On the Nature of Impulsive Electron Acceleration in Solar Hard X-ray Flares. II. A Theory
Abstract
Summary. The suggestion is elaborated that shock wave generated Langmuir waves accelerate electrons in the adjoining plasma. Langmuir wave generation can be achieved in ion- acoustic unstable shocks by induced bremsstrahlung from electrons. A crude model analysis shows the Langmuir waves to have short wavelengths, k kD/4, while propagating almost parallel to the shock plane. It is possible that sufficient power in Langmuir waves is generated to explain the observed scale of electron acceleration. The evolution of the Langmuir wave distribution emerging from the shock is analyzed including the effects of spatial gradients. The initial stage is dominated by nonlinear Landau damping, which isotropizes the waves and increases their wavelengths, leading eventually to the onset of the modulational instability. Virtually all wave energy is now converted into fast electrons by Landau damping. Depending on parameters, these processes are completed within a distance of the order of 100 m from the shock. Next follows an extended nonthermal region in which fast electrons (+ selfconsistent level of Langmuir waves) slowly relax to thermal equilibrium. The length of this region is t0 -t0 km and depends among other factors, on the amount of fast electron scattering. The resulting fast electron velocity distribution is not discussed. However, there is hope for a universal distribution because strong wave-wave interactions take place before acceleration, while redistribution of energy between fast electrons occurs afterwards. Key words: solar flare hard X-rays shock wave stochastic acceleration parametric instability
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- February 1977
- Bibcode:
- 1977A&A....55...31H