Measurements of Electron Anisotropy in Solar Flares Using Albedo with RHESSI X-Ray Data
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Measurements of Electron Anisotropy in Solar Flares Using Albedo with RHESSI X-Ray Data E.C.M. Dickson · E.P. Kontar
Received: 2 December 2011 / Accepted: 19 October 2012 / Published online: 13 November 2012 © Springer Science+Business Media Dordrecht 2012
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. Keywords Flares · X-ray bursts, spectrum · Energetic particles, electrons · Corona
1. Introduction Solar flares belong to the most energetic processes which occur in the solar system. However, the details of the acceleration processes responsible are still poorly known. The X-rays
Advances in European Solar Physics Guest Editors: Valery M. Nakariakov, Manolis K. Georgoulis, and Stefaan Poedts E.C.M. Dickson () · E.P. Kontar SUPA School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ Scotland, UK e-mail: [email protected]
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E.C.M. Dickson, E.P. Kontar
which are observed at the Earth are commonly produced by the accelerated electrons interacting with the solar plasma and producing bremsstrahlung radiation. To understand solar flares it is therefore important to understand the distribution of these accelerated electrons. In general, this distribution will vary in space, with energy, and in pitch angle (see Holman et al., 2011; Kontar et al., 2011 for recent reviews of electron properties in solar flares). Several techniques have been used to estimate the anisotropy in the pitch-angle distribution of X-ray emitting electrons in solar flares (Holman et al., 2011; Kontar et al., 2011). The most commonly used method is to
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