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. Plain Language Summary: Sudden commencement (SC) is an impulse response of the magnetosphere‐ionosphere system caused by an abrupt change in the dynamic pressure of solar wind. However, the generation of SC‐associated field‐aligned currents (FACs) is not well understood. The magnetohydrodynamic (MHD) simulations are employed to investigate the FACs in response to the solar wind dynamic pressure pulse. A new method of tracing Alfvén wave packets is used to identify the generation region and mechanism of the FACs associated with the SCs in the simulation. The results show that the generation of preliminary impulse (PI) FACs is off the equator and related with the acceleration by magnetic pressure force; main impulse (MI) FACs are associated with the deceleration of plasma flow in the equatorial plane by magnetic tension force. The polarity of PI and MI FACs depends on two different mechanisms. Key Points: The origin of field‐aligned currents (FACs) associated with solar wind dynamic pressure pulse is identified by tracing a packet of Alfvén waves in MHD simulationPI FACs are generated off equator at leading edge of compressional wave and MI FACs are generated in equatorial plane behind leading edgeThe polarity of PI FAC is determined by vorticity, while that of MI FAC is determined by the dot product of current density and velocity [ABSTRACT FROM AUTHOR]