Linear damping and energy dissipation of shear Alfvén waves in the interstellar medium

Abstract

The heating of the diffuse interstellar medium by the dissipation of interstellar shear Alfvén waves is an important process for the temperature balance of this gas phase. Following our earlier analysis for fast magnetosonic waves we calculate here the heating rate from the damping of interstellar shear Alfvén waves, because interstellar plasma turbulence most probably is a mixture of fast magnetosonic waves and shear Alfvén waves. Relating the Alfvénic magnetic field fluctuation power spectrum to the observed interstellar electron density fluctuation power spectrum we derive the heating rate allowing for a scale independent anisotropy of the power spectrum. Because the diffuse intercloud medium is a partially ionised medium, the shear Alfvén waves undergo different types of dissipation. Besides collisionless Landau damping we considered various damping mechanisms from collision-effects such as Joule dissipation, electron and ion viscosity and ion-neutral friction. For all individual damping processes we derive the respective damping rates as a function of wavenumber and propagation angles of the Alfvén waves. These damping rates then serve as input in the calculation of the associated heating rates of the inter-stellar medium which results from the integral over the product of the damping rate times the magnetic field fluctuation power spectrum allowing for a scale independent anisotropy of the power spectrum. We demonstrate that for isotropic turbulence and typical diffuse intercloud medium parameters ion-neutral friction provides the dominant contribution to the heating rate of about 10-29 erg cm-3 s-1, which is about four orders of magnitude smaller than the cooling rate of the diffuse intercloud medium. Heating by collisionless Landau damping is 13 orders of magnitude smaller than dissipation by ion-neutral friction, heating by Joule and viscosity dissipation is 10 orders of magnitude smaller. Apart from factors of order unity these heating rates also hold for predominantly parallel turbulence (∧ ≫ 1). In case of predominantly perpendicular turbulence (∧ ≪ 1) the heating rates from collisionless Landau damping and Joule and viscosity dissipation decrease ∞∧1/2, whereas the heating rate from ion-neutral friction increases ∞∧-s/2 as long as ∧ ≥ 10-6. However, compared to the heating rate by collisionless Landau damping of fast magnetosonic waves the heating rate of Alfvén waves is negligibly small. Hence, for the temperature balance of the warm intercloud medium heating from shear Alfvén waves is negligible compared to the heating by collisionless damping of fast magnetosonic waves.