Dominant modes of variability in large‐scale Birkeland currents
Dominant modes of variability in large‐scale Birkeland currents
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DOI：
10.1002/2014ja020462
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发表时间：
2015-08
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通讯作者：
E. Cousins;T. Matsuo;A. Richmond;Brian J. Anderson
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作者：
E. Cousins;T. Matsuo;A. Richmond;Brian J. Anderson
通过对主动磁层与行星电动力学响应实验（AMPERE）一周的数据进行经验正交函数（EOF）分析，研究了大尺度伯克兰电流的变化特性。确定了北半球和南半球的平均分布以及主要变化模式。观察到两个半球结果存在差异，这归因于电导率的季节性差异（研究时段接近冬至或夏至）。通过综合两个半球的结果，得到了一个通用的平均值和一组主要变化模式，并且发现平均值以及前三个变化模式（EOF）占两个半球观测到的总磁扰平方（δB²）的38%。平均分布呈现出标准的1区/2区（R1/R2）电流形态，EOF 1捕捉到平均分布的增强/减弱，并且与行星际磁场（IMF）的南北分量有很好的相关性。EOF 2捕捉到多种效应的混合，包括（R1/R2）电流的扩张/收缩和旋转；这种模式与可能的外部驱动参数只有微弱的相关性。EOF 3捕捉到昼侧尖点区域电流形态的变化，并且与IMF的晨昏分量有很好的相关性。高阶EOF捕捉到伯克兰电流中更复杂、更小尺度的变化，并且通常与外部驱动参数无关。此处描述的EOF分析结果用于描述在一个利用AMPERE数据的数据同化过程中的误差协方差，如一篇相关论文中所述。
Properties of variability in large‐scale Birkeland currents are investigated through empirical orthogonal function (EOF) analysis of 1 week of data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). Mean distributions and dominant modes of variability are identified for both the Northern and Southern Hemispheres. Differences in the results from the two hemispheres are observed, which are attributed to seasonal differences in conductivity (the study period occurred near solstice). A universal mean and set of dominant modes of variability are obtained through combining the hemispheric results, and it is found that the mean and first three modes of variability (EOFs) account for 38% of the total observed squared magnetic perturbations (δB2) from both hemispheres. The mean distribution represents a standard Region 1/Region 2 (R1/R2) morphology of currents and EOF 1 captures the strengthening/weakening of the average distribution and is well correlated with the north‐south component of the interplanetary magnetic field (IMF). EOF 2 captures a mixture of effects including the expansion/contraction and rotation of the (R1/R2) currents; this mode correlates only weakly with possible external driving parameters. EOF 3 captures changes in the morphology of the currents in the dayside cusp region and is well correlated with the dawn‐dusk component of the IMF. The higher‐order EOFs capture more complex, smaller‐scale variations in the Birkeland currents and appear generally uncorrelated with external driving parameters. The results of the EOF analysis described here are used for describing error covariance in a data assimilation procedure utilizing AMPERE data, as described in a companion paper.