1. Castelli, J. P. and Aarons, J.: 1968, Ann. de Geophys. 24, 813.
2. Castelli, J. P., Aarons, J., Michael, G. A., Jones, C., and Ko, H. C.: 1969, in C. de Jager and Z. ?vestka (eds.), Solar Flares and Space Research, North-Holland, Amsterdam, p. 194.
3. Chree, C. and Stagg, I. M.: 1928, Phil. Trans. A227, 21.
4. Covington, A. E.: 1951, J. Roy. Astron. Soc. Canada 45, 49.
5. Das Gupta, M. K. and Basu, D.: 1963, Nature 197, 442.