Estimation of westward drifting rate of non-dipole fields from Holocene geomagnetic secular variation

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A record of the geomagnetic secular variation for the last about 20 ky was obtained from two sediment cores in Lake Qionghai, China. The following four geomagnetic phenomena were deduced from the record. 1) A maximum easterly swing is recorded around 3800yBP,being consistent with the westward drifting model by Hyodo (1993). 2) A rate of 0.14ﾟ/y is newly estimated for westward drifting of a large non-dipole source. 3) A geomagnetic excursion characterized by a large westerly swing is found around 18000 yBP.4) The excursion fields also show clockwise rotational sense of motion, suggesting that westward drifting of non-dipole fields is dominant during the period.A geomagnetic secular variation record from Lake Erhai, China for the last 10 ky has no maximum easterly swing unlike the record from Lake Qionghai. In place, it shows that clockwise rotational motion is dominant from 4000-1700 yBP.This fact is consistent with Hyodo's model for westward drifting of a nondipole field. The absence of the easterly swing may be due to core-twisting. Short period components of the secular variation are well correlated with those of Lake Qionghai.Geomagnetic secular variation records during Holocene time are obtained from 13 areas in the world. Each record has different quality. The records from Britain, North America, and Australia are highly reliable because replicate records have been obtained from several cores. The geomagnetic field motions investigated by use of separation method (Itota et al., 1992) show that periodicities of 1100 and 2400 y/c are dominant at the three sites, and that clockwise rotations are dominant on the whole. Westward drifting rates estimated from the periodicities are 0.32ﾟ/y and 0.15 ﾟ/y. The latter is close to the rate 0.14ﾟ/y estimated from easterly swings.