Coronal Hard X-ray Sources Revisited

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Nugget
Number: 33?
1st Author: Brian Dennis
2nd Author:
Published: 2018
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Introduction

A recent paper attempts to set the record straight by reinterpreting observations of a group of flares that have been reported to have hard X-rays (HXRs) coming predominantly from the corona rather than from the more usual chromospheric footpoints. All of the 26 previously analyzed event time intervals, over 13 flares, were re-examined for consistency with a model in which electrons are accelerated near the top of a magnetic loop that has a sufficiently high density to stop most of the electrons by Coulomb collisions before they can reach the footpoints. Of particular importance in the previous analysis was the finding that the length of the coronal HXR source increased with energy in the 20 - 30 keV range. Such behavior is inconsistent with a thermal source, the size of which generally decreases with increasing energy as the emission becomes more and more dominated by the hottest regions, but it is consistent with the transport of accelerated electrons through a collisional target, since higher energy electrons travel further.

However, after allowing for the possibility that footpoint emission at the higher energies affects the inferred length of the coronal HXR source, and using analysis techniques that suppress the possible influence of such footpoint emission, we conclude that there is no longer evidence that the length of the HXR coronal sources increase with increasing energy. In fact, for the 6 flares and 12 time intervals that satisfied our selection criteria, the loop lengths decreased on average by 1.0 +/- 0.2 arcsec between 20 and 30 keV, with a standard deviation of 3.5 arcsec. We find strong evidence that the peak of the coronal HXR source increases in altitude with increasing energy. For the thermal component of the emission, this is consistent with the standard CHSKP flare model in which magnetic reconnection in a coronal current sheet results in new hot loops being formed at progressively higher altitudes. The explanation for the nonthermal emission is not so clear.

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