VI. The lunar diurnal magnetic variations, and its change with lunar distance

Abstract

In his illuminating article on Magnetism in the ‘Encyclopœdia Britannica’ (9th ed., 1882), Balfour Stewart discussed the origin and mechanism of the short-period magnetic variations, concluding that the only tenable hypothesis was that which attributed them to currents flowing in the upper atmosphere, under the impulse of electromotive forces caused by the motion of the conducting atmosphere across the permanent terrestrial magnetic field. At several stages in the discussion use was made of the phenomena of the lunar diurnal magnetic variation, as disclosed by Broun’s fine study of the subject to these phenomena Balfour Stewart evidently attributed considerable theoretical importance, and an origin similar as regards situation to that of the solar diurnal variations. In 1889 Schuster proved, by the Gaussian potential method, that the solar diurnal magnetic variations arise mainly from causes acting above the earth’s surface, a demonstration which added much weight to Balfour Stewart’s tentative theory Schuster also suggested a connection between the magnetic and barometric variations, an idea which he elaborated and discussed with great cogency eighteen years later (1907). The barometric changes are mainly of thermal origin, and possible differences between the character of the atmospheric motions at the earth’s surface and in the upper regions may have an important bearing on the theory. In his later paper, Schuster suggested that the lunar magnetic variations might throw light on these questions: “It is much to be desired that some systematic attempt should be made to investigate the lunar influence on the magnetic changes, for we possess at-present only the vaguest information as to how the different components are affected. It is quite possible that the effects may depend on a tidal disturbance of the upper regions of the atmosphere. If so, we may expect to get a valuable test of our theory by their investigation” ( loc . cit ., p . 181).