Deviations from standard flare model

2.7 Deviations from standard flare model

A variant of the standard model has been proposed for flares without footpoints (Veronig et al., 2002

; Veronig and Brown, 2004

). The flare loop has been found so dense that accelerated electrons have collisions already in the corona and lose a large fraction of their energy to the flare loop (Figure 15

). A preceding flare at the same location may have produced the high density of the loop (Strong et al., 1984 ; Bone et al., 2007 ).

Figure 15:

RHESSI observations at 6 – 12 keV (red) and 25 – 50 keV (blue) of a coronal flare. The high-energy photons have a non-thermal origin and originate near the loop-top without pronounced footpoints (Veronig and Brown, 2004

There are other indications, that the standard model is not sufficient. In about half of the hard X-ray events, the Neupert behavior is violated in terms of relative timing between soft and hard X-ray emissions (Dennis and Zarro, 1993 ; McTiernan et al., 1999 ; Veronig et al., 2002 ). This is particularly obvious in flares with soft X-rays preceding the hard X-ray emission. Such preheating is well known and cannot be explained by lacking hard X-ray sensitivity (e.g., Benz et al., 1983 ; Jiang et al., 2006 ). Also it has been noted by several authors that the plasma in the coronal source at the top is generally hotter than at the footpoints of the loop.

An alternative interpretation to the standard model is that the soft X-ray emitting plasma is not heated exclusively by high-energy electrons, (e.g., Acton et al., 1992 ; Dennis and Zarro, 1993 ). A likely amendment to the standard model is that some coronal particles get so little energy during flare energy release that they have frequent enough collisions to approximately retain their Maxwellian velocity distribution. Thus their energization corresponds to heating. In a preflare, the heat of the coronal source may reach the chromosphere by thermal conduction. Depending on the rate of the energy release, other particles may gain so much energy that collisions become infrequent (Equation 8 ). These particles then are accelerated further, get a non-thermal velocity distribution, and may eventually leave the energy release region.