1. Antonucci, E.: 1974, Solar Phys. 34, 471.
2. Antonucci, E. and Duvall, T. L.: 1974, submitted to Solar Phys.
3. Babcock, H. W. and Babcock, H. D.: 1955, Astrophys. J. 121, 349.
4. Bumba, V. and Obridko, V. N.: 1969, Solar Phys. 6, 104.
5. Dodson, H. W. and Hedeman, E. R.: 1968, ?Upper Atmospheric Geophysics Report UAG-2?, ESSA Research Laboratories, Boulder, Colorado.