Coronal thick target hard X-ray emissions and radio emissions

Abstract

A distinctive class of hard X-ray (HXR) sources located in the corona was recently found, which implies that the collisionally thick target model (CTTM) applies even to the corona. We investigated whether this idea can be independently verified by microwave radiations which have been known as the best companion to HXRs. This study is conducted on the GOES M2.3 class flare which occurred on 2002 September 9 and was observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager and the Owens Valley Solar Array. Interpreting the observed energy-dependent variation of HXR source size under the CTTM, the coronal density should be as high as 5 × 1011 cm-3 over a distance of up to 12″. To explain the cutoff feature of the microwave spectrum at 3 GHz, however, we require a density no higher than 1 × 1011 cm-3. Additional constraints must be placed on the temperature and magnetic field of the coronal source in order to reproduce the microwave spectrum as a whole. First, a spectral feature called the Razin suppression requires a magnetic field in a range of 250-350 G along with high viewing angles around 75°. Second, to avoid excess fluxes at high frequencies due to the free-free emission that was not observed, we need a high temperature ≥2 × 107 K. These two microwave spectral features, Razin suppression and free-free emissions, become more significant at regions of high thermal plasma density and are essential for validating and determining additional parameters of the coronal HXR sources. © 2013. The American Astronomical Society. All rights reserved..