1. CHERNOSKY, Edwin J., Double sunspot-cycle variation in terrestrial magnetic activity. 1884–1963, J. Geophys. Res., 71, 965, 1966.
2. DODSON, Helen W., E. Ruth HEDEMAN, and O. C. MOHLER, Comparison of activity in solar cycles 18,19, and 20, Rev. Geophys. Space Phys., 12, 329, 1974.
3. FEYNMAN, Joan, The 21st solar cycle: aa index of geomagnetic activity, in Solar-Terrestrial Predictions Proceedings, Vol. 4, U.S. Dept. of Commerce, NOAA Environmental Research Laboratories, Boulder, CO, A-8, 1979.
4. GAO, Meiqing, An estimation of geomagnetic activity maximum in the twenty first solar cycle, Acta Geophysica Sinica, 24, 455, 1981.
5. SARGENT, H. H. III, A prediction for the next sunspot cycle, in Conf. Record of the Vehicular Technology Soc., 28th IEEE Vehicular Technology Conference, Mar 22–24, 1978, Denver, CO, 490, 1978.