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

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.

17 pages, 11 figures, accepted for publication in ApJ

References in corpus (8)

• Impulsive phase flare energy transport by large-scale Alfven waves and the electron acceleration problem
• Double Coronal Hard and Soft X-ray Source Observed by RHESSI: Evidence for Magnetic Reconnection and Particle Acceleration in Solar Flares
• Relations between concurrent hard X-ray sources in solar flares
• The Formation of Kappa-Distribution Accelerated Electron Populations in Solar Flares
• RHESSI Observation of Chromospheric Evaporation
• First detection of >100 MeV gamma rays associated with a behind-the-limb solar flare
• Kappa distribution and hard X-ray emission of solar flares
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Cited by in corpus (12)

• The Acceleration of Charged Particles and Formation of Power-law Energy Spectra in Nonrelativistic Magnetic Reconnection
• Energetic Electron Distribution of the Coronal Acceleration Region: First results from Joint Microwave and Hard X-ray Imaging Spectroscopy
• The Neupert Effect of Flare UltraViolet and Soft X-ray Emissions
• Electron distribution and energy release in magnetic reconnection outflow regions during the pre-impulsive phase of a solar flare
• A study of particle acceleration, heating, power deposition, and the damping length of kinetic Alfvén waves in non-Maxwellian coronal plasma
• Modelling Solar Energetic Neutral Atoms from Solar Flares and CME-driven Shocks
• Automatic Detection of Occulted Hard X-ray Flares Using Deep-Learning Methods
• High-Energy Insights from an Escaping Coronal Mass Ejection with Solar Orbiter/STIX Observations
• Expansion of Hot Plasma into Cold Plasma
• Flare-accelerated Electrons in the Kappa Distribution from X-Ray Spectra with the Warm-Target Model
• GOES class estimation for behind-the-limb solar flares using MESSENGER SAX
• Energy-Containing Electrons in Solar Flares: Improving Hard X-Ray and EUV Diagnostics