Geomagnetic storms are one of the most energetic space weather phenomena. Previous studies have shown that the eigenfrequencies of ultralow frequency (ULF) waves on closed magnetic field lines in the inner magnetosphere decrease during storm times. This change suggests either a reduction in the magnetic field strength and/or an increase in its plasma mass density distribution. We investigate the changes in local eigenfrequencies by applying a superposed multiple‐epoch analysis to cross‐phase spectra from 132 geomagnetic storms. Six ground magnetometer pairs are used to investigate variations from approximately 3 < L  < 7 and across the whole dayside sector. We find that at L  > 4, the eigenfrequencies decrease by as much as 50% relative to their quiet time values. Both a decrease in magnetic field strength and an increase in plasma mass density, in some locations by more than a factor of 2, are responsible for this reduction. The enhancement of the ring current and an increase in oxygen ion density could explain these observations. At L  < 4, the eigenfrequencies increase due to the decrease in plasma mass density caused by plasmaspheric erosion.

中文翻译：

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地磁风暴期间不断变化的本征频率连续谱：对等离子体质量动力学和ULF波耦合的影响

地磁风暴是最活跃的太空天气现象之一。先前的研究表明，在风暴期间，内部磁层中闭合磁场线上的超低频（ULF）波的本征频率会降低。这种变化表明磁场强度降低和/或等离子体质量密度分布增加。我们通过对132个地磁风暴的跨相谱进行叠加多周期分析，研究了局部特征频率的变化。六对地面磁力计用于研究大约3 <  L  <7以及整个日间扇区的变化。我们发现在L > 4，相对于其安静时间值，本征频率最多降低50％。磁场强度的降低和等离子体质量密度的提高（在某些位置超过2倍）都是造成这种降低的原因。环电流的增加和氧离子密度的增加可以解释这些观察结果。在L  <4时，由于等离子层腐蚀引起的等离子体质量密度降低，本征频率增加。