Standard flare model

2.5 Standard flare model

The assumption of a strict causal relation between soft X-ray emission and hard X-ray signatures of energetic electrons has lead to a simple scenario. It postulates that flare energy release consists of accelerating particles. The acceleration process is not part of the model. Energetic particles then precipitate to the chromosphere, where they heat the plasma to the high temperatures observed in soft X-rays. The hot plasma expands along the loop into the corona, a process termed “evaporation”. This simple scenario can explain several phenomena observed in flares:

Correlation of soft X-ray flux with cumulative hard X-ray flux (Neupert effect)
Hard X-rays ( $>$ 25 keV) often originate from sources at the footpoints of the loop emitting soft X-rays.
The coronal hard X-ray source, where reconnection releases energy, is occasionally observed to be above the soft X-ray loop, into which energy was release before and which is still emitting soft X-rays (Masuda et al., 1994 ).
The energy in accelerated electrons tends to be larger than the thermal energy contained in the soft X-ray source (see discussion in Section 4).
The hard X-ray spectrum of non-thermal electrons in the coronal source is considerably softer than in the footpoints, suggesting that the latter is a thick target. In thick targets the particles lose all their kinetic energy, and their bremsstrahlung emission is the result of all collisions until they come to full stop.
The emission measure of the soft X-ray source greatly increases during the impulsive phase, indicating that chromospheric material is evaporating during this period. Evaporation has been observed directly, first in blue-shifted lines of hot material, later also in soft X-ray images (see following section).

Henceforth we refer to this scenario as the standard model (Figure 13 ).

Figure 13:

A schematic drawing of the standard flare scenario assuming energy release at high altitudes.