Relative Timing of Substorm-Associated Processes in the Near-Earth Magnetotail and Development of Auroral Onset Arc

Monday, 15 December 2014
Yukinaga Miyashita1, Akimasa Ieda1, Shinobu Machida1, Yasutaka Hiraki2, Vassilis Angelopoulos3, James P McFadden4, Hans Ulrich Auster5, Stephen B Mende4, Eric Donovan6 and Davin E Larson4, (1)Nagoya University, Nagoya, Japan, (2)National Institute of Polar Research, Tokyo, Japan, (3)University of California Los Angeles, Los Angeles, CA, United States, (4)University of California Berkeley, Berkeley, CA, United States, (5)Technical University of Braunschweig, Braunschweig, Germany, (6)University of Calgary, Calgary, AB, Canada
We have studied the relative timing of the processes in the near-Earth magnetotail and development of auroral onset arc at the beginning of the expansion phase, based on substorm events observed by the THEMIS spacecraft and ground-based all-sky imagers. The THEMIS all-sky imagers can observe auroras over a wide area with temporal and spacial resolutions higher than spacecraft-borne cameras. This enables us to investigate the timing of auroral development in more detail than before. A few min after the appearance and intensification of an auroral onset arc, it begins to form wave-like structure. Then auroral poleward expansion begins another few min later. THEMIS magnetotail observations clearly show that magnetic reconnection is initiated at X~-20 Re at least 1-2 min before the intensification of auroral onset arc. Then low-frequency waves are excited in the plasma sheet at X~-10 Re 2 min before dipolarization, which is simultaneous with the formation of auroral wave-like structure. Dipolarization begins at the same time as the auroral poleward expansion. These results suggest that near-Earth magnetic reconnection plays some role in the development of dipolarization and auroral onset arc.