On the lunar modulation of geomagnetic activity, 1884-1931 and 1932-1959

The lunar modulation of geomagnetic activity for the years 1884–1959 is investigated by means of the magnetic character figure Ci. For the years 1932–1959, the Ci index gives a pattern very similar to that previously found with the Kp index for 1932–1962. This pattern appears also for the interval 1884–1889, but it fails to appear in any of the four sunspot cycles between 1890 and 1933. The absence of this modulation over the years 1890–1931 would seem to indicate that the apparent effect found for earlier and later periods resulted by chance from random fluctuations, but the possibility is also considered that it may arise from a variation in the length of the earth's tail, related in turn to the 80-year cycle in solar activity.