Coulomb Energy Losses in the Solar Corona and the Proton Energy Budget in Flares

Publisher Website
Google Scholar
Abstract
It has recently been proposed, on the basis of measurements of the flux in the 20Ne 1.634 MeV line, that the energy budget for nonthermal protons in solar flares may be significantly larger than previously assumed. The argument is founded on the fact that the 1.634 MeV feature has a (proton) excitation threshold energy significantly lower than that of the C and O lines in the 4-6 MeV range. Hence the observed enhanced level of emission in the 1.634 MeV line requires a higher flux of low-energy (~1 MeV) protons than would be obtained from a backward extrapolation of the ~10 MeV spectrum using canonical (i.e., modified Bessel function) spectral forms and so a greater overall energy content. In this paper we check the effects on this conclusion of two significant factors omitted from the previous analysis, which was based on a "cold" chromospheric target model. While such a model may be appropriate for protons of ~10 MeV energies, protons of ~1 MeV may undergo a significant part of their energy loss in the hot corona, which is ionized and also "warm" for beam protons of these energies. The ionization results in a Coulomb logarithm (and energy loss rate) almost 3 times higher than in the neutral chromosphere. On the other hand, the warm target effect results in energy losses a factor of 1-10 times lower than in a cold target. Thus, if beam protons underwent a substantial part of their energy loss in the corona (depending on the column density encountered), previous conclusions from the 20Ne line flux could be either enhanced or negated, depending on which effect dominates. We show that for likely flare coronal temperatures and column densities that the net consequences for the 20Ne flux are in fact small, unless the low-energy protons are preferentially trapped in an improbably hot dense magnetic island.