3 research outputs found
Stochastic Acceleration of Low Energy Electrons in Plasmas with Finite Temperature
This paper extends our earlier work on the acceleration of low-energy
electrons by plasma turbulence to include the effects of finite temperature of
the plasma. We consider the resonant interaction of thermal electrons with the
whole transverse branch of plasma waves propagating along the magnetic field.
We show that our earlier published results for acceleration of low-energy
electrons can be applied to the case of finite temperature if a sufficient
level of turbulence is present. From comparison of the acceleration rate of the
thermal particles with the decay rate of the waves with which they interact, we
determine the required energy density of the waves as a fraction of the
magnetic energy density, so that a substantial fraction of the background
plasma electrons can be accelerated. The dependence of this value on the plasma
parameter alpha = omega_pe / Omega_e (the ratio of electron plasma frequency to
electron gyrofrequency), plasma temperature, and turbulence spectral parameters
is quantified. We show that the result is most sensitive to the plasma
parameter alpha. We estimate the required level of total turbulence by
calculating the level of turbulence required for the initial acceleration of
thermal electrons and that required for further acceleration to higher
energies
Stochastic Acceleration of Low Energy Electrons in Cold Plasmas
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 non-relativistic and ultra-relativistic
energies are obtained for the acceleration and scattering times of electrons.
These times have a strong dependence on plasma parameter alpha = Omega_pe /
Omega_e (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 certain critical value may interact with higher frequency
electromagnetic plasma waves and this interaction is allowed only in plasmas
with alpha < 1. We show that for non-relativistic and semi-relativistic
electrons in low-alpha 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 which requires
this ratio to be much larger than one 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 alpha <= 0.1.Comment: 22 pages, 13 figures, Latex, submitted to Astrophysical Journa