A numerical simulation of the geomagnetic sudden commencement: 1. Generation of the field‐aligned current associated with the preliminary impulse

The magnetospheric response to a solar wind impulse, a geomagnetic sudden commencement, is studied using an MHD model of the coupled solar wind‐magnetosphere‐ionosphere system. This paper discusses propagation of the first signal launched by the impulse and generation of the field‐aligned current that causes the ground magnetic signal detected as the preliminary impulse (PI). It is revealed that the PI current is first excited as an enhanced Chapman‐Ferraro current in the magnetopause and next turns to the magnetosphere along the wavefront of the compressional signal launched by the impulse. It is finally converted to a field‐aligned current via mode coupling due to plasma nonuniformity. The current in the wavefront region is an inertia current. We present a quantitative model of the PI model presented by Araki [1994] by using a numerical simulation.