Extreme ultraviolet emission lines of Ar XIV in solar active region and flare spectra

Abstract

New R-matrix calculations of electron impact excitation rates for transitions among the 2s 2 2p, 2s2p 2 and 2p 3 levels of Ar XIV are presented. These data are subsequently used to derive the theoretical electron density diagnostic emission-line intensity ratios R 1 = I(187.95 A)/I(194.41 A) and R 2 = I(257.40 A)/I(243.78 A) for a range of densities (N e = 10 9 -10 1 3 cm - 3 ) and electron temperatures (T e = 10 6 . 3 -10 6 . 7 K) appropriate to solar transition region and coronal plasmas. A comparison of these diagnostics with observational data for solar active regions and flares, obtained with the Naval Research Laboratory's S082A spectrograph on board Skylab, reveals that the electron densities determined from R 1 are in good agreement with those estimated from line ratios in Fe XIV or Fe xv, which are formed at similar electron temperatures to Ar XIV. However, there are large discrepancies between densities inferred from the R 2 ratio and those from Fe xiv or Fe xv, confirming that the Ar XIV 243.78- and 257.40-A lines are badly blended with Fe xv 243.79 A and Fe XIV 257.38 A, respectively. Hence, R 2 cannot be employed as a density diagnostic, in contrast to R 1 , which does provide reliable N e estimates.