Effect of Collisions and Magnetic Convergence on Electron Acceleration and Transport in Reconnecting Twisted Solar Flare
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Effect of Collisions and Magnetic Convergence on Electron Acceleration and Transport in Reconnecting Twisted Solar Flare Loops M. Gordovskyy · P.K. Browning · E.P. Kontar · N.H. Bian
Received: 15 November 2011 / Accepted: 4 September 2012 / Published online: 10 October 2012 © Springer Science+Business Media Dordrecht 2012
Abstract We study a model of particle acceleration coupled with an MHD model of magnetic reconnection in unstable twisted coronal loops. The kink instability leads to the formation of helical currents with strong parallel electric fields resulting in electron acceleration. The motion of electrons in the electric and magnetic fields of the reconnecting loop is investigated using a test-particle approach taking into account collisional scattering. We discuss the effects of Coulomb collisions and magnetic convergence near loop footpoints on the spatial distribution and energy spectra of high-energy electron populations and possible implications on the hard X-ray emission in solar flares. Keywords Flares, energetic particles · Magnetic reconnection, theory · Energetic particles, acceleration
1. Introduction Most models of particle acceleration assume that particles gain energy in the corona and then are transported into the chromosphere where they produce non-thermal radiation. This assumption follows from the standard solar flare model (e.g. Shibata, 1996; Yokoyama and Shibata, 1998), with the primary energy release site located above the top of a flaring loop or loop arcade. However, the standard scenario faces a number of difficulties (MacKinnon and Brown, 1989; Benka and Holman, 1994; Brown et al., 2009). The main problem is the large flux of electrons required to produce the observed hard X-ray (HXR) intensity. Also, unless precipitating electrons are accompanied by ions, they may result in a strong return current,
Advances in European Solar Physics Guest Editors: Valery M. Nakariakov, Manolis K. Georgoulis, and Stefaan Poedts M. Gordovskyy () · P.K. Browning Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, UK e-mail: [email protected] E.P. Kontar · N.H. Bian School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
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creating additional electrodynamic challenges, such as preventing the electron beam from reaching the chromosphere. It has been suggested recently that re-acceleration of electrons in the chromosphere may help to overcome the “number problem” (Brown et al., 2009). In the proposed scenario electrons are initially accelerated in the corona and transported to the chromosphere. However, unlike in the standard thick-target model (see Brown, 1971), electrons lose their energy much slower due to re-energization by the electric field in the chromosphere. Indeed, this is possible even with comparably small field strength. In the presence of Coulomb collisions, the value of electric field E required to accelerate electrons depends on their energy Ee as E = qn/(eEe ), where q is an almost consta
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