Acceleration of energetic particles on the Sun, in the Heliosphere, and in the Galaxy

Abstract

This review describes the energetic particle populations observed in the heliosphere including gradual and impulsive solar energetic particle (SEP) events, interplanetary energetic particle enhancements, the anomalous cosmic ray (ACR) component, and galactic cosmic rays (GCRs). Also described briefly are the interstellar pickup ions and the solar wind itself, which are not energetic in the usual sense but are substantially heated and/or accelerated and provide the seed particles for the heliospheric populations. Firstly several overarching themes are presented which are important for all the energetic particles: acceleration mechanisms, particle transport theory, species-dependent transport and acceleration, injection mechanisms, and energetic particles as probes of heliospheric structure and dynamics. Secondly each population of energetic particles is reviewed including our current understanding of the source of the particles, the acceleration mechanism, and the transport, and any puzzles or challenges for theory. Shock acceleration is the dominant acceleration mechanism. Gradual SEP events are accelerated at coronal/interplanetary shocks, the corotating ion events are accelerated at the shocks bounding corotating interaction regions (CIRs) in the solar wind, diffuse ions are accelerated at planetary bow shocks, the ACRs are accelerated at the solar wind termination shock, and GCRs are accelerated at supernova remnant shocks. Only impulsive SEP events do not originate at shocks; they are accelerated by direct electric fields or stochastic acceleration at sites of magnetic reconnection in solar flares. For GCRs in particular, transport in the heliosphere and interstellar space affects their observed energy spectra and composition.