Microphysics of Cosmic Plasmas
Microphysics of Cosmic Plasmas
复制标题
宇宙等离子体的微观物理
DOI：
10.1007/978-1-4899-7413-6_3
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发表时间：
2014
期刊：
影响因子：
--
通讯作者：
Alexandrova O
中科院分区：
文献类型：
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作者：
Alexandrova O
太阳风可能是研究天体物理等离子体湍流的最佳实验室。除了磁场的存在，中性流体各向同性湍流的差异是：（i）碰撞耗散的弱点和（ii）存在几个特征的空间和时间尺度。本文讨论了从磁流体到电子尺度的大范围太阳风湍流的观测特性。在MHD尺度下，在惯性范围内，磁涨落的湍流级联主要在垂直于平均场的平面内发展，垂直级联和平行级联都具有Kolmogorov标度。太阳风湍流本质上是可压缩的：MHD尺度下的密度波动具有柯尔莫哥洛夫谱。速度涨落不遵循磁场涨落：它们的谱是一个幂律，谱指数为-3/2。不同等离子体参数的概率分布函数不是高斯分布，表明存在非线性。目前还没有考虑到惯性范围的所有这些观测特性的全球模型。在离子尺度下，湍流谱发生突变，可压缩性增强，密度涨落谱局部平坦。在离子尺度附近，磁谱是可变的，并且离子不稳定性作为局部等离子体参数的函数发生。在离子和电子尺度之间，似乎建立了一个小尺度的湍流级联。它的特征是在磁场和密度涨落中有一个定义良好的幂律谱，谱指数接近−2.8。接近电子尺度，波动不再是自相似的：通常观察到指数截止（对于没有准平行哨声的时间间隔），表明耗散的开始。离子尺度和电子尺度之间的小尺度惯性范围和电子耗散范围可以用α ε 8/3和耗散尺度ε d来描述，其中α ε 8/3和耗散尺度ε d接近于电子的拉莫尔半径ε d ε ρe。这种小规模级联的性质和可能的耗散机制仍在争论中。
Solar wind is probably the best laboratory to study turbulence in astrophysical plasmas. In addition to the presence of magnetic field, the differences with neutral fluid isotropic turbulence are: (i) weakness of collisional dissipation and (ii) presence of several characteristic space and time scales. In this paper we discuss observational properties of solar wind turbulence in a large range from the MHD to the electron scales. At MHD scales, within the inertial range, turbulence cascade of magnetic fluctuations develops mostly in the plane perpendicular to the mean field, with the Kolmogorov scalingfor the perpendicular cascade andfor the parallel one. Solar wind turbulence is compressible in nature: density fluctuations at MHD scales have the Kolmogorov spectrum. Velocity fluctuations do not follow magnetic field ones: their spectrum is a power-law with a −3/2 spectral index. Probability distribution functions of different plasma parameters are not Gaussian, indicating presence of intermittency. At the moment there is no global model taking into account all these observed properties of the inertial range. At ion scales, turbulent spectra have a break, compressibility increases and the density fluctuation spectrum has a local flattening. Around ion scales, magnetic spectra are variable and ion instabilities occur as a function of the local plasma parameters. Between ion and electron scales, a small scale turbulent cascade seems to be established. It is characterized by a well defined power-law spectrum in magnetic and density fluctuations with a spectral index close to −2.8. Approaching electron scales, the fluctuations are no more self-similar: an exponential cut-off is usually observed (for time intervals without quasi-parallel whistlers) indicating an onset of dissipation. The small scale inertial range between ion and electron scales and the electron dissipation range can be together described by, withα≃8/3 and the dissipation scaleℓdclose to the electron Larmor radiusℓd≃ρe. The nature of this small scale cascade and a possible dissipation mechanism are still under debate.