A stationary convective-like mode in a plasma slab with magnetic shear

The combined effects of resistivity and viscosity on a magnetohydrodynamically unstable plasma slab are shown to be responsible for the existence of a stationary (marginal) mode. This mode gives rise to plasma convection in a shear free situation when non-linear effects are taken into account. In the case of a sheared magnetic field it still extends over the entire width of the slab and is driven by what would be an interchange localised about the resonant (k.B=0) surface in the absence of dissipation. The critical value of the plasma analogue of the Rayleigh number in hydrodynamics is calculated as a function of shear, mode wavenumber and dissipation (represented by the square of the Hartmann number)