AI ix EMISSION LINE RATIOS IN SOLAR FLARES OBSERVED WITH THE S082A SPECTROGRAPH ON BOARD SKYLAB E E KEENAN, E. S. CONLON, D. A. BOWDEN Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 INN, N. Ireland B. N. DWIVEDI Department of Applied Physics, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India and K. G. WIDING Code 4174W, E.O. Hulburt Centerfor Space Research, Naval Research Laboratory, Washington DC 20375-5000, U. S. A. (Received 7 May, 1993; in revised form 2 July, 1993) Abstract. Theoretical electron-density-sensitive emission line ratios in B-like Al IX are presented for R = I(385.01 ,~)/I(392.42 A). A comparison of these with high spectral resolution solar flare data, obtained with the S082A slitless spectrograph on board Skylab, reveals agreement between theory and observation for those spectra that were observed during the later stages of the flares. These results provide experimental support for the accuracy of the line-ratio calculations, and also resolves discrepancies found previously when the theoretical results were compared with solar observations from the S-055 instrument on board Skylab. However, the agreement between theory and observation for a spectrum obtained during the early stages of a flare is very poor, which probably indicates that the 392.42 A line is blended with a transition arising from a species formed at a very high electron temperature. 1. Introduction Emission lines arising from transitions in B-like ions are frequently observed in solar ultraviolet spectra (Vernazza and Reeves, 1978; Sandlin et al., 1986; Feldman and Widing, 1990). The potential usefulness of these lines to infer the electron temperature (Te) and density (Are) of the emitting plasma was first pointed out by Flower and Nussbaumer (1975a), who calculated diagnostic line ratios for O 1v. Since that time, many authors have produced theoretical line ratios for B-like ions applicable to solar spectra, including data for C II (Hayes and Nussbaumer, 1984a, b; Lennon et al., 1985), NlIl (Vernazza and Mason, 1978; Nussbaumer and Storey, 1979; Kastner and Bhatia, 1984a, b), OIv (Vernazza and Mason, 1978; Nussbaumer and Storey, 1982; Kastner and Bhatia, 1984a; Dwivedi and Gupta, 1992), Nevt (Vernazza and Mason, 1978; Dwivedi and Gupta, 1991a), Mg viii (Vernazza and Mason, 1978; Dwivedi and Raju, 1980; Dwivedi, 1988), Six (Flower and Nussbaumer, 1975b; Vernazza and Mason, 1978; Dwivedi and Raju, 1980; Saha and Trefftz, 1982), S XlI (Flower and Nussbaumer, 1975b; Vernazza and Mason, 1978), ArxIv, CaxvI (Dere et al., 1979), and FexxlI (Mason and Storey, 1980). Solar Physics 149: 137-142, 1994. (~) 1994 Kluwer Academic Publishers. Printed in Belgium.