IONOSPHERIC TRAVELING VORTEX GENERATION BY SOLAR-WIND BUFFETING OF THE MAGNETOSPHERE

Traveling ionospheric vortices observed near the polar cusp boundary have been interpreted as signatures of impulsive reconnection (or flux transfer events) on the magnetopause, but neither the sense of motion nor the phase speed of the disturbances is consistent with the assumed generation mechanism. An alternative interpretation as the ionospheric signature of the response to fluctuation solar wind pressure can account for the reported features of the ionospheric perturbations. The solar wind pressure perturbation establishes vortical flow on magnetopause flux tubes and drives a guided shear wave along polar cusp field lines into the ionosphere. The guided shear wave carries the ionospheric disturbance in horizontal flow, electric field, and field-aligned current is dipolar in structure, with the flow near the center predominantly meridional and of the order of 100 km s-1 for pressure perturbations reported as typical in the solar wind. The phase velocity is 3-10 km s-1 in the east-west sense as inferred from mapping of a disturbance carried along the magnetopause at the magnetosheath flow velocity. A significant conclusion of this analysis is that the ionospheric signature of a traveling front across which pressure changes monotonically is a pair of vortices of opposite polarizations. A pressure pulse will produce two pairs of vortices, with the sense of polarization reversed in the trailing pair. Compressional perturbations of the boundary cannot produce isolated vortical flows in the ionosphere, although temporally very nonsymmetric perturbations could produce signatures that appear to have such symmetry.