Generation of Field‐Aligned Currents During Substorm Expansion: An Update.

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. Plain Language Summary: Abrupt intensification of field‐aligned currents (FACs) is one of the manifestations of substorms, which result in bright aurora and auroral electrojet. The FACs are also important to supply energy to the polar ionosphere, in which a large amount of energy (∼1011 W) is consumed during the substorms. The origin of the substorm‐associated FAC is a long‐lasting issue, and not well understood. To identify the processes involved in the generation of the substorm‐associated FACs, we performed global magnetohydrodynamics (MHD) simulation, and traced a packet that is supposed to carry the perturbations associated with the FACs. With this new method, we found that the substorm‐associated FACs are generated in the near‐Earth region near midnight where the plasma pulls the magnetic field lines to excite the Alfvén waves carrying FACs. A series of processes from the near‐Earth reconnection to the sudden intensification of the auroral electrojet (auroral breakup) becomes clearer than ever. Key Points: We identify the origin of substorm‐associated field‐aligned current (FAC) by tracing a packet of Alfvén waves in a global MHD simulationThe FAC is generated in near‐earth plasma sheet where azimuthally moving plasma pulls magnetic field lines (near‐Earth FAC dynamo)The evolution of the substorm is governed by FAC dynamos; flank FAC dynamo for growth phase and near‐Earth FAC dynamo for expansion phase [ABSTRACT FROM AUTHOR]