[P-EM14] [EE] Dynamics in magnetosphere and ionosphere

[PEM14-21] Subsidence of Ionospheric Fast Flows Triggered by Magnetotail Magnetic Reconnection During Transpolar Arc Brightening

*Nowada Motoharu¹、フィアー ロバート²、グロコット エイドリアン³、史 全岐¹、張 春風¹、王 永福⁴、宗 秋剛⁴、魏 勇⁵、傅 綏燕⁴、濮 祖蔭⁴ (1.山東大学 威海校 空間科学与物理学学院 空間科学研究院 空間天気物理与探計測研究中心、2.サウザンプトン大学 天文物理学科、3.ランカスター大学 物理学科 宇宙惑星物理グループ 、4.北京大学 地球与空間科学学院 空間物理与応用技術研究所、5.中国科学研究院 地質与地球物理研究所 地球与行星国家重点実験室)

キーワード：Transpolar Arc, Magnetotail Reconnections, Ionospheric Flows, TRINNIs

A static transpolar arc (TPA), which extended from post-midnight to pre-noon, was seen on 16^th September 2001 in the northern hemisphere. The orientations of the Interplanetary Magnetic Field-B_z and -B_y components were dominantly northward and weakly dawnward, respectively, when the TPA began to brighten. Associated solar wind velocity, density and dynamic pressure were almost stable. The SuperDARN radars detected westward plasma flows whose range was between 0.4 km/s and 0.75 km/s along the poleward edge of the midnight-sector main auroral oval, suggesting that they were confined within closed field lines and identified as the ionospheric plasma flows associated with Tail Reconnection during IMF Northward Non-substorm Intervals (TRINNIs). These TRINNIs’ ionospheric fast plasma flows persisted for at least 50 minutes prior to an appearance of the TPA. Often, TRINNIs are observed even during the period when the TPA is present, but, in this case, the flows associated with TRINNIs subsided beforehand. Additional slower plasma flows, which might cross the open/closed polar cap boundary, were seen at the time of the TPA onset in the same magnetic local time sector as the nightside end of the TPA. These ionospheric flows suggest that magnetotail reconnection significantly contributed to the TPA formation, which is proposed by Milan et al. [2005]. We investigate how magnetotail reconnection actually occurred before and during the TPA appearance by calculating the Joule heating (E・J) based on a global MHD simulation. Evidence for heating associated with magnetotail reconnection was seen during an interval of TRINNIs’ fast flows on the midnight-sector main auroral oval prior to the TPA appearance, but no significant Joule heating due to nightside magnetic reconnection was found during the TPA brightening. This result suggests that the fate (absence or presence) of the TRINNIs’ fast flows on closed field lines (the midnight-sector main auroral oval) during the TPA brightening would depend on a scale of magnetic reconnection, that is, the width of the reconnection line.

Reference:
Milan, S. E., B. Hubert, and A. Grocott (2005), Formation and motion of a transpolar arc in response to dayside and nightside reconnection, J. Geophys. Res., 110, A01212, doi:10.1029/2004JA010835.