[P-EM10] Dynamics of Magnetosphere and Ionosphere

[PEM10-P01] Generation of field-aligned currents intensified at substorm expansion onset

*Yusuke Ebihara¹, Takashi Tanaka² (1.Research Institute for Sustainable Humanosphere, Kyoto University, 2.Faculty of Science, Kyushu University)

Keywords:substorm, field-aligned current, MHD simulation

We investigated generation region and processes of the field-aligned currents (FACs) that are abruptly intensified at the beginning of the substorm expansion phase. Tracing a packet of the Alfvén wave backward in time from the onset position in the ionosphere in the global magnetohydrodynamics (MHD) simulation, we found the generation region in the near-Earth plasma sheet, in which (1) azimuthally moving plasma pulls the magnetic field line, and performs negative work against the magnetic tension force to excite the Alfvén waves, (2) FACs are generated from the requirement of Ampère and Faraday laws, and (3) field-perpendicular current is converted to FACs. We call this near-Earth FAC dynamo. When near-Earth reconnection occurs in the plasma sheet, the plasma originating in the tail lobe region is accelerated earthward by the Lorentz force, and decelerated by the plasma pressure gradient force, followed by the Lorentz force. This process is known as flow braking. 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. We suggest that the evolution of the substorm can be understood in terms of the development of FACs. The growth phase begins when the flank FAC dynamo start working, and the expansion phase begins when the near-Earth FAC dynamo start working.