Contribution of Sudden Solar Wind Compression to Substorm Triggering

The present study investigates the contribution of a sudden solar wind compression of the magnetosphere to the triggering of a substorm on 21 June 2007. We take into account the propagation of shock-associated disturbances from the magnetopause to the magnetotail. Previous studies that identified interplanetary shocks and/or discontinuities as substorm triggers have not examined this propagation in detail, which in this study can be determined by the THEMIS observations. A discontinuity across which the dynamic pressure increases by a factor of "`2 arrived at the magnetopause near 1245 UT. The magnetic field pointed northward in both sides of the discontinuity. The Double Star Program Tan Ce 1 detected the arrival near 1245 UT in the dayside magnetosheath on the dawn side and Cluster at 1248 UT in the flank magnetosheath on the dawn side. Cluster observations indicate that the shock normal was inclined duskward by about 45 deg. Compression-associated magnetic and electric field disturbances were observed by GOES 9 at 1245:50 UT, GOES 12 at 1245:53 UT, and GOES 11 near 12:46:30 UT on the dawn side and by THEMIS near 1246:30 UT on the dusk side, consistent with the duskward inclined shock normal. Polar/UVI observed an auroral breakup at "`1247:00 UT and "`23 MLT followed by a poleward expansion. LANL observations at "`03 MLT show a slightly dispersed electron injection beginning at "`1249:00 UT. We discuss whether or not the disturbances propagating from the dayside magnetopause had the potential to trigger the substorm. The propagation speed and direction of the compression-related disturbances are estimated from local measurements by THEMIS as well as time lags between the GOES spacecraft. The estimate enables us to calculate when the disturbances arrived at a substorm onset site which we determine from aurora intensification, electron injection, and high-latitude negative bays.