Acceleration and Transport of Energetic Charged Particles in Space

Abstract

Energetic charged particles or cosmic rays are found in space wherever the ambient matter density is small enough to allow them to exist. Their observed kinetic energies vary from just above the local thermal energies to in excess of 10²⁰ eV. The energy spectrum is quite smooth, suggesting that a common mechanism is responsible for particles of all energies. Their arrival directions are distributed evenly in solid angle, with the observed anisotropies being quite small even at energies 10¹⁸ eV. Various scenarios for their acceleration are discussed, including 2nd-order Fermi acceleration, shocks and cosmic-ray viscosity. It is concluded that the most likely mechanism for the acceleration of most energetic particles to high energies is diffusive shock acceleration. This mechanism has the benefit of producing a power-law energy spectrum with a spectral index which is very insensitive to parameters and which is close in magnitude to that observed in a variety of contexts. It is also reasonably fast and efficient. Anomalous cosmic rays in the heliosphere are discussed as one example of the success of the shock-acceleration picture.