PARTICLE BEAMS IN THE SOLAR ATMOSPHERE - GENERAL OVERVIEW

An overview of particle beams in the solar atmosphere is separated into discussions of (i) current-carrying beams, (ii) current-neutralized electron beams, and (iii) ion beams. The Alfven-Lawson limit on an electric current implies some severe limitations including the following: the current flowing into the corona cannot exceed about 10(12) A; if the current density is near threshold for a current instability then the current must flow in thin layers; and, the primary electrons and ions cannot be accelerated simply by the particles falling down a parallel potential drop. Considerable progress has been made in understanding how electron beams in type III solar radio events propagate in a way that is consistent with the generation of Langmuir waves, but a completely consistent picture has not yet emerged. Such beams, and more importantly the electron beams that generate hard X-ray bursts require current neutralization; how the required return current is set up is still not entirely clear. There is direct evidence for ion beams with energies greater-than-or-similar-to 10 MeV per nucleon from gamma-ray line emission; there is no unambiguous evidence for ion beams of lower energy. A mechanism is suggested for bulk energization of electrons due to dissipation of a parallel current in solar flares. Some outstanding problems concerning particle beams are identified.