Cosmic-ray driven winds

Abstract

The theory of Galactic Winds, driven by the cosmic-ray pressure gradient, is reviewed both on the magnetohydrodynamic and on the kinetic level. In this picture the magnetic field of the Galaxy above the dense gas disk is assumed to have a flux tube geometry, the flux tubes rising locally perpendicular out of the disk to become radially directed at large distances, with the cosmic-ray sources located deep within the Galactic disk. At least above the gas disk, the magnetic fluctuations which resonantly scatter the cosmic rays are selfconsistently excited as Alf{`e}n waves by the escaping cosmic rays. The fluctuation amplitudes remain finite through nonlinear wave dissipation. The spatially increasing speed of the resulting outflow results in a diffusion-convection boundary whose position depends on particle momentum. It replaces the escape boundary of static diffusion models. New effects like overall Galactic mass and angular momentum loss as well as gas heating beyond the disk appear. Also particle re-acceleration in the distant wind halo suggests itself. The resulting magnetohydrodynamic flow properties and the cosmic-ray transport properties are compared with observations. On the whole they show remarkable agreement. General limitations and generalisations of the basic model arise due to the expected simultaneous infall of matter from the environment of the Galaxy. On an intergalactic scale the combined winds from the Local Group galaxies should form a ``Local Group Bubble``. Its properties remain to be studied in detail.