[P-EM28_29PM2] Magnetosphere-Ionosphere Coupling

[PEM28-19] Relative timing of substorm-associated magnetic reconnection in the magnetotail and formation of auroral onset arc

*Yukinaga MIYASHITA¹, Akimasa IEDA¹, Shinobu MACHIDA¹, Yasutaka HIRAKI², Vassilis ANGELOPOULOS³, James P. MCFADDEN⁴, H. uli AUSTER⁵, Stephen B. MENDE⁴, Eric DONOVAN⁶, Davin LARSON⁴ (1.STEL, Nagoya Univ., 2.NIPR, 3.Univ. of California, Los Angeles, 4.SSL, Univ. of California, Berkeley, 5.Technischen Universitat Braunschweig, 6.Univ. of Calgary)

Keywords:substorm, auroral onset arc, magnetotail, magnetic reconnection, plasmoid, GEMSIS

We have studied the relative timing of magnetic reconnection in the near-Earth magnetotail and formation of auroral onset arc, 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. Based on observations of plasmoids in the near-Earth magnetotail, we clearly show that magnetic reconnection is initiated at X〜-20 Re at least 1-3 min before the appearance of the auroral onset arc. This result suggests that magnetic reconnection plays some role in the formation of auroral onset arc.