DYNAMICS OF SHEAR VELOCITY LAYER WITH BENT MAGNETIC-FIELD LINES

A fully three-dimensional, magnetohydrodynamic simulation of velocity- sheared plasma flow in an ambient transverse magnetic field with bent magnetic field lines has been performed. ''Ionospheric-like'' boundary conditions were used for closing field-aligned currents, the two iono spheres being represented by conducting plates with constant resistivi ty. Compared to the standard plane 2D case with a uniform transverse m agnetic field, the growth rate of the Kelvin-Helmholtz instability dro ps significantly as bending increases. Under conditions representative of the Earth's low latitude boundary layer, the instability may be su ppressed completely by the magnetic field-line tension if the field-li ne bending is sufficiently strong. For weak bending, a combination of the tearing mode instability and the Kelvin-Helmholtz instability lead s to the formation of localized 3D current/vortex tubes, the ionospher ic foot prints of which are possible models of the auroral bright spot s observed by the Viking satellite.