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The cosmic ray spectrum, by species, from 1 to 1012 GeV/particle. To convert to a distribution function in energy, multiply by , where v is particle velocity. The positrons and antiprotons are believed to be secondaries; products of nuclear collisions. The energy ranges of some major laboratory accelerators are shown for comparison, highlighting the importance of cosmic rays as high energy physics probes. Note the large number of different cosmic ray detection experiments, a tribute to the level of innovation and interest in this field. Courtesy of T. K. Gaisser with permission.
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This review paper commemorates a century of cosmic ray research, with emphasis on the plasma physics aspects. Cosmic rays comprise only of interstellar particles by number, but collectively their energy density is about equal to that of the thermal particles. They are confined by the Galactic magnetic field and well scattered by small scale magnetic fluctuations, which couple them to the local rest frame of the thermal fluid. Scattering isotropizes the cosmic rays and allows them to exchange momentum and energy with the background medium. I will review a theory for how the fluctuations which scatter the cosmic rays can be generated by the cosmic rays themselves through a microinstability excited by their streaming. A quasilinear treatment of the cosmic ray–wave interaction then leads to a fluid model of cosmic rays with both advection and diffusion by the background medium and momentum and energy deposition by the cosmic rays. This fluid model admits cosmic ray modified shocks, large scale cosmic ray driven instabilities, cosmic ray heating of the thermal gas, and cosmic ray driven galactic winds. If the fluctuations were extrinsic turbulence driven by some other mechanism, the cosmic ray background coupling would be entirely different. Which picture holds depends largely on the nature of turbulence in the background medium.
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