Acceleration and Propagation of Solar Energetic Particles

Abstract

The acceleration of electrons and charged nuclei to high energies is a phenomenon occuring at many astrophysical sites throughout the universe. In the heliosphere, processes in the solar corona associated with flares and coronal mass ejections (CMEs) are the most energetic natural particle accelerators, sometimes accelerating electrons and ions to relativistic energies. The observation of these particles offers the unique opportunity to study fundamental processes in astrophysics. Particles that escape into interplanetary space can be observed in situ with particle detectors on spacecraft, and their spectra and composition can be used as diagnostic of the acceleration processes. On the other hand, energetic processes on the sun can be studied indirectly, via observations of the electromagnetic emissions (radio, X-ray, gamma-ray) produced by the particles in their interactions with the solar atmosphere. The comparison of interacting and escaping particles can provide valuable information about the question whether there is one dominant energization process in solar events, or whether particles are accelerated in multiple processes or sites. Equally important, the study of the propagation of solar cosmic rays allows to address some fundamental problems in the scattering of charged particles by magnetic fluctuations. In this article, we give an overview on models of stochastic particle acceleration and interplanetary particle transport, and discuss the question what conclusions about those models can be drawn from spacecraft observations.