Theoretical studies of proton beams in solar flare loops

Physical processes associated with energized streams of protons in the solar flare atmosphere are examined. The importance of proton beam momentum deposition in a flare loop is investigated. Consideration of Coulomb collisions between beam protons and the background plasma leads to general expressions for energy and momentum deposition by polyenergetic protons beams in cold and warm hydrogen targets. It is found that momentum deposition forces are unimportant in comparison with the thermal pressure gradients that arise from nonuniform deposition of beam energy in the case of deka-MeV proton beams, but are potentially important in the case of deka-keV proton beams when the beam energy flux is sufficiently large. The stability of proton beams against resonant scattering by Alfven and magnetoacoustic waves in solar flare loops is addressed by considering the conditions for the effective beam-driven growth of these waves. It is found that proton beams of moderate streaming anisotropy, characterized by an excess of forward momentum in relation to perpendicular momentum, are likely to suffer effective resonant scattering in the loop when the root-mean-square speed V(sub O) exceeds a small multiple of the Alfven speed V(sub A), i.e., when the mean beam energy lies toward the high end of the range 10 to 1000 keV under typical flare conditions of temperature, density, and magnetic field strength. Toward the low end of this range, where V(sub O) is less than or approximately V(sub A), moderately anisotropic proton beams are (unconditionally) stable against resonant scattering by magnetoacoustic waves, and, with assistance from thermal damping, also stable against resonant scattering by Alfven waves