Weakly collisional space plasmas are rarely in local thermal equilibrium and often exhibit non-Maxwellian electron and ion velocity distributions that lead to the growth of microinstabilities, that is, enhanced electric and magnetic fields at relatively short wavelengths. These instabilities play an active role in the evolution of space plasmas, as does ubiquitous broadband turbulence induced by turbulent structures. This study compares certain properties of a 2.5 dimensional Particle-In-Cell (PIC) simulation for the forward cascade of Alfvenic turbulence in a collisionless plasma against the same properties of turbulence observed by the Magnetospheric Multiscale Mission spacecraft in the terrestrial magnetosheath. The PIC simulation is of decaying turbulence which develops both coherent structures and anisotropic ion velocity distributions with the potential to drive kinetic scale instabilities. The uniform background magnetic field points perpendicular to the plane of the simulation. Growth rates are computed from linear theory using the ion temperature anisotropies and ion beta values for both the simulation and the observations. Both the simulation and the observations show that strong anisotropies and growth rates occur highly intermittently in the plasma, and the simulation further shows that such anisotropies preferentially occur near current sheets. This suggests that, though microinstabilities may affect the plasma globally , they act locally and develop in response to extreme temperature anisotropies generated by turbulent structures. Further studies will be necessary to understand why there is an apparent correlation between linear instability theory and strongly intermittent turbulence.

中文翻译：

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间歇性和离子温度-各向异性不稳定性：模拟和磁鞘观测

弱碰撞空间等离子体很少处于局部热平衡，并且通常表现出非麦克斯韦电子和离子速度分布，导致微不稳定性的增长，即在相对较短的波长下增强的电场和磁场。这些不稳定性在空间等离子体的演化中起着积极的作用，湍流结构引起的无处不在的宽带湍流也是如此。本研究比较了无碰撞等离子体中阿尔芬湍流前向级联的 2.5 维细胞内粒子 (PIC) 模拟的某些特性与磁层多尺度任务航天器在地面磁鞘中观察到的相同湍流特性。PIC 模拟是衰减湍流，它产生相干结构和各向异性离子速度分布，并有可能导致动力学尺度不稳定性。均匀的背景磁场垂直于模拟平面。使用模拟和观察的离子温度各向异性和离子β值从线性理论计算生长速率。模拟和观察结果都表明，等离子体中强烈的各向异性和增长率高度间歇性地出现，并且模拟进一步表明，这种各向异性优先出现在电流片附近。这表明，虽然微观不稳定性可能会影响等离子体的整体性，但它们会在局部作用并响应湍流结构产生的极端温度各向异性而发展。