An approximate calculation of the effect of opacity on Si III emission line ratios near the solar limb

In this paper we analyse the Si III spectrum observed by the Naval Research Laboratory's spectrograph (S082-B) aboard Skylab as the slit was stepped across the disc and out above the limb. We interpret the ratio between a pair of these lines as due to the effect of opacity, and show that its variation is consistent with a simple model for the atmosphere. In this way we are able to calculate the optical depths of several transitions in Si III, both at the solar limb and disc centre, which are found to compare favourably with those estimated from solar model D of Gabriel, although for model A the discrepancies range up to a factor of 6. All the Sim lines have small optical depths, apart from |3s 3p³ P₂-3p² ³P₂| and the resonance line |3s2 1S₀-3s 3p 1P₁|⁠, which are unsuitable for use in emission line ratio diagnostics at the limb. However, the former may be useful for observations made near to the disc centre. The optical depth of the |3s 3p³ P₂-3p² ³P₂| line at disc centre is used in conjunction with an estimate of the absolute population density of the |3s 3p ³P₂| level to calculate an effective thickness for the Sim emitting region in the solar atmosphere of Heff=26 km, which is in excellent agreement with the value of Heff=22 km deduced from solar transition region model D of Gabriel.