1. Air Force's Air Weather Service Pamphlets, “Solar Optical Observing” (e. g. 1981: 105–60; 1988: 105–10).
2. Bornmann, P L., Kalmbach, D., Kulhanek, D., and Casale, A., “A Study of the Evolution of Solar Active Regions for Improving Solar Flare Forecasts”, Proceedings of the Solar Terrestrial Predictions Workshop held October 1989, vol. 1, p. 301, 1991.
3. Kildahl, K. J. N., “Frequency of Class M and X Flares by Sunspot Class (1969–1976)”, Solar-Terrestrial Predictions Proceedings, vol. 3, Donnelly, R. (ed), p. 166, 1980.
4. McIntosh, P S., “The Classification of Sunspot Groups”, Solar Physics, 125, 251, 1990.
5. Neidig, D. F, Wiborg, P H., Seagraves, P H., Hirman, J. W, and Flowers, W E., “Objective Forecasts for Solar Flares using Multivariate Discriminant Analysis”, Proceedings of the Solar Terrestrial Predictions Workshop, p. 300, 1984.