A reconnection-driven model of the hard X-ray loop-top source from flare 2004-Feb-26

TL;DR

This paper presents a numerical model of a solar flare's X-ray emission, incorporating effects like thermal conduction and chromospheric evaporation, to better understand the origin of the loop-top X-ray source observed during the flare.

Contribution

It advances previous analytic models by numerically solving the thin flux tube equations, including additional physical effects, to more accurately simulate the flare's plasma dynamics and X-ray emission.

Findings

Simulations confirm slow mode shocks produce the observed loop-top source.

Thermal conduction and evaporation increase plasma density and alter temperature.

Model results match observed X-ray light curves and spectra.

Abstract

A compact X-class flare on 2004-Feb-26 showed a concentrated source of hard X-rays at the tops of the flare's loops. This was analyzed in previous work (Longcope et al. 2010), and interpreted as plasma heated and compressed by slow magnetosonic shocks generated during post-reconnection retraction of the flux. That work used analytic expressions from a thin flux tube (TFT) model, which neglected many potentially important factors such as thermal conduction and chromospheric evaporation. Here we use a numerical solution of the TFT equations to produce a more comprehensive and accurate model of the same flare, including those effects previously omitted. These simulations corroborate the prior hypothesis that slow mode shocks persist well after the retraction has ended, thus producing a compact, loop-top source instead of an elongated jet, as steady reconnection models predict. Thermal conduction leads to densities higher than analytic estimates had predicted, and evaporation enhances the density still higher, but at lower temperatures. X-ray light curves and spectra are synthesized by convolving the results from a single TFT simulation with the rate at which flux is reconnected, as measured through motion of flare ribbons, for example. These agree well with light curves observed by RHESSI and GOES and spectra from RHESSI. An image created from a superposition of TFT model runs resembles one produced from RHESSI observations. This suggests that the HXR loop-top source, at least the one observed in this flare, could be the result of slow magnetosonic shocks produced in fast reconnection models like Petschek's.