非洲磁异常对地磁场结构及其长期变化的控制作用

地球非偶极磁场在主磁场结构及其长期变化中起着重要作用.非偶极磁场主要表现为行星尺度磁异常，它们是南大西洋磁异常、非洲磁异常、欧亚大陆磁异常、澳洲磁异常和北美磁异常.在这5块磁异常中，非洲磁异常对磁赤道的形状和位置以及全球长期变化特征有极大的影响.非洲磁异常的重要性主要表现在3方面：第一，由于异常区位于赤道这一特殊的地理位置，所以它极大地影响磁赤道的形状和位置.相对于偶极场的地磁赤道而言，异常区所在的中北非洲和中大西洋地区的磁赤道向北移动，最大移动量可达约15°.第二，非洲磁异常的快速西漂对全球长期变化的分布起着决定性作用，它在该异常区西边的中美洲形成了全球最主要的长期变化区，在1900~2005年期间，最大年变率Z_max超过200 nT/a.第三，非洲负磁异常区与其南面的南大西洋正磁异常区相结合，〖HJ〗它们的变化使西半球地磁场强度大大减弱，也使全球磁场发生显著畸变.这两块磁异常与深部的反极性斑区有着成因联系.

Role of the African magnetic anomaly in controlling the magnetic configuration and its secular variation

The configuration of the main magnetic field and its secular variation are greatly controlled by the nondipole field. It exhibits as 5 planetary-scale magnetic anomalies in Australia, Africa, Southern Atlantic Ocean, Eurasia continent, and North America，as well as a few minor anomalies. Only anomaly in the equatorial region is the African anomaly, which affects seriously the shape and position of the magnetic equator, pushing the equator northward for 15^o at maximum. The fast westward drift of African anomaly results in a remarkable secular variation cell (Z_max>200 nT/a for 1900~2005) in Central America on the west of the anomaly. In addition, the secular variations of African and Southern Atlantic Ocean anomalies strongly distort the geomagnetic field of the western hemisphere, implying its relationship with the polarity reversed patches on the core-mantle boundary.