STOCHASTIC ACCELERATION OF LOW-ENERGY ELECTRONS IN COLD-PLASMAS

We investigate the possibility of stochastic acceleration of backgroun d low-energy electrons by turbulent plasma waves. We consider the reso nant interaction of the charged particles with all branches of the tra nsverse plasma waves propagating parallel to a uniform magnetic field. Numerical results and asymptotic analytic solutions valid at nonrelat ivistic and ultrarelativistic energies are obtained for the accelerati on and scattering times of electrons. These times have a strong depend ence on plasma parameter alpha = omega(pe)/Omega(e) (the ratio of elec tron plasma frequency to electron gyrofrequency) and on the spectral i ndex of plasma turbulence. It is shown that particles with energies ab ove a certain critical value may interact with higher frequency electr omagnetic plasma waves, and this interaction is allowed only in plasma s with alpha < 1. We show that for nonrelativistic and semirelativisti c electrons in low-alpha plasmas, the ratio of the acceleration time t o the scattering time can be less than unity for a wide range of energ ies. From this we conclude that the transport equation derived for cos mic rays that requires this ratio to be much larger than unity is not applicable at these energies. An approximate ''critical'' value of par ticle energy above which the dynamics of charged particles may be desc ribed by this transport equation is determined as a function of plasma parameters. We propose new transport equation for the opposite limit (energies less than this critical value) when the acceleration rate is much faster than the pitch angle scattering rate. This equation is ne eded to describe the electron dynamics in plasmas with alpha less than or similar to 0.1.