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

Frederic Effenberger   Submitted: 2016-12-14 21:42

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.

Authors: Frederic Effenberger, Fatima Rubio da Costa, Mitsuo Oka, Pascal Saint Hilaire, Wei Liu, Vahé Petrosian, Lindsay Glesener, Säm Krucker
Projects: RHESSI

Publication Status: Accepted in ApJ
Last Modified: 2016-12-21 12:21
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Hard X-ray morphology of the X1.3 April 25, 2014 partially occulted limb solar flare  

Frederic Effenberger   Submitted: 2016-05-17 15:32

At hard X-ray energies, the bright footpoint emission from solar flare loops often prevents a detailed analysis of the weaker loop-top source morphology due to the limited dynamic range available for X-ray imaging. Here, we study the X1.3 April 25, 2014 flare with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). This partially occulted limb flare allows the analysis of the loop-top emission in isolation. We present results on the flare light curve at different energies, the source morphology from X-ray imaging and a detailed spectral analysis of the different source components by imaging spectroscopy. The loop-top source, a likely site of particle acceleration, shows a clear composition of different emission components. The results indicate the opportunities that detailed imaging of hard X-rays can provide to learn about particle acceleration, transport and heating processes in solar flares.

Authors: Frederic Effenberger, Fatima Rubio da Costa, Vahé Petrosian
Projects: GOES X-rays ,RHESSI,SDO-AIA

Publication Status: Under Review
Last Modified: 2016-05-17 15:56
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Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles
Hard X-ray morphology of the X1.3 April 25, 2014 partially occulted limb solar flare

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