Seasonal Dependence of Lunar Daily Geomagnetic Variations in Different Regions of the World

抄録

Essential characteristics of the lunar daily geomagnetic variations L at three Japanese stations are presented along with those at 23 other stations in the world. The focus latitudes of L current system and the oceanic dynamo effect on L are examined with respect to the annual mean L. The focus latitudes of L current system estimated from daygraphs are lower than those of the current system of the solar daily geomagnetic variation S. On the other hand, the oceanic dynamo effect on L is remarkable for the vertical intensity Z and the complex features of the observed L (Z) are explained by this effect. In the seasonal mean L, the amplitude of L is not always minimum in local winter, in contrast to S. The data of all three Japanese stations shows the minimum amplitude in equinoctial months, and this anomalous seasonal change is a feature peculiar to the western North Pacific region. The seasonal variation of L shows a longitudinal inequality over the world. Possible causes of the inequality of the seasonal change are discussed, and it is suggested that the lunar tidal movements in the ionosphere may be the most plausible cause. The dependence of L on the sunspot number and magnetic activity is too complex to draw any definitive conclusion.