Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles

TL;DR

This study analyzes RHESSI observations of partially occulted solar flares over two solar cycles, revealing details about coronal electron acceleration, spectral components, and the Neupert effect, enhancing understanding of flare energetics.

Contribution

It provides a comprehensive statistical analysis of 116 flares, combining spectral, imaging, and light curve data, and confirms coronal acceleration and thin-target emission models.

Findings

Most spectra fit with thermal plus broken power-law components.

X-ray imaging shows small spatial separation between thermal and non-thermal sources.

Strong correlation between soft and hard X-ray derivatives supports the Neupert effect.

Abstract

Flares close to the solar limb, where the footpoints are occulted, can reveal the spectrum and structure of the coronal loop-top source in X-rays. We aim at studying the properties of the corresponding energetic electrons near their acceleration site, without footpoint contamination. To this end, a statistical study of partially occulted flares observed with RHESSI is presented here, covering a large part of solar cycles 23 and 24. We perform a detailed spectra, imaging and light curve analysis for 116 flares and include contextual observations from SDO and STEREO when available, providing further insights into flare emission that was previously not accessible. We find that most spectra are fitted well with a thermal component plus a broken power-law, non-thermal component. A thin-target kappa distribution model gives satisfactory fits after the addition of a thermal component. X-rays imaging reveals small spatial separation between the thermal and non-thermal components, except for a few flares with a richer coronal source structure. A comprehensive light curve analysis shows a very good correlation between the derivative of the soft X-ray flux (from GOES) and the hard X-rays for a substantial number of flares, indicative of the Neupert effect. The results confirm that non-thermal particles are accelerated in the corona and estimated timescales support the validity of a thin-target scenario with similar magnitudes of thermal and non-thermal energy fluxes.