Some topics and historical episodes in geomagnetism and aeronomy

Abstract

Both the American Geophysical Union (AGU) and the International Union of Geodesy and Geophysics (IUGG) were established in 1919, to promote the study of physical problems in and above the Earth, for national and international managements. The initial internal structure within IUGG (especially for geomagnetism and space science dealt with in IAGA, the International Association of Geomagnetism and Aeronomy) owes very much to AGU's appropriate proposals. After the IGY (International Geophysical Year 1957‐58), it was necessary for both AGU and IAGA to modify their internal structures to cope with a rapid progress in space physics with technological development for in‐situ observations by spacecraft. A serious discussion was held in 1920's ‐ 30's about the possible existence of a non‐potential part in the geomagnetic field and its time‐variations in connection with the air‐Earth current. The curl‐freeness of the geomagnetic field cannot be experimentally proved insofar as the differential operation is simply substituted by spatial differences of the observed magnetic field values. It was shown that the archeo‐secular variation of the geomagnetic dipole axis in historic time could be known also from a comparison of the records of ancient auroras preserved in the Oriental and Occidental places. In the history of magnetic storm studies, there was a severe debate between two groups of workers, the Scandinavian school (the importance of field‐aligned currents at high latitudes was emphasized, based on an early study of Birkeland, and later advocated by Alfvén and his colleagues) and the British school (headed by Chapman, attributing the ground magnetic effect rather to horizontal currents in the ionosphere). However, their severe debate was rather meaningless without in‐situ magnetic field measurements above the ionosphere. It is emphasized in this paper that we have to pay more attention to the change in the magnetic declination, in particular its seasonal dependence. The declination change at the storm sudden commencements shows specific local‐time and seasonal dependences, reflecting the solar‐wind interaction with the geomagnetic field. The seasonal dependence of the daily declination change on quiet days enables us to infer transequatorial field‐aligned currents in the magnetosphere at low and middle latitudes. A review is given also for the significant contribution of MAGSAT to the study of three‐dimensional electric currents in the near‐Earth environmental space. There are five Appendices to supplement the above subjects, i.e. on the origin of the words “geomagnetism” and “aeronomy”; the transition from IATME to IAGA; Birkeland's last days in Tokyo; and Harang discontinuity; and the VII General Assembly of IUGG in September 1939 in Washington, D. C.