Warm thick target solar γ-ray source revisited

¹ Department of Adult and Continuing Education, University of Glasgow, Glasgow G3 1LP, UK
² Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK e-mail: mark@astro.gla.ac.uk

Corresponding author: A. L. MacKinnon, a.mackinnon@educ.gla.ac.uk

Received: 1 July 2002
Accepted: 18 June 2003

Abstract

The 1.63 MeV γ-ray line of ²⁰Ne is sensitive to protons of lower energies than most other nuclear de-excitation lines. Its unexpected strength has been taken as evidence for a solar flare fast ion distribution that remains steep at low energies, and thus has a large total energy content. It has also been suggested that its strength might instead reflect the enhancement of ion lifetimes that occurs when ambient temperatures exceed 10⁷ K. Here we revisit this idea (a) recognising that ions may be effectively trapped in high temperature regions and (b) taking account of the contribution to the line of all ions above threshold. The strength of the 1.63 MeV line relative to other de-excitation lines has been used to estimate the steepness (e.g. energy power-law index) of the ion distribution. We show that these estimates must be significantly revised if primary ions are contained in a region with temperature in the few 10⁷ K range, lower than found elsewhere. Such a region would almost certainly be coronal, so we also briefly review other arguments for and against coronal γ-ray sources.