Comment [on “Measures of terrestrial‐magnetic activity” by G. van Dijk

Abstract

G. van Dijk presents evidence that the discrepancy between C and u (or u1), especially for the year 1930, may not be due to a break in the homogeneity of C, and stresses the usefulness of the international character‐figure C as a measure of magnetic activity not only for intervals of days and weeks (which is undisputed) but also for months and years. Some remarks on the latter point may be permitted, because it now seems quite feasible to ascribe most of the discrepancy found for 1930 not so much to inherent faults or failures of C and u1, but to the different conceptions of magnetic activity underlying both measures.(1) April 1930 has the highest average monthly character‐figure, C = 1.04, of the whole series from 1906 to 1934. In the lists of “Principal magnetic storms” published in this Journal, however, all observatories, except Sitka, reported “no storm” for this month. On the other hand, May 1921, famous for two of the most intensive magnetic storms ever recorded, has only C = 0.83. The u1‐measure gives 52 for April 1930, and 132 for May 1921 (the second highest value for the series 1872 to 1934). This means that the mean monthly C fails to indicate the conspicuous absence of storms in April 1930 as compared with the outstanding storms in May 1921, while u1 represents these conditions satisfactorily.