Dipolarization fronts and associated auroral activities: 1. Conjugate observations and perspectives from global MHD simulations

Earthward-propagating dipolarization fronts (DFs) are often found to be associated with magnetic reconnection and bursty bulk flows (BBFs) in the magnetotail. Recent THEMIS (Time History of Events and Macroscale Interactions During Substorms) probe observations have shown a DF propagating over 10 R-E from the mid-tail region to the near-Earth tail region, and THEMIS All-Sky Imager data show a north-south auroral form and intensification of westward auroral zone currents. In this study, we examine THEMIS in situ observations of DFs in the magnetotail and simultaneous observations of the proton aurora from ground-based CANOPUS (the Canadian Auroral Network for the OPEN Program Unified Study) Meridian Scanning Photometers (MSPs). We find that earthward-moving DFs are often associated with intensification of proton aurora when the THEMIS probes are conjugate to the meridian of the MSP. The proton auroral intensifications are transient and in some cases detached from the background proton precipitation. Just before the DFs, the ion distribution is anisotropic in the field-aligned direction (mostly earthward) and the ion energy increases. These observations suggest that plasma sheet protons can be reflected and energized by earthward-moving DFs as they propagate through the magnetotail. We postulate that this population of ions is the source of the proton auroral intensification observed on the ground. This conjecture is tested using our global MHD simulation results, where the proton precipitation is calculated with the field-line curvature (FLC) model. The MHD simulation results show that proton precipitation enhancement can be caused by compression of plasma by approaching DFs/BBFs, which is consistent with ion reflection at DFs. Thus, using the conjugate observations from THEMIS spacecraft and MSP in this study, we are able to directly link the magnetotail dynamics, i.e., dipolarization fronts, with ground auroral activities. However, understanding of DF-associated ion energization requires detailed test-particle simulations with an analytical magnetotail model, such as those in our companion paper.