Coronal Hard X-ray Sources Revisited
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Number: | 333 |
1st Author: | Brian Dennis |
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Published: | 2018 |
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Introduction
For the most part RHESSI observes hard X-rays (HXRs) from bright footpoint sources via the Neupert Effect. These footpoint sources lie at the intersections of coronal magnetic fields with the lower solar atmosphere. As many of these Nuggets have discussed, RHESSI also sees a variety of sources in the solar corona.
A recent paper (Ref. [1]) 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.
Example Flare - 14/15 April 2002
The effect of weak footpoint emission at higher energies on the apparent source length is shown during the M3.7 flare in NOAA AR09893 at N16W60 the peaked at 00:14 on 15 April 2002. The time history is shown in Figure 1 and the spectrum for the 5 min. interval at the start of the impulsive emission is shown in Figure 2. The count fluxes are fitted to a thermal component consisting of a differential emission measure that is a power-law in temperature (multi_therm_pow) plus a nonthermal component (thick2_vnorm) with a power-law electron spectrum. With these assumed components, the nonthermal emission dominates at energies up to ~22 keV. This is to be compared to a crossover energy of ~15 keV if an isothermal function is assumed as was done previously. Such an isothermal function is inconsistent with the change in altitude of the coronal source with energy as indicated in Figure 3, where a 12 - 25 keV image is shown with the 25 - 50 keV contours overlaid. The loop structure is clearly evident in both energy ranges but with a ~5 arcsec offset corresponding to a higher altitude in the higher energy band. Two footpoint sources appear in the 25 - 50 keV image at the expected locations with respect to the loop structures.
To minimize the effect of the footpoint emission in estimating the length of the coronal loop source, we defined an arc along the spine of the loop structure that also passed through the two footpoints and determined the flux along this curve shown in Figure 4.
References
[1] "Coronal hard X-ray sources revisited"
RHESSI Nugget Date | 2018 + |
RHESSI Nugget First Author | Brian Dennis + |
RHESSI Nugget Index | 333 + |