Increased destabilization of the parallel velocity shear instability due to reversed magnetic shear

The nonlinear behavior of the parallel velocity shear instability in a shear magnetic field is studied. It is found that the nonlinear fluctuation levels and turbulent momentum transport depend strongly upon the direction of the magnetic shear. When the shear has the same sign as the second derivative of the parallel velocity with respect to the radial coordinate, the fluctuations grow to larger levels than if there were no magnetic shear. The physical mechanism controlling this effect is vortex merging between modes on either side of the velocity peak. It is likely that this behavior may be a general feature of all modes with structure parallel to the magnetic field. [S0031-9007(98)07377-3].