Coronal Thick Target Hard X Ray Emissions and Radio Emissions

TL;DR

This study investigates coronal hard X-ray and radio emissions during a solar flare, using observations to constrain plasma density, temperature, and magnetic field, and to validate the thick target model in the corona.

Contribution

It provides a combined analysis of HXR and microwave data to independently verify the coronal thick target model and constrains key plasma parameters.

Findings

Coronal density estimated at 5×10^11 cm^-3 from HXR source size variation.

Microwave spectrum cut-off at 3 GHz suggests density no higher than 1×10^11 cm^-3.

Magnetic field in the range 250-350 G and temperature ≥2×10^7 K are required to reproduce observed microwave features.

Abstract

Recently a distinctive class of hard X ray (HXR) sources located in the corona was found, which implies that the collisionally thick target model (CTTM) applies even to the corona. We investigated whether this idea can independently be verified by microwave radiations that have been known as the best companion to HXRs. The study is made for the GOES M2.3 class flare occurred on 2002 September 9 that were observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Owens Valley Solar Array (OVSA). Interpreting the observed energy dependent variation of HXR source size under the CTTM the coronal density should be as high as $5\times 10^{11}$ cm$^{-3}$ over the distance up to 12$"$. To explain the cut-off feature of microwave spectrum at 3 GHz, we however, need density no higher than $1\times 10^{11}$ cm$^{-3}$. Additional constraints need to be placed on temperature and magnetic field of the coronal source in order to reproduce the microwave spectrum as a whole. Firstly, a spectral feature called the Razin suppression requires the magnetic field in a range of 250--350 gauss along with high viewing angles around 75$^{\rm o}$. Secondly, to avoid excess fluxes at high frequencies due to the free-free emission that were not observed, we need a high temperature $\geq2\times 10^7$ K. These two microwave spectral features, Razzin suppression and free-free emissions, become more significant at regions of high thermal plasma density and are essential for validating and for determining additional parameters for the coronal HXR sources.