Interaction of dipolarization fronts within multiple bursty bulk flows in global MHD simulations of a substorm on 27 February 2009

Abstract

[1] We performed a global MHD simulation of a well-studied substorm on 27 February 2009 (Runov et al., 2009) to understand the generation and large-scale evolution of dipolarization fronts within bursty bulk flows (BBFs). Conjugate, well-positioned Time History of Events and Macroscale Interactions During Substorms (THEMIS) observations from space and ground observatories provide significant constraints to the simulation model. The main substorm onset auroral brightening, at 0749 UT, was in the field of view of Fort Smith (FSMI), just poleward of a preexisting auroral arc. Two minutes later, the space probes recorded a sharp dipolarization front moving sunward, passing by THEMIS and traversing ∼10 RE along the magnetotail. Our global MHD model, OpenGGCM, driven by real-time solar wind/interplanetary magnetic field conditions, is able to reproduce the key features of these signatures. We show that the auroral breakup is caused by the strong flow shear and the flow vortices formed by the BBF flows. Rebound oscillations of the intruding BBF (consistent with recent observations by Panov et al. (2010a)) and filamentation of the front into 1 RE size undulations are superimposed on the flow pattern. Further investigation of the interaction of the BBF and the dipolarization fronts (DFs) reveals that an observed bipolar Bz signature ahead of the DF is due to the interaction between two distinct plasmas emanating from multiple X lines: antisunward-moving flux tubes from a reconnection region at ∼13 RE and sunward-moving dipolarization region within a BBF from a midtail reconnection region at ∼23 RE.