A Statistical Study of the Solar Wind Dependence of Multi‐Harmonic Toroidal ULF Waves Observed by the Arase Satellite
A Statistical Study of the Solar Wind Dependence of Multi‐Harmonic Toroidal ULF Waves Observed by the Arase Satellite
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DOI：
10.1029/2021ja029840
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发表时间：
2021-12
期刊：
影响因子：
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通讯作者：
K. Yamamoto;K. Seki;A. Matsuoka;S. Imajo;M. Teramoto;M. Kitahara;Y. Kasahara;A. Kumamoto;F. Tsuchiya;M. Shoji;S. Nakamura;Y. Miyoshi;I. Shinohara
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文献类型：
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作者：
K. Yamamoto;K. Seki;A. Matsuoka;S. Imajo;M. Teramoto;M. Kitahara;Y. Kasahara;A. Kumamoto;F. Tsuchiya;M. Shoji;S. Nakamura;Y. Miyoshi;I. Shinohara
环形阿尔芬驻波是地球磁层中经常观测到的超低频波之一。它们有时表现出多谐波频谱，表明磁层中的宽能量范围输入。然而，由于缺乏统计研究，它们的能量来源尚未得到充分了解。在这里，我们使用了2.35年的Arase卫星观测数据，并对多谐波环形波的发生率、波功率和频率与太阳风的关系进行了统计分析。我们自动检测到多谐波，并根据太阳风速度和行星际磁场的锥角将其分为四组。我们发现，多谐波的发生率和波功率的增加与太阳风的速度在侧面。在正午扇区，多谐波的出现率随着锥角的减小而增加。对于小锥角，向阳面上的多谐波的中值频率与由离子束不稳定性理论预测的上游波频率正相关。发生率也随着太阳风动压的波动而增加。因此，我们认为Kelvin-Helmholtz不稳定性，上游波和动压波动是多谐波的可能来源。这项研究揭示了在各种太阳风条件下可以影响辐射带电子的多谐波的活动。
Toroidal standing Alfvén wave is one of the ultra‐low frequency waves that are frequently observed in the terrestrial magnetosphere. They sometimes exhibit multi‐harmonic frequency spectra, indicating wide energy range input in the magnetosphere. However, their energy source has not been fully understood due to the lack of statistical studies. Here we used the data of the Arase satellite observations for ∼3.5 years and conducted a statistical analysis of the solar wind dependence of the occurrence rate, wave power, and frequency of the multi‐harmonic toroidal waves. We automatically detected the multi‐harmonic waves and categorized them into four groups according to the solar wind velocity and the cone angle of the interplanetary magnetic field. We found that the occurrence rate and wave power of the multi‐harmonic waves increase with the solar wind velocity on the flank sides. In the noon sector, the occurrence rate of the multi‐harmonic waves increases with the decrease of the cone angle. The median frequency of the multi‐harmonic waves on the dayside is positively correlated with the upstream wave frequency predicted by the theory of the ion beam instability for a small cone angle. The occurrence rate also increases with the solar wind dynamic pressure fluctuations. Therefore, we suggest that the Kelvin‐Helmholtz instability, the upstream waves, and the dynamic pressure fluctuations are possible sources of the multi‐harmonic waves. This study sheds light on the activity of the multi‐harmonic waves which can affect radiation belt electrons under various solar wind conditions.