Solar particle acceleration and propagation

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Abstract
Large solar flares are the most energetic natural particle accelerators in the solar system, occasionally accelerating ions to many GeV and electrons to ≳102 MeV energies. The accelerated particles can be studied indirectly, via observations of the electromagnetic emissions produced by the particles in their interactions with the solar atmosphere, or directly by spacecraft and ground‐based observations of the particles which escape into the interplanetary medium. The electromagnetic emissions (X‐ray, gamma‐ray, and radio emission) depend critically on ambient conditions in the solar atmosphere where the accelerated particles reside (the density, magnetic field, and temperature), while the interpretation of the particle fluxes observed in the interplanetary medium depends on the characteristics of the escape and interplanetary propagation processes.Radio, X‐ray and gamma‐ray, and energetic particle observations obtained before the 1980 solar maximum suggest that there are at least two acceleration processes associated with solar flares. During the impulsive or flash phase, electrons are often accelerated to ∼10–100 keV energies, even in small flares or subflares. For some flares, the energy contained in these electrons may be a substantial fraction of the total flare energy. Thus the primary energy release mechanism for flares may initially convert stored magnetic energy into energetic electrons. These electrons can produce most of the observed impulsive phase flare phenomena through their interactions with the solar atmosphere. In large flares a second acceleration sometimes occurs which accelerates both ions and electrons to MeV energies and above. This second acceleration appears to have a close association with shock waves in the solar atmosphere as observed by type II radio bursts. Based on observations of escaping particles and hard X‐ray and gamma ray bursts, the impulsive phase acceleration events occur on the order of ∼102 times a month near solar maximum while large solar energetic particle (LSEP) events where >10 MeV ions and relativistic electrons are accelerated to observable levels occur a few times a month.