Astron. Astrophys. 346, L69-L72 (1999) 2. Theoretical line ratiosThe Si IV model ion used in this work consists of the four energetically lowest LS terms (3s^{2}S, 3p^{2}P, 3d^{2}D and 4s^{2}S), 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 , 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): , , , , , , , , , and . to are -sensitive, whereas , and are independent of both and , and are all under conditions applicable to the solar transition region. They are therefore very useful for identifying line blends. Fig. 2 contains two plots showing the -sensitive line ratios to . The large temperature sensitivity of the ratios is clear from the figures, with to increasing by a factor of over the range shown, and to changing by a factor of over the same interval.
© European Southern Observatory (ESO) 1999 Online publication: June 17, 1999 |