Modeling the Varying Location of Field Line Resonances During Geomagnetic Storms
Modeling the Varying Location of Field Line Resonances During Geomagnetic Storms
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DOI：
10.1029/2021ja029804
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发表时间：
2022-01
期刊：
影响因子：
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通讯作者：
T. Elsden;T. Yeoman;S. Wharton;I. J. Rae;J. Sandhu;M. Walach;M. K. James;D. Wright
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文献类型：
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作者：
T. Elsden;T. Yeoman;S. Wharton;I. J. Rae;J. Sandhu;M. Walach;M. K. James;D. Wright
以前的观测研究表明，地磁场线的自然阿尔文频率在磁暴过程中变化很大，从等离子体层外的安静时间值下降了50%。最近，使用地面磁力计对132个地磁暴事件的观测在统计上证明了这一点（Wharton等人，2020年）。这就引发了一个问题：相对于安静时期，风暴时期将在哪里形成场线共振（FLR）。由于风暴时间辐射带动力学在很大程度上取决于波粒相互作用，因此了解FLR位置在风暴过程中的变化将对该领域产生重要影响。使用3D磁流体动力学（MHD）模拟，我们研究了在磁暴过程中地球昼侧磁层的Alfvén连续频率的变化如何影响快-Alfvén波耦合。通过设置模型Alfvén频率与观测结果一致，并允许等离子体层顶/磁层顶位置与风暴时间行为一致的适度变化，我们表明FLR位置在风暴期间可以发生实质性变化。在风暴主相期间，较高的快速波导频率和较低的阿尔文频率的组合效应共同作用，使FLR位置相对于平静时间径向向内移动。对于所考虑的情况，等离子体轨道外的FLR径向向内移动1.7个地球半径。
Previous observational studies have shown that the natural Alfvén frequencies of geomagnetic field lines vary significantly over the course of a geomagnetic storm, decreasing by up to 50% from their quiet time values outside the plasmasphere. This was recently demonstrated statistically using ground magnetometer observations across 132 geomagnetic storm events (Wharton et al., 2020). This then brings into question where field line resonances (FLRs) will form in storm‐time conditions relative to quiet times. With storm‐time radiation belt dynamics depending heavily upon wave‐particle interactions, understanding how FLR locations change over the course of a storm will have important implications for this area. Using 3D magnetohydrodynamic (MHD) simulations, we investigate how changes in the Alfvén frequency continuum of the Earth's dayside magnetosphere over the course of a geomagnetic storm affect the fast‐Alfvén wave coupling. By setting the model Alfvén frequencies consistent with the observations, and permitting a modest change in the plasmapause/magnetopause locations consistent with storm‐time behavior, we show that FLR locations can change substantially during storms. The combined effects of higher fast waveguide frequencies and lower Alfvén frequencies during storm main phases, act together to move the FLR locations radially inwards compared to quiet times. FLRs outside of the plasmasphere are moved radially inward by 1.7 Earth radii for the cases considered.