A Global Perspective of Substorm Onset

We present a case study of the 25 December 2015 substorm which occurred between 08:15 and 08:45 Universal Time. A fortuitous and unique alignment of several independent spacecraft missions near the Earth-Sun line together with ground based measurements, allows a comprehensive and global analysis of the substorm onset. During this interval, fast particle flows and field geometry consistent with magnetic reconnection were detected in the mid-tail region. An ejected plasmoid was observed by the lunar-orbiting ARTEMIS probes and a corresponding dipolarization signature was observed by the THEMIS spacecraft earthward of the reconnection site, which was determined to be approximately -33 RE. Ground signatures indicative of substorm activity were also observed by the THEMIS ground-based observatories during this interval. The MMS probes, which were in the dayside magnetosheath, detected a strong fluctuation in Bz, with a minimum near -35 nT, at 08:00 UT, consistent with the time delay required for propagation from the magnetosheath to the mid-tail. We analyze and discuss these fluctuations and propose that this strong southward component of Bz in the magnetosheath is possibly associated with the substorm trigger. We simulate the entire magnetosphere for this event using the SWMF/BATS-R-US model with a special, high-resolution grid. The simulations qualitatively agree with the observed substorm flows. The results of this work will be highly relevant to future solar wind observation missions, global-scale space weather models, and the ongoing effort to understand how solar wind energy is coupled to the space environment in near-Earth and at lunar distances.