High-Energy Emission from a Solar Flare in Hard X-rays and Microwaves

TL;DR

This study analyzes a large solar flare using simultaneous hard X-ray and microwave observations, revealing complex emission features and challenging existing flare models with new insights into electron spectra and source inhomogeneity.

Contribution

It provides detailed spectral and morphological analysis of a solar flare up to 200 keV, highlighting inhomogeneity and magnetic field strength implications, and challenges standard flare models.

Findings

Microwave emissions may be optically thick at high frequencies.

Magnetic fields in high-frequency radio sources might be stronger than assumed.

Sources over large volumes can show matching hard X-ray spectral evolution.

Abstract

We investigate accelerated electron energy spectra for different sources in a large flare using simultaneous observations obtained with two instruments, the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz, and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) at hard X-rays. This flare is one of the few in which emission up to energies exceeding 200 keV can be imaged in hard X-rays. Furthermore, we can investigate the spectra of individual sources up to this energy. We discuss and compare the HXR and microwave spectra and morphology. Although the event overall appears to correspond to the standard scenario with magnetic reconnection under an eruptive filament, several of its features do not seem to be consistent with popular flare models. In particular we find that (1) microwave emissions might be optically thick at high frequencies despite a low peak frequency in the total flux radio spectrum, presumably due to the inhomogeneity of the emitting source; (2) magnetic fields in high-frequency radio sources might be stronger than sometimes assumed; (3) sources spread over a very large volume can show matching evolution in their hard X-ray spectra that may provide a challenge to acceleration models. Our results emphasize the importance of studies of sunspot-associated flares and total flux measurements of radio bursts in the millimeter range.