A Very Narrow RHESSI X-ray Flare on 25 September 2011

Abstract

The unusually narrow X-ray source imaged with RHESSI during an impulsive spike lasting for $\sim$10~s during the GOES C7.9 flare on 25 September 2011 (SOL2011-09-25T03:32) was only $\sim$2~ arcsec wide and $\sim$10~arcsec long. Comparison with HMI magnetograms and AIA 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 $\sim$10${}^{12}c{m}^{-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 lass than $\sim$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 $\sim$6.

19 pages, 10 figures, accepted for publication in the Astrophysical Journal

References in corpus (2)

• Determination of the total accelerated electron rate and power using solar flare hard X-ray spectra
• A Compressed Sensing-based Image Reconstruction Algorithm for Solar Flare X-Ray Observations

Cited by in corpus (4)

• Thermal-nonthermal energy partition in solar flares derived from X-ray, EUV, and bolometric observations
• Bridging High-Density, Electron Beam Coronal Transport and Deep Chromospheric Heating in Stellar Flares
• Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
• Visibility Interpolation in Solar Hard X-ray Imaging: Application to RHESSI and STIX