Abstract
Abstract The unusually narrow X-ray source imaged with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during an impulsive spike lasting for ∼10 s during the Geostationary Operational Environmental Satellite C7.9 flare on 2011 September 25 (SOL2011-09-25T03:32) was only ∼2″ wide and ∼10″ long. Comparison with Helioseismsic and Magnetic Imager magnetograms and Atmospheric Imaging Assembly images at 1700 Å shows that the X-ray emission was primarily from a long ribbon in the region of positive polarity with little if any emission from the negative polarity ribbon. However, a thermal plasma source density of ∼1012 cm−3 estimated from the RHESSI-derived emission measure and source area showed that this could best be interpreted as a coronal hard X-ray source in which the accelerated electrons with energies less than ∼50 keV were stopped by Coulomb collisions in the corona, thus explaining the lack of the more usual bright X-ray footpoints. Analysis of RHESSI spectra shows greater consistency with a multi-temperature distribution and a low-energy cutoff to the accelerated electron spectrum of 22 keV compared to 12 keV if a single-temperature distribution is assumed. This leads to a change in the lower limit on the total energy in electrons by an order of magnitude, given the steepness of the best-fit electron spectrum with a power-law index of ∼6.
Highlights
The C7.9 flare of interest, designated as SOL2011-09-25T03:32, was first studied by Guo et al (2012b,a, 2013), who treated it as a coronal hard X-ray source since no obvious footpoints were detected
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) imaging is based on the measurement of the solar X-ray flux that is modulated by nine bi-grid collimators as the spacecraft rotates (Hurford et al 2002)
The unusually narrow X-ray source imaged with RHESSI during an impulsive spike lasting for ∼10 s during the GOES C7.9 flare on 25 September 2011 (SOL2011-09-25T03:32) was only ∼2 arcsec wide and ∼10 arcsec long
Summary
The C7.9 flare of interest, designated as SOL2011-09-25T03:32, was first studied by Guo et al (2012b,a, 2013), who treated it as a coronal hard X-ray source since no obvious footpoints were detected. They fitted the observations with a collisional model with an extended electron acceleration region and a sufficiently high coronal density such that the accelerated electrons would lose all of their energy in the legs of the magnetic loop before reaching the chromospheric footpoints. Summed count rates (corrected for attenuator changes) from the front segments of detectors 1, 3, 4, 5, 8, and 9 are plotted with a 4 s cadence to match the spacecraft spin period. The GOES two-channel lightcurves are shown in the lower plot for the same time interval with 2-s cadence
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