行星尺度地磁异常的长期变化

为了综合反映地球表面行星尺度磁异常的展布面积、磁场极值以及磁场分布特征等多种因素及其与磁能的关系，本文用穿过各异常区的“无符号磁通量”为特征参数来表征磁异常区强度.用第八代国际参考地磁场模型(IGRF)，分析了1900年到2000年全球最大的5个磁异常区的长期变化，结果表明，在一百年中，南大西洋(SAT)、大洋洲(AUS)和非洲(AF)3个异常区的磁通量均增加了200MWb以上，欧亚异常(EA)磁通量增加幅度稍小(157MWb)，上述4个异常区磁通量增幅为30%-60%，而北美异常(NAM)的磁通量则减小了50MWb.各异常区面积虽有变化，但最大变化仅为%左右.对磁异常区的西向漂移研究表明，地球表面和核幔界面的西漂明显存在差异：地表磁场有持续而稳定的西向漂移，全球平均西漂速度为0.2°/a；但核幔界面磁场的西向漂移速度要小得多，最大不超过0.1°/a.形成这种差异的原因可能是组成地磁场的不同球谐分量有不同的漂移速度；地表磁场的西漂主要决定于占优势的低阶分量，而核幔界面的西漂则受到高阶分量的重大影响.本文指出，在把地表西漂值用作地核磁流体运动速度的典型值时必须十分谨慎.

SECULAR VARIATIONS OF THE PLANETARY-SCALE GEOMAGNETIC ANOMALIES

The characteristics of a planetary-scale geomagnetic anomaly are described by using‘unsigned magnetic flux' through the whole anomaly region. The eighth generation of the International Geomagnetic Reference Filed (IGRF) is analyzed to study the secular variations of 5 planetary-scale geomagnetic anomalies during 1900—2000 period. The results show that the magnetic fluxes through the southern Atlantic anomaly (SAT),Australian anomaly (AUS),and African anomaly(AF) have increased by more than 200MWb,the increase of the flux through the Eurasian anomaly (EA) is smaller(157MWb),while the flux through the North American anomaly (NAM) has decreased by 50MWb. On the other hand,the variations of the anomaly areas are within 10%.The westward drift at the core-mantle boundary(CMB) is less than 0.1°/a,much slower than that at the Earth surface(0.2°/a). This difference may be attributed to the different westward drift velocities of the harmonics in the geomagnetic field:the westward drift at the Earth surface is controlled by the dominant low-degree harmonics,while the westward drift at the CMB largely depends on high-degree harmonics. Consequently,it should be careful to take the westward drift velocity at the Earth surface as the typical velocity of the fluid flow in the outer core.