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

摘要

We have investigated dynamical effects of an oscillating solar wind dynamic pressure (P sw ) on the magnetosphere-ionosphere (M-I) system using a global magnetohydrodynamic (MHD) numerical simulation. We find that a directly P sw -forced pulsation on the ground is strongly controlled by the P sw -induced plasma convection patterns in the M-I system and the associated three-dimensional current system. When a sinusoidal P sw oscillation with 10-min period is applied over the magnetosphere, it forces the magnetosphere to oscillate in the same manner. As the magnetosphere undergoes gradual compressed (expanded) state, a twin-vortex flow with clockwise (counterclockwise) in the morning and counterclockwise (clockwise) in the afternoon is excited well inside the dayside magnetosphere and then develops into large-scale vortices with a slow tailward motion. Each developed 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 oscillating ionospheric twin-vortex flows result in a global geomagnetic pulsation activity with a latitude-independent period at high latitudes. The major period of the global geomagnetic pulsation period matches that of the P sw oscillation. In either state, the main dynamo (J · E < 0) supplying the electromagnetic energy to the ionosphere via FACs is found to be driven by P sw -induced plasma flows across high-pressure regions around the magnetospheric cusp and in the dawnside/duskside equatorial plane. Our simulation indicates that, the ionospheric flow and FAC patterns in the compressed and expanded states of the magnetosphere are nearly mirror images of each other, while the dynamo regions and the three-dimensional current configurations in the magnetosphere do not always appear as mirror images.