Four-dimensional Frequency–Wavenumber Power Spectrum of a Strong Turbulence Obtained from Hybrid Kinetic Simulations
Four-dimensional Frequency–Wavenumber Power Spectrum of a Strong Turbulence Obtained from Hybrid Kinetic Simulations
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DOI：
10.3847/1538-4357/abb99f
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发表时间：
2020-11
期刊：
影响因子：
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通讯作者：
S. Markovskii;B. Vasquez
中科院分区：
文献类型：
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作者：
S. Markovskii;B. Vasquez
我们对强衰减湍流进行三维混合动力学模拟。湍流由包含低波数阿尔芬波模式的种子光谱引发。根据准稳态阶段模拟输出的时间和空间分布，我们计算了湍流的四维频率-波数谱。我们的分析表明，可以在种子湍流谱附近的波数-频率空间中识别动力学阿尔文波。它们产生与线性色散关系一致的功率峰值。然而，在远离种子光谱（大多数波粒相互作用发生的地方）的地方，阿尔文模式的特征在无法用任何色散关系描述的其他波动中消失。此外，在仍可识别特征的较高波数下，其频率展宽变得与频率本身相当。因此，如果线性波的特征仍然存在，则使用基于传统色散关系的线性波不一定适合描述湍流，当其特征消失时更是如此。我们发现，在湍流的均方根振幅较大时，色散关系的特征仅限于较低的平行波数。在可见光范围内，振幅越大，频率展宽就越大。这表明更强的非线性使得波动表现得不太像波模式，直到传统的波模式方法不再有效。
We carry out three-dimensional hybrid kinetic simulations of a strong decaying turbulence. The turbulence is initiated with a seed spectrum that includes Alfvén wave modes at low wavenumbers. From the temporal and spatial distribution of the simulation output in the quasi-steady phase, we calculate a four-dimensional frequency–wavenumber spectrum of the turbulence. Our analysis shows that kinetic Alfvén waves can be identified in the wavenumber–frequency space in the vicinity of the seed turbulence spectrum. They produce a power peak consistent with a linear dispersion relation. However, further away from the seed spectrum, where most of the wave–particle interaction takes place, the signature of the Alfvén modes disappears among other fluctuations that are not described by any dispersion relations. Furthermore, at higher wavenumbers at which the signature is still identifiable, its frequency broadening becomes comparable to the frequency itself. Therefore, the use of linear waves based on the conventional dispersion relation is not necessarily justified to describe the turbulence if their signature is still present and even more so when it disappears. We find that at larger rms amplitudes of the turbulence, the signature of the dispersion relation is confined to lower parallel wavenumbers. In the range where it is visible, the frequency broadening becomes greater at larger amplitudes. This suggests that stronger nonlinearity makes the fluctuations behave less like wave modes until the conventional wave-mode approach is no longer valid.