Weakening of the solar extreme-ultraviolet line emission by Lyman continuum absorption as derived from line ratios

The intensity ratios of N III, O III, O IV, and O V lines observed by the HCO experiment on Skylab are compared with the results of recent multilevel calculations. It is found that solar transition-region spectra require Lyman continuum absorption. The equivalent optical thickness of the absorbers causing the weakening is found to be 1.6-1.7 for a quiet area, 1.4-1.9 for a coronal hole, and 2.4-2.5 for active regions. These values are consistent with previous estimates from different methods. (orig.)