1. Alfvén, H., “Cosmical Electrodynamics” (Oxford, 1950).
2. Hultqvist, B., Arkiv för geofysik, 3, 63 (1958).
3. Vestine, E. H., Terr. Magn., 49, 77 (1944).
4. Gartlein, C. W., U.S. Visual Observations, News Letter No. 18 (Ithaca, New York, Feb. 23, 1959).