1. Metzger, A. E., Anderson, E. C., Van Dilla, M. A., and Arnold, J. R., Nature, 204, 766 (1964).
2. Rocchia, R., Rothenflug, R., Boclet, D., Ducros, G., and Labeyrie, J., in Space Research (edit. by Smith-Rose, R. L., and King, J. W.), 7, 1327 (North-Holland Publ. Co., Amsterdam, 1967).
3. Okudaira, K., and Hirasima, Y., Canad. J. Phys., 46, 494 (1968).
4. Peterson, L. E., in Space Research (edit. by Smith-Rose, R. L.), 6, 53 (North-Holland Publ. Co., Amsterdam, 1966).
5. Dolan, J. F., and Fazio, G. G., Rev. Geophys., 3, 319 (1965).