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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
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.
