Generation of Field-Aligned Currents in Response to Sudden Enhancement of Solar Wind Dynamic Pressure

We investigated the generation of field-aligned currents (FACs) in response to the sudden enhancement of the solar wind dynamic pressure by tracing backward in time a packet of the Alfvén wave in the global magnetohydrodynamic (MHD) simulation. The generation region is identified from three perspectives, including the continuity of the current, the energy conservation and the time rate of change in the FACs. The generation mechanism is found to be related with the tailward motion of the compressional wave, which is excited when the magnetopause is compressed due to the solar wind dynamic pressure pulse. The compressional wave with a high magnetic pressure center interacts with the Earth's dipole field and forms a protruding part of the wavefront near the equatorial plane. The leading edge of the merged magnetic pressure that pertains to the compressional wave and the Earth starts to generate preliminary impulse (PI) FACs off the equator, as the magnetic pressure force accelerates the plasma and magnetic field lines are bent (FAC dynamo 1). Main impulse (MI) FACs are generated behind the leading edge in the equatorial plane due to the enhanced magnetic tension force (FAC dynamo 2). Magnetic field lines would be extremely curved during the passage due to the increasing magnetic tension force. The polarity of PI FACs is decided by the parallel vorticity of plasma flow, and MI FACs are the result of enhanced perpendicular currents together with the plasma flow in the equatorial plane.