Environments for Magnetic Field Amplification by Cosmic Rays
  We consider a recently discovered class of instabilities, driven by cosmic
ray streaming, in a variety of environments. We show that although these
instabilities have been discussed primarily in the context of supernova driven
interstellar shocks, they can also operate in the intergalactic medium and in
galaxies with weak magnetic fields, where, as a strong source of helical
magnetic fluctuations, they could contribute to the overall evolution of the
magnetic field. Within the Milky Way, these instabilities are strongest in warm
ionized gas, and appear to be weak in hot, low density gas unless the injection
efficiency of cosmic rays is very high.
