We investigate compressive turbulence at sub-ion scales with measurements from the Magnetospheric MultiScale Mission. The tetrahedral configuration and high time resolution density data obtained by calibrating spacecraft potential allow an investigation of the turbulent density fluctuations in the solar wind and their three-dimensional structure in the sub-ion range. The wave-vector associated with the highest energy density at each spacecraft frequency is obtained by application of the multipoint signal resonator technique to the four-point density data. The fluctuations show a strong wave-vector anisotropy $k_{\perp }\gg k_{\parallel }$ where the parallel and perpendicular symbols are with respect to the mean magnetic-field direction. The plasma frame frequencies show two populations, one below the proton cyclotron frequency $\omega <{\mathrm{\Omega }}_{ci}$ consistent with kinetic Alfvén wave (KAW) turbulence. The second component has higher frequencies $\omega >{\mathrm{\Omega }}_{ci}$ consistent with ion Bernstein wave turbulence. Alternatively, these fluctuations may constitute KAWs that have undergone multiple wave-wave interactions, causing a broadening in the plasma frame frequencies. The scale-dependent kurtosis in this wave-vector region shows a reduction in intermittency at the small scales which can also be explained by the presence of wave activity. Our results suggest that small-scale turbulence exhibits linear-wave properties of kinetic Alfvén and possibly ion-Bernstein (magnetosonic) waves. Based on our results, we speculate that these waves may play a role in describing the observed reduction in intermittency at sub-ion scales.

中文翻译：

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太阳风中亚离子级压缩湍流中动力学Alfvén模式和离子Bernstein模式的可能共存

我们通过磁层多尺度任务的测量研究了亚离子尺度的压缩湍流。通过校准航天器的电势获得的四面体配置和高时间分辨率密度数据可以研究太阳风中的湍流密度波动及其在亚离子范围内的三维结构。通过将多点信号谐振器技术应用于四点密度数据，可以获得与每个航天器频率上最高能量密度相关的波矢量。波动显示出很强的波矢各向异性${ķ}_{\perp }\gg {ķ}_{\parallel }$其中平行和垂直符号相对于平均磁场方向。等离子体的框架频率显示出两个总体，一个低于质子回旋加速器频率$\omega <{\mathrm{\Omega }}_{C\mathrm{一世}}$与动力学Alfvén波（KAW）湍流一致。第二部分频率较高$\omega >{\mathrm{\Omega }}_{C\mathrm{一世}}$与离子伯恩斯坦波湍流一致。可替代地，这些波动可以构成经历了多次波-波相互作用的KAW，从而导致等离子体帧频率变宽。在此波矢区域中，与比例有关的峰度显示出小尺度上的间歇性降低，这也可以通过存在波活动来解释。我们的结果表明，小尺度湍流表现出动力学Alfvén和可能的离子-Bernstein（磁声）波的线性波特性。根据我们的结果，我们推测这些波可能在描述观察到的亚离子级的间歇性降低方面发挥了作用。