1. Van Allen, J. A., and Frank, L. A., Nature, 183, 430 (1959).
2. Vernov, S. N., Chudakov, A. Ye., Vakulov, P. V., and Logachev, Yu. I., Doklady Akademia Nauk S.S.S.R., 125, 304 (1959).
3. Billings, Prescott and Dolder, Preliminary Report of Solar Activity TR 392 of March 6, 1959, High Altitude Observatory, Boulder, Colorado.
4. Trotter, Dorothy, E., “Solar Activity preceding the Geomagnetic Storms of 25 February and 26 March, 1959” (private communication).
5. The trajectory of Pioneer III in Fig. 5 of ref. 1 was mistakenly not converted to geomagnetic co-ordinates. The corrected version of Fig. 5 differs only slightly from the one published, however.