ON HYDROMAGNETIC CRITICAL LAYERS.

When a horizontal magnetic field B(z) is sheared vertically on a lengthscale L in a diffusionless fluid, critical layers occur at z//c where the local Alfven speed V(z//c) matches the phase speed c of the wave. However, when a vertical field B//z is introduced, all the critical layers disappear. The present study investigates the solution in the neighborhood of z//c when B//z/B is very small, in order to clarify the manner in which the vertical field annihilates the critical layers. It is found that the solution across the critical layer is adjusted in a thin magnetic layer whose thickness is determined by the parameter epsilon **2( equals U/ alpha V, where U, V are measures of the vertical and horizontal components of the Alfven velocity and alpha /L is the horizontal wavenumber). The influence of the vertical field on the resistive instabilities present in its absence is also examined.