The lag between solar activity and magnetic activity
Authors: Fisk H. W. 
Published: February 2008

The question of an immediate connection between the existence of sunspots and magnetic disturbance on the Earth has been studied by many investigators, among them the late Dr. Charles Chree, who presents his conclusion? in his “Studies in Terrestrial Magnetism” (Macmillan and Co., London, 1912). In this he affirms his earlier conclusion that an immediate connection is not shown by the facts, and says: “The conclusion which I drew in 1908 from these facts was that unless a time‐lag exceeding one month be allowed, there must be agencies not associated with sunspots which powerfully affect the range of the magnetic needle.” He further states (loc. cit., p. 201) that such evidence as there is of a relationship between sunspot‐area and absolute magnetic range on individual days favors the view that the phenomenon on Earth is influenced by the state of the Sun some days previous to the day considered, rather than the view that the terrestrial effect occurs simultaneously with the appearance of sunspots.

In discussing the Earth's magnetism and solar activity in 1918 (Terr. Mag., v. 23, p. 66), L. A. Bauer says: “The pronounced lag in the magnetic curves with respect to the sunspot‐maximum of 1905 and 1907 is also shown by the so‐called international magnetic character‐numbers. There is comparatively little lag, however, in all the curves, with respect to the period of minimum solar activity as shown both by sunspot‐numbers and solar‐constant values. Thus we are led once more to the suggestion that the lag in magnetic effect is a variable quantity and appears to depend on the degree of solar activity.” The graph by means of which Bauer illustrated the argument just quoted made use of character‐numbers only from 1906 to 1916, as no others were then available. The longer series now at hand shows the lag pointed out by both Chree and Bauer, but the supposition that this lag was excessive, at times of sunspot‐maxima is not confirmed at the maximum of 1917. The mean annual magnetic character‐numbers are available for twenty‐two years, namely, from 1906 to 1927, inclusive. The relative sunspot‐numbers of course have been compiled for a much longer period. In order to make a direct graphical comparison between these two series of numbers they should be reduced for convenience to approximately the same numerical range. The sunspot‐numbers vary between one and 104 for these twenty‐two years, while the magnetic character‐numbers are fractional and have approximately 0.45 and 0.75 for their lower and upper limits, respectively. A suitable formula for modification was found to be

2.7 R = (C ‐ 0.515) 103

in which C is the magnetic character‐number and R the relative sunspot‐number for the same year. The quantities thus modified and called for convenience C′ and R′, are plotted in Figure 1. Aside from the general conformity of these two graphs, two features are worthy of attention. The character‐numbers for 1910 and 1911 are unusually high as compared with the general trend of the curve and with the sunspot‐numbers for those years. It was in this feature that Bauer found confirmation of his supposition that the lag at time of maximum was much greater than at time of minimum. The other feature to notice is the persistent lag of C′ behind R′.