1. Fedoretz, V. A.: 1952,Uch. Zap. Kharkovsk. Gos. Univ., Nos. 42, 49.
2. Gehrels, T., Coffee, T., and Owings, D.: 1964,Astron. J. 69, 826.
3. Kenknight, G. E., Rosenberg, D. L., and Wehner, G. K.: 1967,J. Geophys. Res. 72, 3117.
4. Lane, A. and Irvine, W.: 1972, Contribution Number 159 of the Five College Observatories.
5. Mahmoud, F. M.: 1971, M. Sc. Thesis, Cairo University.