中文摘要：

地球主磁场B和它的长期变化^·B都起源于地球外核的磁流体发电机过程，但是，它们的空间结构和时间演化特征却有很大差异．本文采用全球平均的“无符号年变率”^-·X、^-·Y、^-·Z、^-·H和^-·F来表征长期变化场虫的总体强度，利用第9代国际参考地磁场模型IGRF-9，研究^·B场的变化特征．结果表明，在1900～2000年的100年当中，^·B场经历了3幕变化，最大年变率分别发生在1910～1920、1940-1950、1970-1980年，显示出清晰的30年周期变化，而且，每一周期的上升段比其下降段短得多．研究结果还表明，非偶极场对^·B的贡献约为偶极场的2倍，因此，决定唐场周期特征的主要因素是非偶极场（特别是四极子场），而不是偶极子场．这一特点与主磁场^·B中偶极场占绝对优势的特点完全不同．

英文摘要：

The main geomagnetic field, B, and its secular variation, b, originate from same dynamo process in the outer core. Their spatial structure and temporal behaviors, however, are drastically different. Using ＇globally averaged unsigned annual rate＇, ^-·X, ^-·Y,^-·X,^-·H and ^-·F, to express the magnitude of the ^·B-field, we studied periodicity in the ^·B-field on the basis of IGRF-9 models. The results show that the ^·B-field experienced a three-episode variation during the centennial period from 1900 to 2000. The maximum annual rates occurred around 1910 - 1920, 1940 - 1950, and 1970 - 1980, respectively, showing a 30-year period. In addition, the rising phase in each episode is much shorter than declining phase. The governing factor of this periodic variation is the non-dipole field, mainly the quatrupole, instead of the dipole field, although the dipole field is dominative over all other multiples in the main field B.