1. Anderson, K. A. and Winckler, J. R.: 1962, J. Geophys. Res. 67, 4103.
2. Bowen, P. J., Norman, K., Pounds, K. A., Sanford, P. W., and Willmore, A. P.: 1964, in Proc. Roy. Soc. A. 281, 538.
3. Chubb, T. A., Friedman, H., and Kreplin, R. W.: 1960, Space Research, I. (ed. by H. K. Bijl). North-Holland Publ. Co., Amsterdam, p. 695.
4. Chubb, T. A., Kreplin, R. W., and Friedman, H.: J. Geophys. Res. 71, 3611.
5. De Jager, C.: 1965, in IAU Symposium 23 (Astronomical Observations from Space Vehicles), 45 (=Ann. Astrophys. 28, 263).