1900-2000年地球主磁场的本征模分析

利用1900-2000年国际参考地磁场模型（IGRF），对地球主磁场的空间结构和长期变化进行了自然正交分量分析，得到了组成地磁场的主要成分（本征模）及其随时间的长期变化（SV）．第一本征模描述主磁场稳定结构的总体特征，其强度随时间单调减小；第二本征模主要描述印度洋和南大西洋地区正异常特征，其强度也随时间单调变化；第三本征模对应于加勒比地区正异常，其强度呈100a周期的变化，目前正处于下降阶段；第四本征模的主要特点是北美正异常和南亚负异常，其强度呈80a周期的变化；第五本征模对应尺度较小的局部磁异常，其强度的变化周期约60a；第六及其以后的本征模则对应空间尺度更小和时间变化更快的磁异常．分析表明，太平洋地区的磁场经历着缓慢的单调减小变化，其结构比较稳定，几乎完全包括在第一本征模中．但是，印度洋磁场结构较复杂，它是由空间结构不同和变化周期各异的多个本征模组成的．大西洋的北部、中部和南部分属不同本征模，它们有完全不同的时间变化特性．

EIGEN MODE ANALYSIS OF EARTH'S MAIN MAGNETIC FIELD

Method of natural orthogonal components(MNOC) is used to analyze the spatial structure and secular variation of the earth's main magnetic field on the basis of IGRF models for 1900-2000. The main components (eigen modes) of the main field and their secular variation are obtained. The first eigen mode describes the overall characteristics in the stable structure of the field, and its intensity decreases monotonously. The second eigen mode, with monotonously decreasing intensity,describes the characteristics of the positive anomalies in indian Ocean and Atlantic.The third eigen mode corresponds to positive Caribbean anomaly, varying with a period of 100 years. The fourth eigen mode is characterized by positive North American anomaly and negative South-Asian anomaly, varying with 80 year period.The fifth eigen mode represents smaller scale and faster-varying anomalies. The results show that the magnetic field in the Pacific Ocean monotonously decreases, and has quite stable structure. It can be represented by the 1st mode. On the other hand, the structure in the indian Ocean is more complicated, it is composed of different eigen modes with different periods and spatial structure. The northern, central and southern partS of Atlantic are described by different modes, they have quite different periods.