Particle acceleration in solar flares observations versus numerical simulations.pdf

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2017-04-06 发布于江苏
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Particle acceleration in solar flares observations versus numerical simulations.pdf

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Particle acceleration in solar flares observations versus numerical simulations

Particle acceleration in solar flares: observations versus numerical simulations A O Benz, P C Grigis, and M Battaglia Institute of Astronomy ETH Zu?rich CH-8092 Zurich Switzerland E-mail: benz@astro.phys.ethz.ch Abstract. Solar flares are generally agreed to be impulsive releases of magnetic energy. Reconnection in dilute plasma is the suggested trigger for the coronal phenomenon. It releases up to 1026 J, accelerates up to 1038 electrons and ions, and must involve a volume that greatly exceeds the current sheet dimension. The Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) satellite can image a source in the corona that appears to contain the acceleration region and can separate it from other X-ray emissions. The new observations constrain the acceleration process by a quantitative relation between spectral index and flux. We present recent observational results and compare them to theoretical modelling by a stochastic process assuming transit-time damping of fast-mode waves, escape and replenishment. The observations can only be fitted if additional assumptions on trapping by an electric potential and possibly other processes such as isotropization and magnetic trapping are made. Keywords: Particle acceleration, hard X-ray observations, transit-time damping simulation, reconnection, astrophysics 21. Introduction Solar flares release energies up to 1026 Joule. Some are accompanied by coronal mass ejections having similar or greater energies (Gopalswamy et al. 2005). A large fraction of the flare energy initially appears as kinetic energy of electrons with 20 keV and higher (Saint-Hilaire Benz 2002). The number of accelerated electrons in a flare sometimes exceeds 1038. Some flares accelerate also ions to MeV energies with total energy comparable to electrons. It is generally assumed that reconnection in a dilute medium (corona) is the cause of the impulsiveness of the process. As the relevant current layers in the solar corona are predicted to have a thick