ar X iv : a st ro - p h / 98 10 08 9 v 1 6 O ct 1 99 8 Solar Flares : Gamma Rays

OVERVIEW Electromagnetic radiation extends over a very broad range of wavelengths, from gamma rays at the shortest wavelengths to radio waves at the long-wavelength end of the spectrum. In terms of photon energies, gamma rays are at the high end of the spectrum, starting at a few tenths of an MeV. This unit of energy (1 MeV=1.6×10 −13 joule) is used throughout this article. In solar flares, as at many other astrophysi-cal sites, gamma-ray emission results from interactions of fast particles with an underlying ambient medium. These fast particles, protons, α particles, heavier nuclei and electrons, are accelerated from the ambient plasma by the electric fields associated with the complex and varying magnetic fields in the flaring solar atmosphere. Thus, solar flare gamma rays can teach us about the mechanisms that accelerate the particles, in particular those which yield particles with energies in excess of about 1 MeV, the minimum energy needed to produce gamma rays. This is quite important for the understanding of flare mechanisms, because such protons and α particles, along with lower energy electrons, contain the bulk of the energy released in flares. In addition, the solar flare gamma-ray emission exhibits characteristic spectral lines which provide information on the elemental composition of the ambient solar atmosphere. Solar flare gamma-ray emission exhibits both lines and continuum (Fig. 1). This theoretical spectrum extends over the entire energy range in which gamma rays from flares were observed. The lines appear at energies from about 0.5 to 8 MeV, whereas the continuum extends up to at least 1000 MeV. Up to about 1 MeV, and again from about 10 to 50 MeV, the continuum is dominated by bremsstrahlung produced by the braking of the accelerated electrons in the Coulomb fields of the ambient nuclei and electrons. The bremsstrahlung produced by ultrarelativistic electrons is strongly collimated along the direction of motion of the electrons. The lines result from the deexcitation of nuclei, from the capture of neutrons , and from the annihilation of positrons. The relevant nuclear cross sections are available from accelerator measurements. Deexcitation lines are either narrow or broad. Narrow lines result from the bombardment of ambient nuclei by accelerated protons and α particles, while broad lines result from the inverse reactions in which accelerated C and heavier nuclei collide with ambient H and He. The strongest narrow deexcitation lines are at 6.129 MeV from 16 O, 4.438 MeV from …