Electron acceleration theories

6.1 Electron acceleration theories

The free mobility of charged particles in dilute plasma and the difference in inertia between electrons and ions make it likely that electrons are accelerated in every major impulsive process in the corona. Convertible (free) magnetic energy requires the presence of an electric current and an associated electric field before the flare-like process. Waves of various types from MHD to collisionless are expected to be excited by the flare and are also capable to accelerate particles in resonance. From a plasma physics point of view, acceleration is not surprising; controversial is, however, which process dominates.

The conversion of magnetic energy into accelerated particles can be accomplished by several processes. It is likely that more than one occurs during a flare and its secondary effects. The most widely discussed can be grouped into 3 types (e.g., Melrose, 1990 ; Benz, 2002 ):

stochastic acceleration (e.g., resonant acceleration by waves),
electric field parallel to the magnetic field,
perpendicular and parallel shocks (first and second order Fermi acceleration).

The preferred but not generally accepted acceleration model for solar flares is stochastic acceleration by the magnetic field component of low-frequency waves (Miller et al., 1997 ; Schlickeiser and Miller, 1998 ; Petrosian et al., 2006 ). Particles near Cerenkov resonance ( $v ≈ ω ∕k ∥$ ) are mirrored by the waves. The process is known as Transit-Time Damping of waves. It acts as a diffusion process of the particle distribution f(p) in momentum space, described by the Fokker–Planck equation

The diffusion coefficients D_ij and F_i contain the physics: the action of accelerating waves and decelerating Coulomb collisions.

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Figure 29:

mpg-Movie (2079 KB) The particle distribution vs. energy in time (blue) as described by Equation (9

). A tail develops out of the thermal distribution (purple). The solution becomes stationary after about one second when acceleration balances particle escape out of the acceleration region. The spectral index resulting from fitting a power-law in the region limited by dashed red lines is also shown (from Grigis and Benz, 2006 ).