Transonic Magnetohydrodynamic Flows in Laboratory and Astrophysical Plasmas

Abstract

Magnetohydrodynamic (MHD) waves control the dynamics of plasma, the main constituent of the universe. They occur as the natural response to global excitation. Frequency and wave forms are determined by the magnetic confinement geometry and distribution of background equilibrium variables. Hence, measurement of the spectrum of MHD waves gives direct information on the internal state of the plasma, provided a theoretical model is available to solve the forward and inverse spectral problems. This activity has been called MHD spectroscopy, [1, Goedbloed et al. Phys. Control. Fusion 35 B277 (1993)] in analogy with quantum mechanical spectroscopy which also involves eigenvalue problems of linear operators. The terminology also entails a program, viz. to improve the accuracy of our knowledge of plasmas, both in the laboratory and in astrophysics.While such a program is highly desirable a new angle to the study of MHD waves coming from the effects of transonic background flows requires a complete revision of all previous spectral results. Transonic flows in laboratory plasmas result from neutral beam heating (causing toroidal rotation) whereas they are commonplace for astrophysical plasmas (flows in coronal flux tubes, stellar winds, rotating accretion disks, jets emitted from radio galaxies, etc.). Such flows upset the standard theoretical approach to plasma dynamics, consisting of a separate study of equilibrium state and of the perturbations of this background. We will discuss a new approach to the dichotomy consisting of a study of the similarities of the nonlinear stationary flow patterns and the different linear waves structures that occur when the background speed traverses the full range of critical speeds (from "slow magnetosonic" to "Alfvén" to "fast magnetosonic"). This requires the development of new computational tools which yield a plethora of new waves and instabilities. We will illustrate this with results on tokomak and accretion disk plasmas.