The evolution of a linearly polarized, long-wavelength Alfvén wave propagating in a collisionless magnetized plasma with a sheared parallel-directed velocity flow is here studied by means of two-dimensional hybrid Vlasov–Maxwell (HVM) simulations. The unperturbed sheared flow has been represented by an exact solution of the HVM set of equations of (Malara et al., Phys. Rev. E, vol. 97, 2018, 053212), thus avoiding spurious oscillations that would arise from the non-stationary initial state and inevitably affect the dynamics of the system. We have considered the evolution of both a small and a moderate amplitude Alfvén wave, in order to separate linear wave–shear flow couplings from kinetic effects, the latter being more relevant for larger wave amplitudes. The phase mixing generated by the shear flow modifies the initial perturbation, leading to the formation of small-scale transverse fluctuations at scales comparable with the proton inertial length/Larmor radius. By analysing both the polarization and group velocity of perturbations in the shear regions, we identify them as kinetic Alfvén waves (KAWs). In the moderate amplitude run, kinetic effects distort the proton distribution function in the shear region. This leads to the formation of a proton beam, at the Alfvén speed and parallel to the magnetic field. Such a feature, due to the parallel electric field associated with KAWs, positively compares with solar wind observations of suprathermal ion populations, suggesting that it may be related to the presence of ion-scale KAW-like fluctuations.

中文翻译：

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通过无碰撞等离子体中的速度剪切产生动力学 Alfvén 波

本文通过二维混合 Vlasov-Maxwell (HVM) 模拟研究了在具有剪切平行定向速度流的无碰撞磁化等离子体中传播的线性极化长波长 Alfvén 波的演化。无扰动剪切流已由 (Malara) 的 HVM 方程组的精确解表示等。,物理。牧师。E，卷。97, 2018, 053212），从而避免了由非平稳初始状态引起的寄生振荡并不可避免地影响系统的动力学。我们已经考虑了小幅度和中等幅度 Alfvén 波的演变，以便将线性波 - 剪切流耦合与动力学效应分开，后者与较大的波幅更相关。剪切流产生的相混合改变了初始扰动，导致在与质子惯性长度/拉莫尔半径相当的尺度上形成小尺度横向波动。通过分析剪切区域中扰动的极化和群速度，我们将它们识别为动力学 Alfvén 波 (KAW)。在中等幅度运行中，动力学效应扭曲了剪切区域中的质子分布函数。这导致质子束的形成，以阿尔文速度并平行于磁场。由于与 KAW 相关的平行电场，这种特征与太阳风对超热离子群的观测结果呈正相关，这表明它可能与离子尺度 KAW 类波动的存在有关。