On the geomagnetic secular variation in the Pacific region

Reexamination of the geomagnetic field for the past 400 years indicates that in the early 17th century the non-dipole field was not weak, nor the secular variation was low in the Pacific region, implying that the hemispherical asymmetry is not a permanent phenomenon. This paper examines the secular variation for the past 400 years from viewpoint of drifting and standing field model. It has been found that the low secular variation in the Pacific region is mainly caused by the drift of the drifting field. At the Earth's surface the drifting field is dominated by spherical harmonics Y22(θ,ϕ) with two pairs of ridges and troughs with their axes aligned in the meridional direction. The two pairs are not equal; one pair is stronger than the other. Movements of the different magnitude of the pairs are considered to have caused the hemispherical asymmetry of the secular variation for recent years, giving rise to the non-weak field in the Pacific region in the early 17th century.