High-energy protons produced by a large-amplitude magnetosonic wave and associated gamma radiation through neutral pion decay

The spectra of ions accelerated by a large-amplitude magnetosonic wave and the spectra of gamma radiation from the decay of neutral pions which are produced in energetic ion collisions were studied through particle simulations and numerical calculations. A large-amplitude magnetosonic wave can promptly accelerate ions to relativistic energies in a rather strong magnetic field (ωce > or approx. ωpe). This acceleration is much more rapid and efficient than the usual stochastic acceleration and can occur in solar flares. The ion spectra, thus generated, can have various shapes, depending on various plasma parameters. The maximum energy is, however well defined, and the ion spectra have strong downward curvatures near the maximum values. Further, a particle simulation shows that this mechanism can produce a (truncated) power-law form with a spectral index of s=1.2. The energy spectra of the gamma radiation were then numerically obtained for this type of ion energy spectrum and for modified Bessel function forms derived from a stochastic acceleration model. The power-law forms of the ion spectra produce much harder photon energy-spectra than do the modified Bessel function forms. (author)