Coherent emission and stochastic acceleration processes associated with nonthermal electrons in astrophysical plasma

Four problems involving wave-particle interactions of nonthermal electrons in astrophysical plasmas are examined. The coherent emission of plasma waves, radiation, or both by the energetic electrons is the fundamental physical process in three of these studies. Stochastic acceleration of electrons by plasma turbulence is treated in the other study.;Firstly, the production of plasma waves by a nonthermal beam of electrons injected into a thick-target thermal plasma is evaluated by solving the coupled kinetic equations for the plasma wave and particle distributions. The effects of the wave-particle interactions on the overall electron distribution are determined and the radiation signature is found.;Secondly, the correlation of solar hard X-ray and type III radio bursts is examined using statistical methods to analyze tabulated data. The observed intensity distributions of hard X-ray and type III events and their correlations are shown to be satisfactorily described by a bivariate distribution consistent with the assumption of statistical linear dependence of the X-ray and type III radio burst intensities.;Thirdly, a numerical solution of the time dependent kinetic (Fokker-Planck) equation describing the evolution of electron distributions in magnetized plasma is developed. The numerical method, its calibration, and the FORTRAN code are given.;Fourthly, the effects of Coulomb collisions of nonthermal electrons with the ambient plasma in magnetic loops is studied to determine the subsequent effects on the induced cyclotron maser and plasma wave growth. Growth rates of instabilities are found to be significantly reduced for modest values of the loop column depth suggesting that maser action is only possible in low density regions in solar flares and primarily at the flare onset before chromospheric evaporation occurs. This explains why the spike bursts are usually seen during the rise of the impulsive emission.;Finally, stochastic acceleration of electrons by plasma turbulence in the presence of competing energy loss processes is studied numerically using the Fokker-Planck code. This extends previous models which ignore energy losses, time dependence, or use simple injected particle distributions. Stochastic acceleration of electrons by whistlers is shown to be an important first phase acceleration mechanism in solar flares.