The relative intensities of CI lines in the solar EUV spectrum

Abstract

The relative intensity of two Ci lines at 1993.6 A and 1657.4 A, observed in the limb spectrum of the sun, is a factor 2.6 × 103 larger than that expected if both lines were optically thin. It is shown that the observed intensity ratio may be explained in terms of the transfer of photons from λ 1657.4 A to λ 1993.6 A due to a large optical depth in the line at 1657.4 A. The observed upper limit on the relative intensity of two further lines at 1992.0 A and 1657.0 A has been used to show that the line at 1993.6 A is optically thin. Hence it is shown that τ(1657.4 A) = 1300, and τ(1993.6 A) = 0.44. These values provide an independent evaluation of optical depths against which chromospheric models may be checked. Assuming a mean temperature of Te = 8000 °K, and a mean scale height of 350 km, the optical depths lead to a mean hydrogen-particle density of N (H) = 1.4 × 1012 cm−3.