Coronal Hard X-ray Sources Revisited

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For the most part RHESSI observes hard X-rays (HXRs) from bright ''footpoint'' sources via the  
For the most part RHESSI observes hard X-rays (HXRs) from bright ''footpoint'' sources via the  
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[http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/EVE/ESP_and_the_Neupert_Effect Neupert Effect].
+
[http://solar.physics.montana.edu/nuggets/2000/000303/000303.html Neupert Effect].
These footpoint sources lie at the intersections of coronal magnetic fields with the lower solar atmosphere.
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.
As many of these Nuggets have discussed, RHESSI also sees a variety of sources in the solar corona.

Revision as of 15:55, 18 September 2018


Nugget
Number: 332
1st Author: Brian Dennis
2nd Author:
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.

Figure 1: RHESSI light curves for the flare on 15 April 2002. The color-coded curves are for the five indicated energy ranges. Counts from the front segments of all detectors except for detectors #2 and #7 were summed and divided by the summed live times and the total effective sensitive area of 35.59 cm2 per detector to give the plotted values with a 4 s cadence to match the spacecraft spin period. The thin attenuators were in place above all detectors limiting the useful energy range to > 6 keV. The blue shaded areas show the first of the three time intervals between 00:00 and 00:05 UT used here and by Guo et al. (2012a,b, 2013) to determine the source dimensions.
Figure 2: RHESSI count flux spectrum for the five-minute time interval shown in Figure 1. The black histogram is the background-subtracted count flux in the front segment of Detector #4. The red histogram is the function that was fitted to the data between 6 and 50 keV. It is the sum of the following components: a multi-thermal bremsstrahlung function (green), a power-law nonthermal thick-target function (yellow), an albedo function for isotropic emission (pink), the estimated pulse pile-up contribution (purple), and two Gaussian instrumental lines (olive and brown). The background spectrum (green) determined from the nighttime period immediately prior to the are is shown with +/-1 sigma error bars. The values of all parameters used for the fit are given for each functional component.

References

[1] "Coronal hard X-ray sources revisited"

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