Warm thick target solar (gamma)-ray source revisited

The 1.63 MeV (gamma)-ray line of  20Ne is sensitive to protons of lower energies than most other nuclear deexcitation 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 10^7 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 (gamma)-ray sources.