About definition of modes and magnetosonic heating in a plasma's flow: Especial cases of perpendicular and nearly perpendicular wave vector and magnetic field

Abstract

Dynamics of hydrodynamic perturbations in a plasma depend strongly on an angle between the wave vector and equilibrium straight magnetic field. The case of perpendicular propagation is especial. There are only two (fast) magnetosonic modes since two (slow) ones degenerate into the stationary one with zero speed of propagation. This demands individual definition of wave modes by the links of hydrodynamic relations. These links are not limiting case of the relations in the case of non‐zero angle. The nonlinear excitation of the entropy mode in the field of intense magnetosonic perturbations is also unusual. Bulk and shear viscosity and thermal conduction are considered as the damping mechanisms in a weakly nonlinear flow. The leading‐order dynamic equation is derived which governs perturbation of density in the entropy mode. The links of magnetosonic perturbations and magnetosonic heating may be indicators of plasma‐β$$ \beta $$, geometry of a flow, damping coefficients and type of wave motion. The “almost resonant” character of magnetosonic heating excited by the slow magnetosonic wave in the course of nearly perpendicular wave propagation, is discussed.