The 27-day period (interval) terrestrial magnetic disturbance

The object of the present research is to show that the existence of a 27-day period or interval, in a series of magnetic disturbances, is connected with a restricted area of solar disturbance, and not with a periodic disturbed condition of the whole solar surface. This question is raised in Dr. Chree’s exhaustive study of the 27-day period in Terrestrial Magnetism (‘Roy. Soc. Proc.,’ A, vol. 101, p. 390, 1922). Dr. Chree likewise notes "that low sun-spot frequency and a low latitude of spots both conduce to the development of the 27-day period” (loc. cit .,p. 382). This conclusion is entirely in accordance with my own results for the last three solar cycles, as set forth in former papers (e.g., This conclusion is entirely in accordance with my own results for the last three solar cycles, as set forth in former pap (e.g., Monthly Notices, R. A. S. 83, p. 214, 1923). Accordingly the years 1921, 1922 and 1923 to July have been selected for study, in which years both the conditions mentioned by Dr. Chree have been fulfilled. Two long series of magnetic disturbances mark this period of comparative solar calm, which extend from 1921 (May) to 1923 (July), in which latter year the sun was almost entirely free from solar disturb­ance. The details are set forth in the following Tables. The first concerns a series of magnetic disturbances, at 27-day intervals, which covered the period 1921 (May 12) to 1923 (April 13), and the second deals with a similar series, extending from 1921 (October 27) to 1923 (July 11). 2. In these Tables, the first column gives the serial numbers of the magnetic disturbances, the second the dates of their occurrence, the third the intervals in days between successive disturbances, and the fourth and the fifth the extreme ranges of declination angle in minutes of arc, and of horizontal force in units Ƴ, where lϒ = 10-5 C. G. S. Units. The next six columns, from the seventh to the eleventh, are concerned with the magnetic character of the days of disturbance, and since disturbed conditions may not unfrequently persist for three or four, or even more days, also with the magnetic character of the two days immediately preceding, and of the two days immediately following that actual day of disturbance, which forms a member of the series. For the series days the magnetic character is given in two forms, the international figure and the Stonyhurst letter, according to the method which has been adopted in our Annual Reports since the year 1889. In the international scheme of character figures, 0 means a quiet day, 1 a moderately disturbed day, and 2 a highly disturbed dav. In the Stonyhurst nomenclature,c means a calm day,s a day of small magnetic disturbance,m of moderate disturbance,g of great disturbance, andv. g . of very great disturbance. From a comparison of these character letters with the figures published from the central international station at De Bilt for the years 1921, 1922, the mean values of the letters in international figures are e = 0.2,s = 0.6,m = 0.9,g = 1.3 andv. g . = 1.5. These values have been employed in the Tables, with slight modifications in particular cases, depending on the character of the disturbance, for the year 1923, for which the international figures from De Bilt were not available at the time of writing.