Discussion of paper by M. E. Rassbach, A. J. Dessler, and A. G. W. Cameron, ‘The lunar period, the solar period, andKp’

In a recent issue of this Journal, Rassbach et al. [1966] argue that the variation of geomagnetic activity with lunar phase, attributed by some investigators [Bell and Defouw, 1964 (paper 1); 1966a; Stolov and Cameron, 1964; Stolov, 1965] to an interaction between the moon and the magnetosphere, is actually due to the sun. They further state that these various authors were led astray because, in calculating statistical significance, they did not take account of the 27-day recurrence tendency or solar period in the indices of geomagnetic activity. Most of the statistical significances derived from 5-day running means of Kp in the Bell and Defouw papers are indeed too high, because our equation for the standard error did not include terms for the solar period. However, we do not find that consideration of the 27-day solar period by Rassbach et al. demolishes all persuasive evidence for the existence of a lunar modulation of geomagnetic activity, nor do we find that it establishes the apparent effect as resulting from the solar period.