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

TL;DR

This paper reports on an extremely narrow X-ray source observed during a solar flare, revealing a coronal origin of accelerated electrons and providing insights into electron energy distribution and flare emission mechanisms.

Contribution

It presents the first detailed analysis of a very narrow RHESSI X-ray source, suggesting a coronal origin and multi-temperature electron distribution in a solar flare.

Findings

X-ray source width was about 2 arcsec, indicating a very narrow emission region.

The source was primarily from a positive polarity ribbon with little negative polarity emission.

Electron energy distribution has a steep power-law index of ~6, with a low energy cutoff around 22 keV.

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~\AA~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}~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 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.