Theoretical emission line ratios for Si XIII compared to solar observations

The electron collision excitation rates recently calculated for transitions in Si XIII by Keenan et al. (1987) are used to derive the electron temperature sensitive ratio G(=(f + i)/r and the density sensitive ratio R(=f/i), where i, f, and r are the intercombination (1s²¹S − 1s2p³P_{1, 2}) forbidden (1s²¹S − 1s2s³S), and resonance (1s²¹S − 1s2p¹P), transitions respectively. Also estimated are the values of R in the low-density limit (R₀) as a function of electron temperature. The theoretical G ratio at the temperature of maximum emissivity for Si XIII, G(T_m) = 0.70, is in much better agreement with the observed G for the 1985, May 5 flare determined by McKenzie et al. (G = 0.60 ± 0.07) than is the earlier calculation of Pradhan, who derived G(T_m) = 0.85. The error in the observed R₀ ratio is so large that both our result and Pradhan's fall within the acceptable limits of uncertainty and hence one cannot estimate which of the two is the more accurate.