Generation of Field‐Aligned Currents During Substorm Expansion: An Update
Generation of Field‐Aligned Currents During Substorm Expansion: An Update
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DOI：
10.1029/2022ja031011
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发表时间：
2023-01
期刊：
影响因子：
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通讯作者：
Y. Ebihara;Takashi Tanaka
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文献类型：
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作者：
Y. Ebihara;Takashi Tanaka
我们通过在全球磁流体动力学（MHD）模拟中从电离层中的起始位置沿时间向后追踪Alfvén波包，研究了在亚暴扩展阶段开始时突然增强的场向电流（FACs）的生成过程。产生区域位于近地等离子体片中，其中（a）方位角移动的等离子体拉动磁场线，并对磁张力执行负功以激发阿尔芬波，（B）根据安培和法拉第定律的要求产生FACs，以及（c）场垂直电流转换为FACs。我们称之为近地FAC发电机所涉及的等离子体起源于尾叶区域。当近地重联发生在等离子体片中时，等离子体被洛伦兹力加速向地球，并被等离子体压力梯度力减速，然后是洛伦兹力。在等离子体压力梯度力和洛伦兹力的作用下，气流向东西方向偏转，从而激发阿尔文波和FACs。阿尔文波在运动等离子体的静止坐标系中沿着磁场传播。当它到达电离层时，极光电喷流开始发展，亚暴扩展阶段开始。近地FAC发电机可以与近地发电机区分开来（J · E < 0，其中J是电流密度，E是电场）。我们认为，亚暴的演变可以理解的发展方面的FAC。
We investigated generation processes of field‐aligned currents (FACs) that are abruptly intensified at the beginning of the substorm expansion phase by tracing a packet of the Alfvén wave backward in time from the onset position in the ionosphere in the global magnetohydrodynamics (MHD) simulation. The generation region is found in the near‐Earth plasma sheet, in which (a) azimuthally moving plasma pulls the magnetic field line, and performs negative work against the magnetic tension force to excite the Alfvén waves, (b) FACs are generated from the requirement of Ampère and Faraday laws, and (c) field‐perpendicular current is converted to FACs. We call this near‐Earth FAC dynamo. The plasma involved originates in the tail lobe region. When near‐Earth reconnection occurs in the plasma sheet, the plasma is accelerated earthward by the Lorentz force, and decelerated by the plasma pressure gradient force, followed by the Lorentz force. The flow is deflected to the west and east directions by the plasma pressure gradient force and the Lorentz force, resulting in the excitation of Alfvén waves and FACs. The Alfvén waves propagate along the magnetic field in the rest frame of the moving plasma. When it arrives at the ionosphere, the auroral electrojet starts developing and the substorm expansion phase begins. The near‐Earth FAC dynamo can be distinguished from the near‐Earth dynamo (J · E < 0, where J is the current density and E is the electric field). We suggest that the evolution of the substorm can be understood in terms of the development of FACs.