The formation of the Mg II resonance lines in the solar atmosphere

Synthetic LTE profiles, calculated for the HSRA, VAL, and the so-called 'Ca II and Mg II' model atmospheres introduced by Ayres and Linsky, are compared with the Mg II profiles (2760 A < approximately lambda < approximately 2820 A) of the average quiet Sun observed at μ = 1.0, 0.6 and 0.43. As a prerequisite for calculating profiles, for each model atmosphere the author derives empirically the total opacity as function of the height so that the continuum intensity and limb darkening observed at lambda 2660 A and lambda 2940 A is reproduced. For each model atmosphere the most appropriate value of Aγ* (A: abundance of Mg, γ*: multiplying factor applied to the collisional broadening calculated from Unsold's expression) is derived from the best fit of the calculated profiles and the central parts of the profiles observed at μ = 1.0 and 0.6. These values Aγ* reproduce the far wings of the Mg II flux profile. The comparison of the computed profiles and the observed profiles indicates that none of the one-component, static model atmospheres of the average quiet Sun reproduces the central parts of the Mg II profiles in their centre to limb variation. The proposal by Ayres and Linsky of higher minimum temperatures is not supported by our observations. (Auth.)