Magnetosonic waves, with the frequency between proton cyclotron frequency and lower hybrid frequency, mainly occur near the Earth's magnetic equatorial plane and play an important role in the magnetospheric dynamics. In this paper, we report unusual magnetosonic waves with nonlinear harmonics observed by Magnetospheric Multiscale mission in the magnetotail. These magnetosonic waves have multiband enhanced electromagnetic power spectral densities (B _w/B ₀ up to 1/45) with the frequency from below to above the lower hybrid frequency and are quasi‐perpendicular propagating and linearly polarized. The frequency of the fundamental band is much higher than the proton cyclotron frequency f _ci (i.e., ~35 f _ci). We identified these emissions as high‐frequency magnetosonic waves with nonlinear harmonics. These are the first observations of such high‐frequency magnetosonic waves with the frequencies of harmonics higher than the lower hybrid frequency in the Earth's magnetosphere to the best of our knowledge. Given the absence of ring distributions for the protons and the easy coupling between compressed mode and its electromagnetic term, we propose that these nonlinear harmonic structures of the observed magnetosonic waves are likely generated by the nonlinear wave‐wave coupling among electromagnetic terms of the fundamental and higher harmonic waves.

中文翻译：

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具有非线性谐波的磁声波的初步观测

频率介于质子回旋频率和较低混合频率之间的磁声波主要发生在地球的赤道平面附近，并在磁层动力学中起重要作用。在本文中，我们报告了磁尾中的磁层多尺度任务观测到的具有非线性谐波的异常电磁波。这些磁声波具有多频带增强的电磁功率谱密度（B _w / B ₀高达1/45），其频率从较低到较低的混合频率，并且是近似垂直传播的并且是线性极化的。基带的频率比质子回旋加速器频率f _ci（即_〜35 f_ci）。我们将这些发射确定为具有非线性谐波的高频磁声波。据我们所知，这是对这种高频磁声波的首次观测，其谐波频率高于地球磁层的较低混合频率。考虑到质子的环分布不存在以及压缩模式与其电磁项之间的容易耦合，我们建议观察到的磁声波的这些非线性谐波结构很可能是由基本波和电磁波之间的非线性波-波耦合产生的。高次谐波。