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sess20 - 20 Cosmic rays and shock acceleration The Milky...

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20. Cosmic rays and shock acceleration The Milky Way galaxy, and other galaxies, are filled with energetic charged particles called cosmic rays. Most of the cosmic rays are hydrogen nuclei, about 10% are helium nuclei, the rest are heavier nuclei and electrons. We measure cosmic rays with energies up to about 10 20 eV, that is 16 Joule! Obviously cosmic rays are not in Local Themodynamical Equilibrium. They play an important role in the Galaxy, as they ionize and heat the interstellar gas. In the Galaxy the energy density of cosmic rays is comparable to that of magnetic field, radiation, and the thermal energy density of the interstellar gas. Highly relativistic particles are also found in active galactic nuclei, very luminous systems that involve supermassive black holes and relativistic outflows. We now discuss the acceleration of charged particles. The process in question was first pro- posed by Enrico Fermi and is therefore dubbed Fermi acceleration of type I. Here we study its application to hydromagnetic shocks. We already know that energetic charged particles propagates diffusively and have an isotropic distribution on account of scattering on magnetic fluctuations or MHD waves. Very often the mean free path for scattering is larger for energetic particles than it is for the bulk of the plasma. Then the energetic particles see the shock as a discontinuity, even if it is collisionless
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