INTERPLANETARY PARTICLE BEAMS

The purpose of this paper is to review the observations of particle beams of the kind that are frequently observed in the interplanetary medium, usually but not always accompanying a solar flare. Most frequent are beams of electrons. They are generally associated with radio bursts of type III and only sometimes with flares and X-ray bursts. The properties of these electron beams have been well studied using quasi-linear and nonlinear theory, in situ observations of electrons and of plasma waves, and remote observations of radio waves Thanks to the interaction between theory and observation, the decade of the 1980s has been one of great progress in understanding the main features of these beams and their associated plasma waves and radio bursts. However, uncertainties remain in terms of (1) whether fine scale features, filamentary structures or wave condensations, occur together with the beams, (2) whether quasi-linear or nonlinear wave emission is the dominant process, and (3) if wave condensations are important, what is the mechanism of conversion of some Langmuir wave energy into radio emission. Other particle beams are composed of protons, of neutrons, of helium ions (sometimes with a large excess of He-3), and of heavy ions with varying concentrations. Sometimes the observations seem to require the fractionation of certain ions, followed by resonant acceleration of certain species. Objects other than the Sun that are the source of interplanetary particle beams include comets and planets, especially the Earth and Jupiter.