Heating of the Interstellar Diffuse Ionized Gas via the Dissipation of Turbulence
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Abstract
We have recently published observations that specify most of the turbulent and mean plasma characteristics for a region of the sky containing the interstellar di use ionized gas (DIG). These observations have provided virtually all of the information necessary to calculate the heating rate from dissipation of turbulence. We have calculated the turbulent dissipation heating rate employing two models for the interstellar turbulence. The rst is a customary modeling as a superposition of magnetohydrodynamic waves. The second is a uid-turbulencelike model based on the ideas of Higdon. This represents the rst time that such calculations have been carried out with full and specic interstellar turbulence parameters. The wave model of interstellar turbulence encounters the severe difficulty that plausible estimates of heating by Landau damping exceed the radiative cooling capacity of the interstellar DIG by 34 orders of magnitude. Clearly interstellar turbulence does not behave like an ensemble of obliquely propagating fast magnetosonic waves. The heating rate due to two other wave dissipation mechanisms, ion-neutral collisional damping and the parametric decay instability, are comparable to the cooling capacity of the di use ionized medium. We nd that the uid-like turbulence model is an acceptable and realistic model of the turbulence in the interstellar medium once the e ects of ion-neutral collisions are included in the model. This statement is contingent on an assumption that the dissipation of such turbulence because of Landau damping is several orders of magnitude less than that from an ensemble of obliquely propagating magnetosonic waves with the same energy density. Arguments as to why this may be the case are made in the paper. Rough parity between the turbulent heating rate and the radiative cooling rate in the DIG also depends on the hydrogen ionization fraction being in excess of 90% or on a model-dependent lower limit to the heating rate being approximately valid. We conclude that the dissipation of turbulence is capable of providing a substantial and perhaps major contribution to the energy budget of the di use ionized medium.