Stochastic acceleration of charged particles in astrophysical plasmas

One of the primary problems of high energy astrophysics is the determination of the mechanisms for acceleration of particles. The usual mechanisms considered are the acceleration by electric fields, shocks and turbulence. In most studies it is assumed that some population of energetic particles is present and the above mechanisms are used to derive the resultant energy spectrum of the particles. The main goal of our research has been to investigate the possibility of accelerating thermal (low energy) particles from the background plasma. At such low energies the most efficient acceleration is due to interaction between particles and plasma turbulent waves. Plasma waves are postulated as agents for acceleration; they can be produced by accelerated beams of high energy particles and can act as energy loss (or diffusion) agents as well. We believe that this kind of acceleration is the most promising scenario in many astrophysical plasmas, in particular in solar flares.; We consider the resonant interaction of thermal electrons with the whole transverse branch of plasma waves propagating along and perpendicular to the magnetic field lines. Numerical results for the whole energy range and asymptotic analytic solutions valid at non-relativistic and ultra-relativistic energies are obtained for the acceleration and scattering times of electrons. From comparison of the acceleration rate of the thermal particles with the decay rate of the waves we find that a substantial fraction of the background plasma electrons can be accelerated if a sufficient amount of turbulence is present. We estimate the required level of the turbulence as a function of a fraction of the accelerated electrons for different plasma parameters. In both cases considered we find that the fundamentals of the electron dynamics obtained for a cold plasma regime does not change significantly when taking into account the finite temperature of the plasma.