地磁场能量在地球内部的分布及其长期变化

DISTRIBUTION OF GEOMAGNETIC ENERGY IN THE EARTH'S INTERIOR AND ITS SECULAR VARIATION

用国际参考地磁场模型 (IGRF)分析了地磁场能量在地球内部的分布及其长期变化 .结果表明 ,从 1 90 0年到 2 0 0 5年 ,地核以外地磁场总能量由 6 .81 8× 1 0 1 8J减少到 6 .5 94× 1 0 1 8J,减小了 3 .3 % ,地表以外地磁场总能量由 8.6 5 8× 1 0 1 7J减小到 7.6 73× 1 0 1 7J,减小了 1 1 .4% .分析地球内部不同圈层地磁场能量的变化表明 ,地壳 (A层 )、上地幔 (B层 )、转换带 (C层 )、下地幔D′层的地磁场总能量在减小 ,但是下地幔D″层的地磁场总能量却在快速增加 .磁能密度随时间的变化更清楚地显示出磁能增加和减小的分界面在r=3 840km处 .上述结果表明 ,地核和地表以外地磁场总能量在趋势性减小的同时 ,也在进行重新分配 .进一步分析表明 ,下地幔D″层磁能快速增长 ,主要是由高阶磁多极子的增强引起的 .在地磁场倒转前 ,偶极矩减小而多极性相对增强在能量分布上的表现就是磁能向下地幔底部 (特别是D″层 )集中 .

The eighth generation of the International Geomagnetic Reference Filed (IGRF)is used to study distribution of geomagnetic energy in the Earth's interior and its secular variations. The results show that during the 105 year interval from 1900 to 2005 the total geomagnetic energy beyond the core mantle boundary (CMB) decreases by 3.3%,from 6.818×10 18 J in 1900 to 6.594×10 18 J in 2005,while the magnetic energy above the Earth's surface decreases in this period by 11.4%,from 8.658×10 17 J to 7.673×10 17 J. The magnetic energy in each layer varies with time,and the amounts of the energy in the crust(layer A),upper mantle (layer B),transition belt(layer C) and D′ layer of the lower mantle decrease,however,the energy increases rather rapidly in the D″ layer. The time variations of the energy density at different depths indicate a clearly demarcation at r =3840km,above and below which the energy decreases and increases,respectively. The characteristics mentioned above suggest that as the total energy of the geomagnetic field steadily decreases, a redistribution of the energy takes place in the Earth's interior,and the magnetic energy is gradually concentrates towards the bottom of the mantle, mainly the layer D″. The rapid increase of the magnetic energy in the D″ may be ascribed to growth of high order multipoles in the geomagnetic field. This resembles,to a certain extent,the behaviours prior to a geomagnetic polarity transition,when the dipole moment decreases and,at same time,the multipoles intensify.