends clearly on the interplanetary magnetic field (IMF) Bx and By components, and that the effect on the cusp location is opposite in the two hemispheres. While the longitudinal dependence is expected from previous studies, our statistical results show also a clear latitudinal dependence on IMF Bx and By. The best correlation with IMF Bx and By is found for summer hemisphere events.

Superposed epoch analysis plots show that in average, the auroral cusp becomes visible after IMF Bz drops from strongly to weakly northward IMF. Mapping the auroral cusp location to the magnetopause with help of the T96 magnetospheric model for different IMF inputs confirms what could be expected: the auroral cusp brightening appears after the magnetospheric cusp has moved from high to lower latitudes. Mapping results for different time shifts between IMF input and auroral signatures indicate a 15 min time delay between IMF and auroral cusp occurrence, which is in agreement with previous reports.