The evolution of plane switch-on and switch-off shocks in the presence of small normal disturbances is examined. (Normal disturbances are those in which the perturbed quantities are functions only of time and the distance from the plane of the shock wave.) In the situation under discussion the magnetic diffusivity of the ambient gas is much greater than each of the viscous diffusivities and the thermal diffusivity.

It is shown that small normal disturbances can eventually cause appreciable changes in the flow pattern around either switch-on or switch-off shocks. Numerical computations are carried out for both a specific null switch-on and a specific null switch-off shock, in the presence of a known disturbance. The results of each of these computations, for large times, are in agreement with analytic, asymptotic solutions which are obtained.

A mechanism is suggested whereby switch-on and switch-off shocks, across which an appreciable deflexion of the direction of the magnetic field lines takes place, can adjust themselves to small normal disturbances.