Cosmic Accelerators

Relativistic Astrophysics generates a number of problems where plasma physics plays an essential role. The acceleration of cosmic rays, and of the nonthermal particle distributions present in astrophysical sources of synchrotron radiation, are famous and long standing examples. The acceleration, transport and termination of magnetized relativistic flows emerging from compact objects is an almost equally hoary astrophysical problem, first recognized in the discovery of apparently super-luminal jets emerging from active galactic nuclei in the early 70s, as well as implied by modeling of the energy losses from rapidly rotating magnetized neutron stars (pulsars), also beginning in the late 60s-early 70s. Plasma physical results play a central role in the theoretical modeling of these phenomena. Many have their roots in non-relativstic plasma studies, often driven by space plasma physics experiments. Experiments from satellites immersed in the solar wind drove the development of quasi-linear theories, and later computational models, of high energy particle diffusion in long wavelength magnetic turbulence, with particles undergoing strong scattering from waves with wavelengths comparable to the particle Larmor radius. For cosmic rays and other high energy (supra-thermal) particles, the Larmor radii are large compared to the thermal Larmor radii of the underlying plasma, causing the particles to resonate with long wavelength waves-for ions, these are MHD modes, thus coupling particle transport to the properties of MHD turbulence in the medium (the solar wind, in the space plasma experimental context.) Outside the solar system, experimental information on MHD turbulence in the media in which cosmic rays propagate-the interstellar medium of our and other galaxies, and the intergalactic medium within and between clusters-was and is largely lacking, except at the longest wavelengths where the turbulence is forced. The fact that cosmic rays themselves can generate MHD turbulence, through a resonant cyclotron instability driven by their streaming through the medium, was a significant discovery in the late 60s (Lerche 1967, Kulsrud and Pearce 1969), augmented by the discovery in the last 10 years that the cosmic ray electric current near a cosmic ray source can drive a powerful non-resonant instability (Bell 2004) perhaps responsible for the strong magnetic fields inferred in supernova remnants (e.g., Berezhko 1