Solar wind pressure forced oscillation of the magnetosphere-ionosphere coupling system: A numerical simulation of pressure-forced geomagnetic pulsations

We have investigated dynamical effects of an oscillating solar wind dynamic pressure (Psw) on the magnetosphere-ionosphere (M-I) system using a global magnetohydrodynamic (MHD) numerical simulation. When a sinusoidal Psw oscillation with a Pc5-range period (10 minutes) is applied over the magnetosphere, it forces the magnetosphere to oscillate in the same manner. We find that the tightly coupled M-I plasma convection and the associated three-dimensional current system play a crucial role in the Psw-forced M-I system process. As the magnetosphere undergoes gradual compressed (expanded) state, the large-scale twin-vortex flow is excited well inside the dayside magnetosphere and decays with moving tailward. Each twin- vortex flow pattern in the magnetosphere is mapped to the polar ionosphere via a pair of field-aligned currents (FACs), one flowing into (out of) the morning ionosphere and the other out of (into) the afternoon. Spatial and temporal variations of the corresponding ionospheric current pattern result in a global coherent Pc5 geomagnetic pulsation at high latitudes. In either state, the main magnetospheric dynamos maintaining the current system are induced around the cusp throat and in the dawn/dusk-side off-equatorial plane. In this simulation, it is shown that the ionospheric flow and FAC patterns in the expanded state are almost mirror images of them in the compressed state, but the dynamo regions and the three-dimensional current loop configurations in the magnetosphere are different between the two states.