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Measurements of Electron Anisotropy in Solar Flares Using Albedo with RHESSI X-Ray Data

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Abstract

The angular distribution of electrons accelerated in solar flares is a key parameter in the understanding of the acceleration and propagation mechanisms that occur there. However, the anisotropy of energetic electrons is still a poorly known quantity, with observational studies producing evidence for an isotropic distribution and theoretical models mainly considering the strongly beamed case. We use the effect of photospheric albedo to infer the pitch-angle distribution of X-ray emitting electrons using Hard X-ray data from RHESSI. A bi-directional approximation is applied and a regularised inversion is performed for eight large flare events to deduce the electron spectra in both downward (towards the photosphere) and upward (away from the photosphere) directions. The electron spectra and the electron anisotropy ratios are calculated for a broad energy range, from about ten up to ∼ 300 keV, near the peak of the flares. The variation of electron anisotropy over short periods of time lasting 4, 8 and 16 seconds near the impulsive peak has been examined. The results show little evidence for strong anisotropy and the mean electron flux spectra are consistent with the isotropic electron distribution. The 3σ level uncertainties, although energy and event dependent, are found to suggest that anisotropic distribution with anisotropy larger than ∼ three are not consistent with the hard X-ray data. At energies above 150 – 200 keV, the uncertainties are larger and thus the possible electron anisotropies could be larger.

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Acknowledgements

This work is supported by STFC, UK. The European Commission is acknowledged for funding from the HESPE Network (FP7-SPACE-2010-263086) (EPK).

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  1. SUPA School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, Scotland, UK

    E. C. M. Dickson & E. P. Kontar

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Correspondence to E. C. M. Dickson.

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Advances in European Solar Physics

Guest Editors: Valery M. Nakariakov, Manolis K. Georgoulis, and Stefaan Poedts

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Dickson, E.C.M., Kontar, E.P. Measurements of Electron Anisotropy in Solar Flares Using Albedo with RHESSI X-Ray Data. Sol Phys 284, 405–425 (2013). https://doi.org/10.1007/s11207-012-0178-3

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