Fast electron slowing-down and diffusion in a high temperature coronal X-ray source

¹ Department of Physics and Astronomy, Kelvin Building, University of Glasgow, Glasgow G12 8QQ, UK e-mail: ross@astro.gla.ac.uk
² Department of Adult and Continuing Education, St. Andrews Building, University of Glasgow, Glasgow G3 6NH, UK e-mail: a.mackinnon@educ.gla.ac.uk
³ Department of Physics and Astronomy, Kelvin Building, University of Glasgow, Glasgow G12 8QQ, UK e-mail: eduard@astro.gla.ac.uk
⁴ UKAEA, Culham Science Centre, Abingdon OX14 3DB, UK e-mail: per.helander@ukaea.org.uk

Received: 7 October 2004
Accepted: 18 April 2005

Abstract

Finite thermal velocity modifications to electron slowing-down rates may be important for the deduction of solar flare total electron energy. Here we treat both slowing-down and velocity diffusion of electrons in the corona at flare temperatures, for the case of a simple, spatially homogeneous source. Including velocity diffusion yields a consistent treatment of both “accelerated” and “thermal” electrons. It also emphasises that one may not invoke finite thermal velocity target effects on electron lifetimes without simultaneously treating the contribution to the observed X-ray spectrum from thermal electrons. We present model calculations of the X-ray spectra resulting from injection of a power-law energy distribution of electrons into a source with finite temperature. Reducing the power-law distribution low-energy cutoff to lower and lower energies only increases the relative magnitude of the thermal component of the spectrum, because the lowest energy electrons simply join the background thermal distribution. Acceptable fits to RHESSI flare data are obtained using this model. These also demonstrate, however, that observed spectra may in consequence be acceptably consistent with rather a wide range of injected electron parameters.