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2.7 Deviations from standard flare model

A variant of the standard model has been proposed for flares without footpoints (Veronig et al., 2002Jump To The Next Citation PointVeronig and Brown, 2004Jump To The Next Citation Point). 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 15View Image). A preceding flare at the same location may have produced the high density of the loop (Strong et al., 1984Bone et al., 2007).
View Image

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, 2004Jump To The Next Citation Point).

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, 1993Jump To The Next Citation PointMcTiernan et al., 1999Jump To The Next Citation PointVeronig et al., 2002Jump To The Next Citation Point). 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., 1983Jiang et al., 2006Jump To The Next Citation Point). 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., 1992Dennis 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 8View Equation). These particles then are accelerated further, get a non-thermal velocity distribution, and may eventually leave the energy release region.


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