SECULAR VARIATION IN THE GEOGRAPHIC LOCATION OF THE DIP EQUATOR

The geographic location of the dip equator on the globe has been deter mined for several epochs between 1550 and 1990 A.D. It is shown that t wo regions near 165 degrees E and 285 degrees E are devoid of migrator y changes whereas between 300 and O degrees E there is a large northwa rd movement. The period between 1945 and 1990 is marked by a nearly li near change in the latitude of the dip equator while earlier epochs sh ow more complex trends. Consistent with the earlier observations of th e movement of the magnetic center towards or away from the centre of t he earth, before and after 1800 A.D., the secular curves in certain lo ngitude zones indicate a clear change in pattern. The observed secular variations in the vertical component at several low latitude stations are generally in agreement with the direction of the movement of the dip equator. Examples of unexpected behaviour are highlighted. The ecc entric dipole model appears to be adequate to delineate the dip equato r for any epoch.