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Astron. Astrophys. 346, L69-L72 (1999)

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2. Theoretical line ratios

The Si IV model ion used in this work consists of the four energetically lowest LS terms (3s2S, 3p2P, 3d2D and 4s2S), giving a total of six fine-structure levels. Energy levels were taken from Martin & Zalubas (1983), electron impact excitation rates from Dufton & Kingston (1994) and Einstein A -coefficients from Maniak et al. (1993). Level populations were calculated using the equilibrium code of Dufton (1977). Relative emission line strengths and resulting line ratios were calculated for electron temperatures ranging from [FORMULA], in steps of 0.1 dex.

The Si IV transitions considered in this paper are illustrated in Fig. 1, and the emission line ratios we consider are as follows (using the reference numbers in Fig. 1): [FORMULA], [FORMULA], [FORMULA], [FORMULA], [FORMULA], [FORMULA], [FORMULA], [FORMULA], [FORMULA], [FORMULA] and [FORMULA]. [FORMULA] to [FORMULA] are [FORMULA]-sensitive, whereas [FORMULA], [FORMULA] and [FORMULA] are independent of both [FORMULA] and [FORMULA], and are all [FORMULA] under conditions applicable to the solar transition region. They are therefore very useful for identifying line blends. Fig. 2 contains two plots showing the [FORMULA]-sensitive line ratios [FORMULA] to [FORMULA]. The large temperature sensitivity of the ratios is clear from the figures, with [FORMULA] to [FORMULA] increasing by a factor of [FORMULA] over the [FORMULA] range shown, and [FORMULA] to [FORMULA] changing by a factor of [FORMULA] over the same [FORMULA] interval.

[FIGURE] Fig. 1. Term diagram for Si IV showing the six energy levels and transitions considered in this paper. Each level is labelled with its electron configuration, and a reference number from 1 to 6.

[FIGURE] Fig. 2. Theoretical Si IV line intensity ratios [FORMULA] to [FORMULA], plotted as a function of electron temperature. We note that [FORMULA] and [FORMULA] are indistinguishable from [FORMULA] and [FORMULA] respectively, so only one ratio is plotted in each of these instances.

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999