1. Biermann, L.: 1951, Z. Astrophys.
29, 274.
2. Billings, D. E.: 1966, A Guide to the Solar Corona, Academic Press, New York.
3. Bray, R. J. and Loughhead, R. E.: 1964, Sunspots, John Wiley and Sons, New York.
4. Broxon, J. W.: 1942, Phys. Rev.
62, 508.
5. Chapman, S.: 1943, Monthly Notices Roy. Astron. Soc.
103, 117.