Transonic MHD flows
Flow in laboratory and astrophysical plasmas
We started the study of the effects of background flow on waves and instabilities of laboratory and astrophysical plasmas in Chapter 12. We also considered the modifications of the equilibrium caused by the flow. These modifications were rather trivial for plane shear flows, but considerable for rotating plasmas due to centrifugal forces. However, except for the forebodings of Chapter 18, the most substantial effects have not been faced yet. The adjective “substantial” on background flows obviously should refer to some standard on what is a sizeable velocity. For transonic gas dynamics, it is clear that the appropriate standard velocity is the sound speed. For the macroscopic description of plasmas, which incorporates the dynamics of ordinary gases, the three MHD speeds (slow, Alfvén and fast) collectively take over the role of the sound speed. This implies that trans“sonic” MHD flows will be characterized by different flow regimes depending on the speed of the background flow relative to those three MHD speeds. In addition, the relative direction of the background velocity, v0, with respect to the direction of the background magnetic field, B0, introduces an anisotropy in plasma dynamics that is not present in ordinary gas dynamics.