Spatial and temporal power spectra of the geomagnetic field
Spatial and temporal power spectra of the geomagnetic field
复制标题
地磁场的时空功率谱
DOI：
10.1029/95jb03042
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发表时间：
1996
影响因子：
--
通讯作者：
M. G. Mcleod
中科院分区：
文献类型：
--
作者：
M. G. Mcleod
地磁场的时空功率谱对于地磁场及其长期变化的随机反演和最佳预报是有用的。根据1910-1983年期间的地磁观测年平均值和年平均值的一阶差分，确定了磁场及其一阶时间导数的球谐系数。分析中包括了9度的内部球谐系数和1、3、5和7度地磁坐标中的外部纬向系数。由这些系数估计了地磁场球谐系数的自协方差函数。估计的内部自协方差函数的特征时间常数取决于球谐度和自协方差函数是场的还是场的一阶导数。磁场的特征时间常数约为10，000年除以（n + 1/2）的平方，其中n是球谐次数。一阶导数的特征时间常数大约小10倍。第一个时间常数被认为与磁扩散有关，第二个时间常数被认为与核心流体速度相关时间有关。还有一个约为5年的第三时间常数与二阶时间导数相关。度1和度3的外部自协方差函数都显示出太阳活动周期分量的明显证据。用这些数据拟合了地磁场空间和时间功率谱的简单解析表达式，并与实验结果吻合良好。
Spatial and temporal power spectra of the geomagnetic field are useful for stochastic inversion and optimal prediction of the geomagnetic field and its secular variation. Spherical harmonic coefficients for the field and its first time derivative were determined from magnetic observatory annual means and first differences of annual means for each year in the time interval 1910–1983. Internal spherical harmonic coefficients through degree 9 and external zonal coefficients in geomagnetic coordinates for degrees 1, 3, 5, and 7 were included in the analysis. Autocovariance functions for spherical harmonic coefficients of the geomagnetic field were estimated from these coefficients. Characteristic time constants for the estimated internal autocovariance functions depend upon both spherical harmonic degree and whether the autocovariance function is for the field or its first derivative. The characteristic time constant for the field is approximately 10,000 years divided by the square of (n + 1/2), where n is the spherical harmonic degree. The characteristic time constant for the first derivative is about a factor of 10 smaller. The first time constant is thought to be related to magnetic diffusion, and the second time constant is thought to be related to core-fluid velocity correlation time. There is also a third time constant of approximately 5 years associated with the second time derivative. External autocovariance functions for degree 1 and degree 3 both show clear evidence of a solar cycle component. A simple analytic expression for the spatial and temporal power spectra of the geomagnetic field was fitted to these data and agrees well with the experimental results.