A1

講演予稿:
　太陽風の条件の突然の変化が、サブストームの開始をトリガーで きるかどうかは磁気圏物理において最近興味が持たれるようにな ってきた。特に、惑星間磁場の北向きへの変化がサブストームを トリガーする可能性について多くの経験的な研究が行われた。私 たちはその可能性を太陽風と地球磁気圏相互作用の３次元グロー バルＭＨＤシミュレーションから調べた。ＩＭＦを北向きから南 向きに変化させて、南向きを保つ場合と約４０分後再び北向きに 変化させた場合の尾部リコネクションの開始時間の差は大きくは なかった。ＩＭＦのみの北向き変化は、サブストームのトリガー にＭＨＤモデルの範囲では効果的ではないことが示唆される。

abstract:
　Recently, there has been a strong interest in determining whether abrupt changes in solar wind conditions can trigger substorm expansion onsets. In particular, the triggering capability of northward IMF turnings has been the topic of many empirical studies. Here, the question is addressed from the standpoint of global magnetohydrodynamic (MHD) simulation. A set of MHD simulation runs have been compiled to test whether step-like changes in solar wind dynamic pressure and/or IMF Bz conditions can trigger an early onset of the substorm expansion phase. A reference simulation is run to estimate the standard length of a substorm growth phase. The growth phase duration is found by measuring the length of the time interval marked at the beginning by southward turning of the IMF at the dayside magnetopause and at the end by formation of the near-Earth neutral line. The baseline simulation run is initiated by setting the IMF northward at +5 nT for 4-hr then by flipping the IMF southward to -5 nT. For this run, the expansion onset time occurred about 60-min after southward IMF reached the nose of the magnetosphere. In later simulations, a solar wind trigger (a northward IMF turning and/or solar wind pressure step) is introduced at the 40-min mark of the baseline run and diagnostics are checked to see if the onset timing is affected. For a northward turning without an associated solar wind pressure change, the results show that onset timing is unchanged. This suggest that northward triggers are not effective expansion phase triggers for an MHD magnetosphere. In fact, the dynamics following onset for the northward turning case are weaker, the associated flows are slower and a smaller plasmoid is produced which stalls in the deep tail. The simulation run with a step-like solar wind pressure increase (also introduced 40-min after the IMF southward turning) induces the near-Earth neutral line to form about 10 minutes faster the formation time found for the reference case run with no external trigger. The onset time delay is shortened by compression of the tail plasma sheet as the magnetosphere reconfigures in response to the increase in solar wind dynamic pressure. A case run with a solar wind pressure increase and a northward turning, both introduced at the 40-min time step, produces an accelerated onset time matching that for an isolated pressure step change.

キーワード:
サブストーム開始、太陽風の変化、惑星間磁場の北向き変化、電磁流体力学的シミュレーション、トリガー

Keywords:
substorm onset, solar wind pressure changes, IMF northward turnings, MHD simulation, triggering