Dynamics of shear velocity layer with bent magnetic field lines

A fully three‐dimensional, magnetohydro‐dynamic 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 ionospheres being represented by conducting plates with constant resistivity. Compared to the standard plane 2D case with a uniform transverse magnetic field, the growth rate of the Kelvin‐Helmholtz instability drops significantly as bending increases. Under conditions representative of the Earth's low latitude boundary layer, the instability may be suppressed completely by the magnetic field‐line tension if the field‐line bending is sufficiently strong. For weak bending, a combination of the tearing mode instability and the Kelvin‐Helmholtz instability leads to the formation of localized 3D current/vortex tubes, the ionospheric foot prints of which are possible models of the auroral bright spots observed by the Viking satellite.