6.2 Comparing theories with observations
Flare particle acceleration occurs in the wake of reconnection in low density plasma. The details are still
obscure. Current flare observations contain numerous information that limits the scenarios for particle
acceleration. Concerning the three types mentioned in the previous section, observations exclude more than
they confirm. In particular:
- High-frequency waves near the plasma frequency can be excluded as drivers for stochastic
acceleration. They would couple into radio decimeter waves and be present in every flare. On
the other hand, acceleration by Transit-Time Damping is compatible with the frequent lack
of radio emission because the frequency of the postulated turbulence is far below the plasma
frequency and does not cause radio emission.
- The number of electrons that can be accelerated in current sheets is limited by
the current, and observed values are difficult to match with a classic current sheet
(Holman, 1985; Litvinenko, 1996).
- Large-scale, stationary electric fields would cause major distortions in velocity space, susceptible
to velocity space instabilities and beam driven high-frequency waves observable as radio waves.
Such radio emission should correlate well with hard X-ray emission, which is not seen in every
flare.
- Shock acceleration plays an important role in interplanetary space, which is not the issue here.
More directly related to flares are shocks possibly produced by the reconnection process. Radio
signatures of such shocks have been reported by Aurass et al. (2002); Mann et al. (2006),
but are still disputed and, if confirmed, extremely rare. Nevertheless, slow shocks have been
predicted at the interface between inflow and outflow regions of reconnection, as well as fast
shocks at the bow of the reconnection outflows. High energies could be readily be achieved
(100 MeV ions in
1 s, Ellison and Ramaty, 1985; Tsuneta and Naito, 1998).
The Transit-Time Damping scenario is consistent with present observations, although there is no direct
evidence for it. Thus it must be considered as a still unproven hypothesis. In fact, there are other viable
scenarios: as an alternative to acceleration by fluctuating magnetic fields (and also to large-scale
quasi-electric fields) low-frequency fluctuating electric fields parallel to the magnetic field have been
proposed. Such fields may originate from low-frequency and high-amplitude turbulence, such as kinetic
Alfvén waves. They accelerate and decelerate electrons, leading to a net diffusion in energy space (Arzner
and Vlahos, 2004).
