CHARACTERISTIC SPEEDS IN HIGH-BETA ISOTROPIC ANISOTROPIC PLASMAS

Given the importance of linear mode properties (e.g., characteristic s peeds) in identification/classification of discontinuities, a detailed comparison between the mode properties in fluid theory and kinetic th eory in high beta plasmas is carried out. It is found that conventiona l fluid theories of linear modes in both isotropic and anisotropic pla smas do not yield the correct mode properties, even in the long-wavele ngth Limit. In particular, fluid phase velocities are very sensitive t o the model and parameters (polytropic indices) employed. Because of t his, fluid theory loses its predictive power. In linear kinetic theory , modes cannot be ordered according to their phase velocities. For ins tance, at small and moderate propagation angles, the slow/sound (S/SO) mode can have the fastest phase velocity. In such cases, a (quasipara llel) fast shock would be associated with the S/SO mode rather than th e usual fast/magnetosonic (F/MS) model. This has important implication s for fast shocks. Since it is the F/MS rather than S/SO mode that con nects to the whistler branch, low Mach number quasiparallel shocks ass ociated with S/SO would not be expected to have a phase standing whist ler wave train upstream, and their thickness is determined by dissipat ion rather than dispersion. The consequences of the kinetic mode prope rties are demonstrated via hybrid simulations (fluid electron, kinetic ions) using the quasiparallel shock as an example. (C) 1995 American Institute of Physics.