Euv Emission Line Ratios for Si IV in the Solar Transition Region

Abstract

AbstractTheoretical electron temperature sensitive emission line ratios in Si IV involving the 3d2D − 3p2P and 4s2S - 3p2P multiplets at ~1125A and 816A respectively are derived using R-matrix electron excitation rate calculations. A comparison of these with observational data for a solar active region obtained with the Harvard S-055 spectrometer on board Skylab reveals that there is good agreement between theory and observation for the ratio that includes the2D3/2, 5/2 −2p3/2 transition at 1128.3A. This is in contrast to the findings of Keenan, Dufton and Kingston, and provides support for the atomic data adopted in the calculations. However the2D3/2 −2pl/2 line at 1122.5A appears to be severely blended, as it leads to electron temperature estimates that differ significantly from that expected in ionisation equilibrium. The fact that the I(1122.5A)/I(1128.3A) intensity ratios determined from several flare spectra are closer to theory than that for the active region indicates that the blending is probably due to species with relatively low ionisation potentials, such as an Fe III transition at 1122.53A, as suggested previously by Burton and Ridgeley, and Feldman and Doschek. Electron temperatures deduced for a sunspot are much lower than that predicted from ionisation balance calculations, in agreement with earlier results, and imply that a cooling flow may be present.