Abstract:
IT is well known that moderate geomagnetic storms tend to recur after 27 days. Such storms are not related to sunspots, are almost always of gradual commencement type and are well marked only during two or three years immediately preceding the sunspot minima. A large number of such sequences, consisting of seven or eight storms, have been recorded; a few much longer sequences have been mentioned by Bartels1. The longest sequence of which we are aware in the literature consisted of seventeen recurrences between December 9, 1929, and March 13, 1931.
