MODE PROPERTIES OF LOW-FREQUENCY WAVES - KINETIC-THEORY VERSUS HALL-MHD

In fluid theory, the ordering of low-frequency modes in a homogeneous plasma is based on the phase velocity, since modes do not intersect ea ch other in dispersion diagrams as a function of wavenumber or other p arameters. In linear kinetic theory, modes cross each other. Thus a co nsistent and useful classification should be based on the physical pro perties of the modes instead. This paper attempts such a classificatio n by documenting the dispersion and general mode properties of the low -frequency waves (omega much-less-than (OMEGA(ci)OMEGA(ce))1/2, where OMEGA(ci), OMEGA(ce) are the cyclotron frequencies of the ions and ele ctrons, respectively) in kinetic theory, and by comparing them to the results of two-fluid theory. Kinetic theory gives a separate Alfven/io n-cyclotron (A/IC) wave with phase speed omega/k almost-equal-to v(A) cos theta for omega much-greater-than OMEGA(ci), where v(A) is the Alf ven velocity and theta the angle of propagation between wave vector k and background magnetic field B(o). For a given wavenumber, the magnet osonic mode is a double-valued solution with a singular point in theta , beta parameter space, where beta is the ratio of thermal pressure to magnetic pressure. It is shown that a branch cut starting at the sing ular point theta approximately 30-degrees, beta approximately 3 and le ading to larger beta gives a practical and consistent separation of th is double-valued magnetosonic solution. Selection of this branch cut r esults in a moderately damped fast/magnetosonic and a heavily damped s low/sound wave. A comprehensive review of the polarization, compressib ility and other mode properties is given and shown to be consistent wi th the selected branch cut. At small wavenumbers, the kinetic mode pro perties typically start to deviate significantly from their fluid coun terparts at beta approximately 0.5. At larger beta, there is no longer a consistent correspondence between the fluid and kinetic modes. Kine tic theory also dictates the use of different mode properties to disti nguish between them in observational data. For example, the phase betw een the density and magnetic field perturbations may become useless at high beta, whereas the direction of the magnetic field perturbation w ith respect to k and B(o) remains a useful characteristic. Two quantit ies based on this characteristic are suggested and are shown to be use ful also to distinguish between the mirror mode and A/IC waves in a pl asma with temperature anisotropy.