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5.1 Coronal hard X-ray signatures

The coronal hard X-ray source is not necessarily identical to the acceleration region, but a thin or thick target for accelerated electrons. The relation between coronal hard X-rays and acceleration is not clear. The centroid position of the coronal hard X-ray emission has been reported to be higher with increasing photon energy, and to be located above the thermal soft X-ray loop (Masuda et al., 2000). In some cases it has been reported to move out (Shibata et al., 1995). In another case, the speed of the outward motion of several hundred km s–1 was correlated with the flux of accelerated electrons (Sui et al., 2004Jump To The Next Citation Point, Figure 26View Image). The shape of the hard X-ray emission is not always plain circular. Elongated tongues stretching out from the loop-top are frequently reported (e.g., Sui et al., 2004Jump To The Next Citation Point). The altitude of this source decreases from the start of the hard X-rays until peak time at 23:12 UT, while a source above the loop-top moves upwards at speeds of 23 km s–1. Well contained emission is also seen (e.g., Figure 24Watch/download Movie).

With higher spectral resolution it became possible to determine also the shape of the photon energy distribution. The coronal hard X-rays have been found to be well represented by a power-law in energy (Emslie et al., 2003Battaglia and Benz, 2006), similar to what is well known for footpoint sources. The predominant near power-law distribution of accelerated electron energies is a stringent characteristic of the acceleration process, and not the result of propagation.

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Figure 26: Top: Light curves in two energy bands (upper curve, 6 – 12 keV ; lower curve, 25 – 50 keV) of the April 15, 2002 flare observed by RHESSI. Bottom: Altitude of the loop-top centroid obtained using the 60% contour for the images in the 6 – 12 (crosses) and 12 – 25 keV (diamonds) bands. The triangles show the altitude of the lower source above the flare loop. The horizontal bars on each point represents the integration time of the corresponding image. The lines show linear fits to the altitudes vs. time for two time ranges and two energy bands (from Sui et al., 2004).

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