II. Recurrence phenomena in terrestrial magnetism

Abstract

1. The international scheme with headquarters at De Bilt, Netherlands, which supplies for each Greenwich day a magnetic character figure varying from 0·0 (very quiet) to 2·0 (very disturbed), has been in operation since January 1, 1906. The publication in 1926 of the results for 1925 brought up to 20 the number of complete years for which data are available. Of late, owing partly to a supposed connection between wireless and magnetic phenomena, the existence of a 27-day interval in magnetic disturbance has received increased attention. Further, Dr. Deslandres, of Meudon Observatory, has put forward the view that, in addition to the 27-day interval T, there are shorter intervals i T/6, where i is integral. Thus the time seemed to have arrived for carrying out a more exhaustive enquiry than was possible when the international character figures were first used to prove the existence of the 27-day interval. The length of the interval has been generally accepted as evidence that its ultimate cause is resident in the sun. In accordance with ideas prevalent since the time of the late Prof. Kr. Birkeland, it is supposed that magnetic disturbance is due to the discharge from the sun of some form of electricity carrier, and it is often assumed, following Birkeland, that sunspots are the areas where the discharge originates. During the 20 years 1906 to 1925, sunspot frequency and the mean annual solar latitude of spots have varied as shown in Table I. In our treatment of the subject, the 11 years 1906 to 1909, 1915 to 1920 and 1925 are regarded as years of many sunspots, the remaining 9 years as years of few sunspots.Again, 1906, 1907, 1913 to 1918, and 1923 to 1925 are regarded as years of high spot latitude, the remaining 9 years as years of low spot latitude. The reason for grouping the years was the large part played by “accident” in individual years.