Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt.........4l&link_type=abstract
Thesis (PhD). UNIVERSITY OF ALABAMA IN HUNTSVILLE, Source DAI-B 60/05, p. 2175, Nov 1999, 108 pages.
Astronomy and Astrophysics
Astrophysics
Alfven Waves, Fast Mode Waves, Magnetohydrodynamic Turbulence
Scientific paper
Impulsive solar flares generate a wide range of photon and particle emissions and hence provide an excellent backyard laboratory for studying particle acceleration processes in astrophysical plasmas. The source of the acceleration remains unidentified, but the basic observations are clear: (1)Hard X-ray and gamma-ray line emission occur simultaneously, indicating that electron and ion acceleration must occur simultaneously; (2)the electron and ion precipitation rates at the foot-points of the flare must be extremely large to account for the photon emission (~1037 electrons s -1 and ~1035 protons s-1, respectively), which means that replenishment of the acceleration region (which contains ~1037 fully ionized hydrogen atoms) is a crucial issue; and (3)there are enhancements of the heavy ion abundances relative to normal coronal values. The basic model proposed assumes the generation of extremely low levels of magnetohydrodynamic (MHD) turbulence in the form of Alfvén waves and fast mode waves which then stochastically accelerate the ions (Alfvén waves) and electrons (fast mode waves). In this dissertation electron acceleration is considered (independent of ion acceleration) through a process known as transit-time acceleration, in which the electrons are accelerated by the compressive magnetic component of the fast mode waves in a second order Fermi process. A time- independent continuous spectrum of waves with a given energy density and mean wavenumber is assumed and the effects of Coulomb collisions, transport, and replenishment of the acceleration region are included in this model. Spatially resolved hard X-ray spectra generated by the resulting electron distribution are calculated in anticipation of the High Energy Solar Spectroscopic Imager (HESSI) mission, which will be the first to observe spatially resolved flare loops. Spatially integrated hard X-ray spectra are also calculated for comparison to the general characteristics of present day observations.
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