The Role of Pressure Anisotropy in Cosmic Ray Hydrodynamics
  Cosmic ray propagation in the Milky Way and other galaxies is largely
diffusive, with mean free path determined primarily by pitch angle scattering
from hydromagnetic waves with wavelength of order the cosmic ray gyroradius. In
the theory of cosmic ray self confinement, the waves are generated by
instabilities driven by the cosmic rays themselves. The dominant instability is
due to bulk motion, or streaming, of the cosmic rays, parallel to the
background magnetic field B, and transfers cosmic ray momentum and energy to
the thermal gas as well as confining the cosmic rays. Classical arguments and
recent numerical simulations show that self confinement due to the streaming
instability breaks down unless the cosmic ray pressure and thermal gas density
gradients parallel to B are aligned, a condition which is unlikely to always be
satisfied We investigate an alternative mechanism for cosmic ray self
confinement and heating of thermal gas based on pressure anisotropy
instability. Although pressure anisotropy is demonstrably less effective than
streaming instability as a self confinement and heating mechanism on global
scales, it may be important on mesoscales, particularly near sites of cosmic
ray injection.
