1. Athay, R. G.: 1970, Solar Phys. 11, 347.
2. Avrett, E. H.: 1981, in L. Cram and J. H. Thomas (eds.), The Physics of Sunspot. Sacramento Peak Observatory, p. 235.
3. Babij, B. T. and Rikalyuk, R. E.: 1981, Astron. Zh. 58, 825.
4. Beckers, J. M. and Milkey, R. W.: 1975, Solar Phys. 43, 289.
5. Carlsson, M.: 1986, Uppsala Astron. Report, No. 33, 1.