On the analysis of paleomagnetic secular variation
On the analysis of paleomagnetic secular variation
复制标题
古地磁长期变化分析
DOI：
10.1029/95jb00609
复制
发表时间：
1995
影响因子：
--
通讯作者：
Peter Kelly
中科院分区：
文献类型：
--
作者：
D. Gubbins;Peter Kelly
考虑到古地磁结果相对稀疏和不准确的特点，现代地磁场分析方法可以应用于古地磁资料。古地磁倾向于产生以倾角测量为主的数据集，这不能提供一个独特的磁场模型。例如，发现两个磁场具有非常相似的倾斜度，但完全不同的赤纬和强度。定向测量很难处理，因为它们与场模型非线性相关。通过对轴向偶极子的场进行线性化，简化了方程；这种近似对于古地磁数据是足够的，并且可以用来证明VGP散射函数即使在经度上平均也不是对非轴偶极子场的适当估计。在古地磁文献中所作的四个断言在这种线性化中是正确的，但在一般情况下是不正确的。特别地，VGP散射函数在独立对称场（“四极”）中的赤道对称性保持在近似范围内。现代地磁场的虚磁极散射函数具有强烈的南北不对称性，但历史磁场模型表明这是一个相对较新的特征。磁场对称部分Sq的VGP散射函数由赤道偶极子主导，近似等于地磁极与地理极之间的角距离；对于磁场的反对称部分，Sd由2次和1次的地磁系数主导。令人惊讶的是，轴对称的四极和八极项贡献很小，突出了不考虑偏离轴对称的古磁场建模的危险。
Modern methods of geomagnetic field analysis can be applied to paleomagnetic data provided account is taken of the relatively sparse and inaccurate nature of paleomagnetic results. Paleomagnetism tends to produce data sets dominated by inclination measurements, which do not provide a unique field model. As an example, two magnetic fields are found with closely similar inclinations but completely different declinations and strengths. Directional measurements are difficult to deal with because they are nonlinearly related to the field model. The equations are simplified by linearizing the field about an axial dipole; the approximation is adequate for paleomagnetic data and can be used to show that the VGP scatter function is not an appropriate estimate of the non-axial-dipole field even when averaged over longitude. Four assertions made in the paleomagnetic literature are shown to be correct within this linearization but are not true in general. In particular, equatorial symmetry of the VGP scatter functions for the separate symmetric (“quadrupole”) field holds within the approximation. The virtual geomagnetic pole (VGP) scatter functions for the modern geomagnetic field has strong north-south asymmetry, but historical field models show it to be a relatively recent feature. The VGP scatter function for the symmetric part of the field, Sq, is dominated by the equatorial dipole and is approximately equal to the angular distance between the geomagnetic and geographic poles; that for the antisymmetric part of the field, Sd, is dominated by geomagnetic coefficients of degree 2 and order 1. Surprisingly, the axisymmetric quadrupole and octupole terms contribute very little, highlighting the dangers of modeling the paleomagnetic field without regard to departures from axial symmetry.