Magnetosonic jet flow

We present a nozzle equation for isentropic, magnetohydrodynamic (MHD) flow in a channel, where the critical flow speed is the magnetosonic speed. A similar equation for a jet from a young stellar object (YSO) is obtained, where the jet flow is produced from a toroidal field, embedded in an accretion disc, which surrounds a YSO. For at least one case, the critical flow point for a YSO jet occurs on or near the surface of the accretion disc. We use this result to model how the stability of the toroidal fields, in the jet region, may be enhanced by the ram pressure of the jet.