Persistent solar rotation‐period of 26 7/8 days and solar‐diurnal variation in terrestrial magnetism

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Abstract
An examination of the daily means of the horizontal force at Godhavn Greenland, during the years 1926 to 1941 shows that there seems to exist a recurrence‐tendency for the mean values of H with a period of 26 7/8 days during that epoch, the values being rather low in the first half of the period and rather high in the latter half. The phenomenon is most pronounced during the four summer months where the values increase in a rather regular way from a minimum in the first half‐period to a maximum 40 γ higher in the last half. A further investigation shows that this variation is due to the disturbance daily variation, SD, that varies in a regular way during the period so that the amplitude for the eleventh day in the period is about four times greater than that for the twentieth day. Even when a sequence of only three days is examined it turns out that there is a systematic difference between the SD for days belonging to the eleventh‐day group and the SD for days of the twentieth‐day group. A similar recurrence‐tendency is not found at Sodankylä. The author assumes that the effect found at Godhavn is due to the fact that the maximum magnetic disturbance at Godhavn occurs at noon which implies that the maximum stream of charged corpuscles from the Sun hits a sunlit atmosphere. This phenomenon is restricted to places north of the auroral zone. The persistence of a fixed period during 15 years points to the possibility that the origin of this effect is located in a layer on the Sun with a fixed rotation‐period during a long time.

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