Transition to kinetic turbulence at proton scales driven by large-amplitude kinetic Alfvén fluctuations
Transition to kinetic turbulence at proton scales driven by large-amplitude kinetic Alfvén fluctuations
复制标题
由大振幅动能阿尔芬涨落驱动的质子尺度动能湍流的转变
DOI：
10.1051/0004-6361/201629240
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发表时间：
2016
影响因子：
6.5
通讯作者：
F. Pucci
中科院分区：
文献类型：
--
作者：
Francesco Valentini;Christian L. Vásconez;O. Pezzi;S. Servidio;F. Malara;F. Pucci
空间等离子体主要存在大振幅波、大规模不均匀性、动力学效应和湍流。除了均匀湍流之外，小尺度波动的产生也可能发生在其他现实配置中，即当扰动叠加到不均匀背景磁场时。当阿尔文波在阿尔文速度沿平均场横向变化的介质中传播时，它会经历相位混合，逐渐弯曲波前，在横向上产生小尺度。一旦横向尺度达到质子惯性长度 d p 的数量级，就会自然产生动能阿尔文波（KAW）。 KAW 属于阿尔文波的分支，几乎垂直于环境磁场传播，尺度接近 d p 。许多数值、观测和理论工作表明，这些波动可能在太阳风湍流级联的发展中发挥决定性作用。在本文中，通过混合 Vlasov-Maxwell 直接数值模拟研究了非均匀背景下大幅 KAW 涨落的产生及其对质子的影响。施加压力平衡磁剪切，通过改变磁构型和初始扰动的幅度来研究质子的动力学。这里特别令人感兴趣的是从准线性到湍流状态的转变，特别关注由 KAW 波动驱动的质子分布函数中重要的非麦克斯韦特征的发展。提出了几个量化质子与热力学平衡偏差的指标。这些数值结果可能有助于解释不均匀和湍流天体物理等离子体的复杂动力学，例如日光层电流片、磁层边界层和日冕。
Space plasmas are dominated by the presence of large-amplitude waves, large-scale inhomogeneities, kinetic effects and turbulence. Beside the homogeneous turbulence, the generation of small scale fluctuations can take place also in other realistic configurations, namely, when perturbations superpose to an inhomogeneous background magnetic field. When an Alfven wave propagates in a medium where the Alfven speed varies in a direction transverse to the mean field, it undergoes phase-mixing, which progressively bends wavefronts, generating small scales in the transverse direction. As soon as transverse scales become of the order of the proton inertial length d p , kinetic Alfven waves (KAWs) are naturally generated. KAWs belong to the branch of Alfven waves and propagate almost perpendicularly to the ambient magnetic field, at scales close to d p . Many numerical, observational and theoretical works have suggested that these fluctuations may play a determinant role in the development of the solar-wind turbulent cascade. In the present paper, the generation of large amplitude KAW fluctuations in inhomogeneous background, as well as their effect on the protons, have been investigated by means of hybrid Vlasov-Maxwell direct numerical simulations. Imposing a pressure balanced magnetic shear, the kinetic dynamics of protons has been investigated by varying both the magnetic configuration and the amplitude of the initial perturbations. Of particular interest here is the transition from quasi-linear to turbulent regimes, focusing in particular on the development of important non-Maxwellian features in the proton distribution function driven by KAW fluctuations. Several indicators to quantify the deviations of the protons from thermodynamic equilibrium are presented. These numerical results might help to explain the complex dynamics of inhomogeneous and turbulent astrophysical plasmas, such as the heliospheric current sheet, the magnetospheric boundary layer, and the solar corona.