Abstract

A substorm onset at approximately 10 UT on December 12 2007 was well observed from both the THEMIS constellation and ground observatories in eastern Canada, most emplaced to support the THEMIS project. Clear step-like changes in the solar wind parameters on this day aid in relating them to magnetospheric responses. With the large set of ground magnetometers available, quantitative inversion can be done using the Automated Regional Modelling (ARM) technique which includes both auroral zone and subauroral stations. Not only can the substorm current wedge parameters be well determined, but the response of the driven system morning electrojet to southward IMF during the growth phase can be quantified. A clear subauroral Y component magnetic bay observed in eastern North America before onset can be attributed to this driving, as can stretching at GOES 11 (W) and evening sector bays. The onset appears to have been triggered by a northward turning. Analysis of tail response to the onset must take into account the fact that simultaneously the solar wind northward velocity component reversed from approximately -30 km/s to +40 km/s. The THEMIS C and B spacecraft, located 12 and 14 Re downtail, respectively, showed slow magnetic field X component reversals around the time of onset that are most likely mainly related to the solar wind Vz velocity blowing the tail across the spacecraft, as opposed to direct effects of the substorm. This result is confirmed by MHD modeling and studied using an event-oriented mapping approach.