VII. On the diurnal variations of the earth's magnetism produced by the Moon and Sun

Abstract

1. While the observational study of terrestrial magnetism is receiving ever more and more attention, and being rewarded with success by the acquisition of new and important data, the theoretical side of the subject shows a much less rapid advance. The search for a physical theory of the earth’s magnetism and its changes is fascinating but elusive. Perhaps in one case only—that of Schuster’s important theory of the diurnal variations of the magnetic state of the earth—has there been put forward a clearly outlined theory which promises to explain the real mechanism of any magnetic phenomenon. On this theory, the solar diurnal variations are attributed to the action of electromotive forces produced in masses of conducting air in the upper atmosphere, by their motion across the permanent magnetic field of the earth. The magnetic field of the resulting electric currents is identified with that which produces the observed diurnal changes. Schuster has shown that if the motion of the air is taken to he substantially that which is indicated by the barometric variations, the atmosphere being supposed to oscillate as a whole, the conductivity required by the theory is not unreasonable, considering the ionization of the tenuous upper atmosphere by ultra-violet radiation from the sun. The fundamental assumptions are in accordance with Schuster’s demonstration! that the magnetic variations are principally due to a system of currents above the earth’s surface. In order to explain the relative magnitudes of the diurnal and semi-diurnal terms in the magnetic potential, it is necessary to suppose that the conductivity of the atmosphere varies with the solar hour angle, which is certainly à priori probable: the great excess of the summer variation over the winter variation is unexplained, however, as the usual rapid rate of recombination of ions makes it difficult to believe that the solar ionization is slowly cumulative.