1. Benz, A. O. and Wentzel, D. G.: 1981, Astron. Astrophys. 94, 100.
2. Bruzek, A.: 1974, in G. Newkirk (ed.), ‘Coronal Disturbances’, IAU Symp. 57, 323.
3. De Groot, T., Loonen, I., and Slottje, C.: 1976, Solar Phys. 48, 321.
4. Elgerøy, Ø. and Ugland, O.: 1970, Astron. Astrophys. 5, 372.
5. Fokker, A. D.: 1960, Doctoral Thesis, University of Leiden.