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Cosmic Ray Heating of the Warm Ionized Medium
Wiener, Joshua ( Department of Physics, University of California, Santa Barbara, CA 93106, USA ); Zweibel, Ellen G. ( Departments of Astronomy and Physics, and Center for Magnetic Self-Organization, University of Wisconsin-Madison, Madison, WI 53706, USA ); Oh, S. Peng ( Department of Physics, University of California, Santa Barbara, CA 93106, USA ) show affiliations
The Astrophysical Journal, Volume 767, Issue 1, article id. 87, 8 pp. (2013).
Published in Apr 2013
Observations of line ratios in the Milky Way's warm ionized medium suggest that photoionization is not the only heating mechanism present. For the additional heating to explain the discrepancy, it would have to have a weaker dependence on the gas density than the cooling rate, \Lambda n_e^2. Reynolds et al. suggested turbulent dissipation or magnetic field reconnection as possible heating sources. We investigate here the viability of MHD-wave mediated cosmic ray heating as a supplemental heating source. This heating rate depends on the gas density only through its linear dependence on the Alfvén speed, which goes as n_e^{-1/2}. We show that, scaled to appropriate values of cosmic ray energy density, cosmic ray heating can be significant. Furthermore, this heating is stable to perturbations. These results should also apply to warm ionized gas in other galaxies.
Astronomy: H II regions; ISM: general; cosmic rays; galaxies: ISM; galaxies: halos; magnetohydrodynamics: MHD
arXiv: Astrophysics - Astrophysics of Galaxies; Astrophysics - High Energy Astrophysical Phenomena