荻野/竜樹

講演予稿:
　北向ＩＭＦで定常な磁気圏構造が得られている時、ＩＭＦを南向に すると磁気リコネクションの起る場所はカスプの後から太陽直下点 に移り、磁束が昼側から夜側に運ばれローブに蓄積される。プラズ マシートは薄くなって約１時間後に尾部リコネクションが起る。そ の尾部リコネクションを太陽風磁気圏相互作用の高分解能グローバ ルＭＨＤシミュレーションから調べた。尾部リコネクションの開始 直前では、プラズマシートは極端に薄くなり、ダイポール型からテ ール型への遷移が急峻に起る。尾部リコネクションは、地球方向と 尾部方向への強い流れを数分で発生させる。その地球方向の流れは 最初の２分間程度は圧縮的であり、発散が零ではない。

abstract:
　When the interplanetary magnetic field (IMF) suddenly turns southward in the quasi-steady magnetospheric configuration for a northward IMF, reconnection site shifts from the high latitude tail to the subsolar region at the magnetopause to carry magnetic flux from the dayside magnetosphere to the tail lobes. Plasma sheet becomes thinner and thinner and tail reconnection starts to occur at x=-(15-20)Re about 1 hour later to generate strong earthward and tailward flow, which may imply expansion phase of substorms. This story on magnetotail dynamics was argued by many scientists and was already presented by several global MHD simulation works. However, the previous global MHD simulation did not have enough spatial resolution to discuss how much the plasma sheet became thin. We have studied the near-tail reconnection in detail by using a 3-dimensional global magnetohydrodynamic (MHD) simulation of interaction between the solar wind and the earth's magnetosphere with a high spatial resolution of 0.15Re grid spacing and (482,222,222) grid point. Moreover, we sampled 3-dimensional simulation data every 2 minutes and 2-dimensional data in the cross section every 0.5 minutes to watch full temporal variation of the near-tail reconnection. The number density and velocity of the solar wind are 46/cc-185/cc and 412km/s. The IMF suddenly turns from a northward IMF of Bz=18.2nT to a southward IMF of Bz=-18.2nT. Just before the onset of near-tail reconnection, the plasma sheet becomes very thin up to about 0.5Re. At the same time transition from dipole-like to tail-like field configuration occurs at about x=-6Re and becomes very sharp. The onset of near-tail reconnection generates strong earthward and tailward flow in a few minutes. The strong earthward flow is compressive in the first 2 minutes, that is, divergence of the earthward flow is not zero. Generation of the compressive earthward flow corresponds to sudden enhancement of divergent flow toward the lower latitudes in the midnight region in the polar cap. The divergent flow expands the auroral oval in the midnight region and simultaneously generates region 1 type field aligned currents due to vorticity and region 2 type field aligned current due to pressure gradient. The divergent flow soon decays and sunward convection is formed in the middle magnetosphere to return magnetic flux from the nightside to the dayside magnetosphere. A large plasmoid is simultaneously ejected down the tail. After the plasmoid ejection, tail reconnection does not occur in steady manner but does in intermittent and patchy manner. Tail reconnection intermittently occurs in a narrow region of plasma sheet and moves sides. A wavy structure appears at the dayside magnetopause and propagate down the tail to show oscillations with about 2-4 minute period.

キーワード:
尾部磁気再結合, ＭＨＤシミュレーション, 太陽風磁気圏相互作用, 磁気圏構造

Keywords:
Magnetotail Reconnection, MHD Simulation, solar wind-magnetosphere interaction, magnetospheric structure