A preliminary study on variations in the Gauss coefficients of the geomagnetic potential over several hundred years

Geomagnetic secular variations are examined in terms of time variation in the Gauss coefficients. Major parts of the variations over several hundred years can be represented by a two mode model which consists of a standing and a drifting field. When the Gauss coefficients are plotted on a diagram with g_n^m in the abscissa and h_n^m in the ordinate, the drifting component describes a circle. However, some of the observed coefficients depict an elliptical trajectory rather than a circular one. Improvement of the model is attempted in two different ways. One is to assume time variability of the amplitude of the drifting component. The other is to introduce another drifting mode. Selecting a few spherical harmonic terms, variations in the Gauss coefficients since A.D. 1600 are analysed.When the amplitude of the drifting field is assumed to vary, the observed nature of the elliptical trajectory is well represented. In this case, phase velocity also changes with time. It is large while the amplitude is small, and it is small while the amplitude is large. Three mode models, in which an eastward drifting mode is incorporated, approximate the observed variations as well, not only for the period over several hundred years but also for the last several decades. In this model the westward drifting mode dominates the eastward mode.