On the latitude dependence of drift velocity of the geomagnetic main field and its secular variation

Abstract

There is an apparent difference in the westward drift between the geomagnetic main field and its time derivative, secular variation. The drift velocity of the main field is about 0.2°/year, definitely lower than that of the secular variation, 0.3°/year. The drift velocity of the main field appears to change with latitude, being low at high latitudes and higher at low latitudes, whereas the velocity of the secular variation is nearly constant irrespective of latitude. This paper examines what causes this difference by adopting the drifting and standing field model that assumes the geomagnetic field consists of the field steadily drifting westwards and the field remaining at nearly the same location.In this study, we confirm that the existence of the non-drifting standing field significantly affects the estimate of the drift velocity of the total field (i.e., the main field), and makes it slower than that of the secular variation. The drifting field is intense in low latitudes with its maximum at the equator, while the standing field dominates in higher latitudes. As a consequence, reduction of the apparent drifting velocity of the total field by the standing field is conspicuous in higher latitudes and less so in low latitudes. This creates the observed latitudinal structure of the drift velocity of the main field. On the other hand, the drift velocity of the secular variation is less affected by existence of the standing field, and mostly reflects the velocity of the drifting field that is almost constant with latitude. The velocity of the secular variation thus becomes almost uniform independent of latitude. The observed difference between the main field and the secular variation is naturally derived from the drifting and standing field model. This implies that physical mechanisms to generate the drifting and standing fields can be considered independently.