  When present, extended disks of neutral hydrogen around spiral galaxies show
a remarkably uniform velocity dispersion of approx 6 km/s. Since stellar winds
and supernovae are largely absent in such regions, neither the magnitude nor
the constancy of this number can be accounted for in the classical picture in
which interstellar turbulence is driven by stellar energy sources. Here we
suggest that magnetic fields with strengths of a few microgauss in these
extended disks allow energy to be extracted from galactic differential rotation
through MHD driven turbulence. The magnitude and constancy of the observed
velocity dispersion may be understood if its value is Alfvenic. Moreover, by
providing a simple explanation for a lower bound to the gaseous velocity
fluctuations, MHD processes may account for the sharp outer edge to star
formation in galaxy disks.
