Fe XIV green/Fe XIII infrared line ratio diagnostics

Abstract

We consider the first 27-level atomic model of Fe XIII (5.9 K < log Te < 6.4 K) to estimate its ground level populations, taking account of electron as well as proton collisional excitations and de-excitations, radiative cascades, radiative excitations and de-excitations. Radiative cascade is important but the effect of dilution factor is negligible at higher electron densities. The 3/SupP/Sub1 - 3/SupP/Sub0 and 3/SupP/Sub2 -3/SupP/Sub1 transitions in the ground configuration 3s/Sup23p/Sup2 of Fe XIII result in two forbidden coronal emission lines in the infrared region, namely 10747 and 10798 Ao While the 5303 Aogreen line is formed in the 3s/Sup2 3p ground configuration of Fe XIV as a 2/SupP/Sub3/2-2/SupP/Sub1/2 magnetic dipole transition. The line-widths of simultaneously observed Fe XIV green and Fe XIII infrared forbidden coronal emission lines can be a useful diagnostic tool to de- duce temperature and nonthermal velocity in the largescale coronal structures using intensity ratios of the lines as the temperature signature, instead of as- suming ion temperature to be equal to the electron temperature. Since the line intensity ratios IG5303 /II R10747 and IG5303 /II R10798 have very weak density de- pendence, they are ideal monitors of temperature mapping in the solar corona. The computed ratios will be compared with the recently obtained observations in our next paper.