Electron distribution and energy release in magnetic reconnection outflow regions during the pre-impulsive phase of a solar flare

Abstract

We present observations of electron energization in magnetic reconnection outflows during the pre-impulsive phase of solar flare SOL2012-07-19T05:58. During a time-interval of about 20 minutes, starting 40 minutes before the onset of the impulsive phase, two X-ray sources were observed in the corona, one above the presumed reconnection region and one below. For both of these sources, the mean electron distribution function as a function of time is determined over an energy range from 0.1~keV up to several tens of keV, for the first time. This is done by simultaneous forward fitting of X-ray and EUV data. Imaging spectroscopy with RHESSI provides information on the high-energy tail of the electron distribution in these sources while EUV images from SDO/AIA are used to constrain the low specific electron energies. The measured electron distribution spectrum in the magnetic reconnection outflows is consistent with a time-evolving kappa-distribution with $\kappa =3.5-5.5$. The spectral evolution suggests that electrons are accelerated to progressively higher energies in the source above the reconnection region, while in the source below, the spectral shape does not change but an overall increase of the emission measure is observed, suggesting density increase due to evaporation. The main mechanisms by which energy is transported away from the source regions are conduction and free-streaming electrons. The latter dominates by more than one order of magnitude and is comparable to typical non-thermal energies during the hard X-ray peak of solar flares, suggesting efficient acceleration even during this early phase of the event.

9 pages, 5 figures, accepted for publication in ApJ

References in corpus (6)

• Relations between concurrent hard X-ray sources in solar flares
• Kappa distribution and hard X-ray emission of solar flares
• On the variation of solar flare coronal x-ray source sizes with energy
• Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles
• Differential emission measure Analysis of a limb solar flare on 2012 July 19
• Where is the chromospheric response to conductive energy input from a hot pre-flare coronal loop?

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• On the source of the Fe K-alpha emission in T Tauri Stars. Radiation induced by relativistic electrons during flares. An application to RY Tau
• Energy-Containing Electrons in Solar Flares: Improving Hard X-Ray and EUV Diagnostics