The 27-day period in magnetic phenomena

§ 1. In two earlier papers, termed here for brevity SM1 * and SM2 , I have discussed data supporting the existence of a period of about 27 days in magnetic phenomena. The results have, I think, put it beyond question that there has been—at all events since 1890—a decided tendency for the magnetic character of a day, whether disturbed or quiet, to resemble that of a day either 27 days earlier or 27 days later more closely than it does that of the average day. There is no period—or, if there is, its amplitude is very small—in the usual sense of the word, the phenomena do not go on repeating themselves in an indefinite sequence like the phenomena of day and night. Various ways suggest themselves of prosecuting the enquiry further, with a view to ascertaining its connection with other natural occurrences, especially with sunspots or other solar phenomena. Having at hand a long series of magnetic records, the method that appeals to me as a practical man is the utilisation of these records. The next question is how to use them to best advantage. If the 27-day period, as we may term it for brevity, is directly associated with sunspots, we should expect it to be influenced by the difference between the solar latitude of sunspots in years of many and few sunspots. My previous work has shown that the period, though ordinarily far from insignificant, may sometimes be little in evidence. There is much that in the present state of our knowledge must be regarded as “accidental” in magnetic phenomena, and a few “ accidents ” conspiring together may render the 27-day period unapparent in an individual year. Thus, for an enquiry into the nature of the relationship, if any, between the period and sunspots, or any other solar or lunar phenomenon, data are wanted from a period so long that accidental phenomena will be largely eliminated from the groups of years representative of any one given set of conditions such as sunspot maximum or minimum. As a preliminary to any such general enquiry, the measure of magnetic disturbance to be adopted claims consideration. The measure adopted in SM2, and also mainly in SM1 , was the “character” of the day on the international scale 0 (quiet), 1 (moderately disturbed), 2 (highly disturbed). Since 1906 mean “character” figures taken out to 0.1, based on the data from a large number of stations, have been published at De Bilt. These enable the days of each month to be arranged easily, and probably with adequate accuracy, in the order of their disturbance. It is, however, impossible to say how the day whose character in the international lists is, say, 0 5 compares with the day whose international figure is, say, 1.5. If one takes the “activity” of the day as defined by Prof. Bidlingruaier, which represents practically the integral of the squares of instantaneous departures from a normal value, the day whose international figure is l-8 is much more than twice as disturbed as the day whose international figure is 0'9. There is a good deal to be said for “activity” as a measure of disturbance, but activity data are practically non-existent, and deriving them for any considerable period of years would represent an enormous deal of labour. A second objection is that there are a few days whose “ activity ” is so enormously greater than that of the average day that an “accidental” occurrence of two of these might introduce a false period into the records of even a number of years.