Stochastic Acceleration of Low‐Energy Electrons in Cold Plasmas

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Abstract
We investigate the possibility of stochastic acceleration of background low-energy electrons by turbulent plasma waves. We consider the resonant interaction of the charged particles with all branches of the transverse plasma waves propagating parallel to a uniform magnetic field. Numerical results and asymptotic analytic solutions valid at nonrelativistic and ultrarelativistic energies are obtained for the acceleration and scattering times of electrons. These times have a strong dependence on plasma parameter α = ωpe/Ωe (the ratio of electron plasma frequency to electron gyrofrequency) and on the spectral index of plasma turbulence. It is shown that particles with energies above a certain critical value may interact with higher frequency electromagnetic plasma waves, and this interaction is allowed only in plasmas with α < 1. We show that for nonrelativistic and semirelativistic electrons in low-α plasmas, the ratio of the acceleration time to the scattering time can be less than unity for a wide range of energies. From this we conclude that the transport equation derived for cosmic rays that requires this ratio to be much larger than unity is not applicable at these energies. An approximate "critical" value of particle energy above which the dynamics of charged particles may be described 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 needed to describe the electron dynamics in plasmas with α 0.1.
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