Attenuation of Magnetohydrodynamic Waves

Abstract

The existence of viscosity, thermal conductivity, and electrical resistivity results in the dissipation of energy in MHD waves; the wave energy is consequently absorbed and the wave amplitude progressively attenuated. Dissipation mechanisms cause the amplitude of the waves to vary slowly over a large scale length L. When L is greater than the wavelength λ, WKB solutions can be obtained to compare the absorption rates for the three modes of MHD waves. Among the three dissipation mechanisms, thermal diffusion plays the dominant role, while electrical resistivity has negligible effects. The absorption rate of slow waves is greater than that of fast waves by a factor of the order of 10, in agreement with Barnes's theory on strong Landau damping of slow waves in kinetic scale. On average, the absorption rates for fast and Alfvén waves are of the same order of magnitude; the absorption rate for slow waves is a few tens of times greater than that of the Alfvén wave. The slow-mode absorption rate is particularly strong when β is small. Thus, absorption of slow waves can contribute more to the heating and acceleration of the solar wind near the Sun than can the absorption of Alfvén waves and fast waves. The fast-mode and the Alfvén-mode fluctuations have nearly equal chances of surviving the dissipation mechanism. Fast-mode fluctuations could play a nonnegligibile role in interplanetary fluctuations.