The Effect of . Fast Normal Mode Structure and Magnetopause Forcing on FLRs in a 3-D Waveguide
The Effect of . Fast Normal Mode Structure and Magnetopause Forcing on FLRs in a 3-D Waveguide
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DOI：
10.1029/2018ja026222
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发表时间：
2019-01-01
影响因子：
2.8
通讯作者：
Wright, A. N.
中科院分区：
文献类型：
--
作者：
Elsden, T.;Wright, A. N.
本文研究了非均匀偶极子平衡条件下波导模的激发及其与场线共振的耦合。波导模是一种快速压缩超低频波，其结构取决于磁层平衡和太阳风驱动条件。通过磁流体力学模拟，我们考虑了不同形式的磁层顶驱动对激励波导模结构的影响。我们发现，该波导支持一组由平衡决定的简正模。然而，被激发的特定正常模式由磁层顶驱动器的结构决定。需要对简正波的空间结构有充分的了解，以便预测将在哪里发生耦合到FLR。结果表明，子午线附近的对称压力驱动比反对称驱动可以激发出持续时间更长的简正模，从而驱动共振。此外，有效耦合的临界量是与共振对齐的磁压梯度。简单地说，地球的磁场一直受到太阳(太阳风)外流的干扰。这种干扰会产生地球磁场的周期性振荡，这可以通过地面和太空中的仪器来测量。这些振荡(波)已经被证明对极光和辐射带(地球周围被捕获的高能粒子的区域)内的高能粒子的运动产生影响。在这项研究中，我们使用地球磁场的计算模型来研究这些波是如何传播的。我们考虑了驱动系统的不同方式，以及不同的密度分布，并测量了所产生的波浪响应。我们确定该系统支持所谓的“快速正常模式”，即系统的自然波。这些可以被认为是以相同的方式在打开的瓶子上吹气，这将产生特定频率的声音。我们讨论了这些模式的结构如何影响能量在整个地球磁环境中的传播和积累。
This paper investigates the excitation of waveguide modes in a nonuniform dipole equilibrium and, further, their coupling to field line resonances (FLRs). Waveguide modes are fast compressional ultralow frequency (ULF) waves, whose structure depends upon the magnetospheric equilibrium and the solar wind driving conditions. Using magnetohydrodynamic simulations, we consider how the structure of the excited waveguide mode is affected by various forms of magnetopause driving. We find that the waveguide supports a set of normal modes that are determined by the equilibrium. However, the particular normal modes that are excited are determined by the structure of the magnetopause driver. A full understanding of the spatial structure of the normal modes is required in order to predict where coupling to FLRs will occur. We show that symmetric pressure driving about the noon meridian can excite normal modes which remain around to drive resonances for longer than antisymmetric driving. Further, the critical quantity in terms of efficient coupling is the magnetic pressure gradient aligned with the resonance.Plain Language Summary Earth's magnetic field is constantly being disturbed by the outflow from the Sun (the solar wind). Such disturbances generate periodic oscillations of the Earth's magnetic field, which can be measured by instruments on the ground and in space. These oscillations (waves) have been shown to have an impact on the aurora and the motion of energetic particles within the radiation belts (regions of trapped energetic particles around the Earth). In this study, we use a computational model of the Earth's magnetic field to investigate how these waves propagate. We consider various ways of driving the system, together with different density distributions and measure the resulting wave response. We identify that the system supports what are called "fast normal modes," which are the natural waves of the system. These can be thought of in the same way as blowing across an open bottle, which will produce a specific frequency of sound. We discuss how the structure of these modes affects how energy propagates and accumulates throughout the Earth's magnetic environment.