The nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is being investigated extensively nowadays, both theoretically and observationally. Although recent observations gave evidence of the dominance of kinetic Alfvén waves (KAWs) at sub-ion scales with ω ωci, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., ω ωci) is more relevant. Here, we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically β i β e ~ 1 and for high oblique angles of propagation 80° ≤ Θ kB 90° as observed from the Cluster spacecraft data. The linear properties of the plasma modes under these conditions are poorly known, which contrasts with the well-documented cold plasma limit and/or moderate oblique angles of propagation (Θ kB 80°). Based on linear solutions of the Vlasov kinetic theory, we discuss the relevance of each plasma mode (fast, Bernstein, KAW, whistler) in carrying the energy cascade down to electron scales. We show, in particular, that the shear Alfvén mode (known in the magnetohydrodynamic limit) extends at scales kρ i 1 to frequencies either larger or smaller than ωci, depending on the anisotropy k ∥/k ⊥. This extension into small scales is more readily called whistler (ω ωci) or KAW (ω ωci), although the mode is essentially the same. This contrasts with the well-accepted idea that the whistler branch always develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter the more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small-scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW.
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机译：质子陀螺规模以下的太阳风（SW）湍流的性质是当今一个广泛的话题，无论是在理论上还是在观察上。尽管最近的观察提供了在ω<ωci的亚离子尺度上动力学Alfvén波（KAWs）占优势的证据，但其他研究表明KAW模式不能将湍流级联降低到电子尺度，并且吹口哨模式（即ω >ωci）更相关。在这里，我们研究了短波长等离子体模式在有限但切合实际的SW条件（通常为βiβe〜1）以及对于高倾斜传播角80°≤ΘkB <90°的情况下的关键性质，数据。在这些条件下，等离子体模式的线性特性鲜为人知，这与文献充分记载的冷等离子体极限和/或中等倾斜传播角（ΘkB <80°）形成鲜明对比。基于弗拉索夫动力学理论的线性解，我们讨论了每种等离子模式（快速，伯恩斯坦，KAW，吹口哨）在将能量级联传递到电子尺度时的相关性。我们特别表明，根据各向异性k∥/ k the，剪切Alfvén模式（在磁流体动力学极限中已知）以尺度kρi 1扩展到大于或小于ωci的频率。尽管模式基本相同，但这种扩展到小规模的方法更容易称为惠斯勒（ω>ωci）或KAW（ω<ωci）。这与公认的想法相反，即惠斯勒分支总是在快速磁声模式的高频下继续发展。此外，我们还表明，惠斯勒分支比KAW分支更受阻尼，这使KAW成为将能量级联传递到电子尺度的更相关的候选者。我们讨论了这些新发现如何促进有关小规模湍流性质的争议的解决，并且我们讨论了西南偏航对当前和未来航天器波测量的影响。
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