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

The present analysis of the Si III spectrum obtained by the Skylab spectrograph as the slit was stepped across the solar disk, and out above the limb, interprets the ratio between a pair of these lines to be due to the effect of opacity; its variation is consistent with a simple model for the atmosphere, allowing the calculation of optical depths for several of the Si III transitions at the limb and solar center. The results obtained are noted to compare well with those estimated on the basis of the solar model D or Gabriel (1976). An effective thickness for the Si III-emitting region of the solar atmosphere is calculated to be 26 km, in excellent agreement with the 22-km value deduced from Gabriel's solar transition region model D.