太阳风磁化等离子体中各向异性湍流的级联模型

中文摘要 AI 辅助

我们提出了一个用于阿尔文太阳风湍流的物理框架，其中等离子体被建模为具有关于域平均磁场局部旋转对称性的离散域。利用这种对称性，我们构建了由两个特征时间尺度（非线性和阿尔文）控制的极简级联模型，这两个时间尺度分别与相对于域平均磁场的垂直和平行方向相关。在这种部分对称性下，我们还通过一个额外的几何参数来表征每个域的各向异性，即域平均速度与磁场之间的对齐角。我们引入了一个基于这两个时间尺度的双峰等待时间分布的随机更新过程，从而得到总能量级联的双分支重正化解：一个统计稳健的分支，具有类似伊罗什尼科夫-克赖奇南的$k^{-3/2}$谱；以及一个统计边缘的分支，具有类似科尔莫戈罗夫的$k^{-5/3}$谱。利用因果性和级联稳定性的原理，我们表明当能量通量波动变得超临界时，系统会在两个可用的级联速率中选择更快的那个，从而防止间歇性通量积累。因此，在太阳风膨胀过程中，平衡域（具有低交叉螺旋度）经历从慢的$k^{-3/2}$级联到快的$k^{-5/3}$级联的一级相变。该转变通过开关处的异质成核加速。结合一个从属于能量级联的前向磁螺旋度级联，我们表明大尺度谱解耦为平坦的$k^{-3/4}$磁谱和$k^{-3/2}$动能谱。来自旅行者、尤利西斯、太阳神、风神和帕克太阳探测器的数据证实了这些谱特征在多样日球层区域中的存在。

英文摘要

We present a physical framework for Alfvénic solar wind turbulence in which the plasma is modeled as discrete domains with local rotational symmetry about the domain-mean magnetic field. Using this symmetry, we construct minimalist cascade models governed by two characteristic time scales, nonlinear and Alfvénic, associated, respectively, with the perpendicular and parallel directions relative to the domain-mean magnetic field. Within this partial symmetry, we also characterize the anisotropy of each domain by a single additional geometrical parameter, the alignment angle between the domain-mean velocity and magnetic fields. We introduce a stochastic renewal process with a bimodal waiting-time distribution based on these two time scales, yielding a two-branch renormalization solution for the total energy cascade: a statistically robust branch with an Iroshnikov-Kraichnan-like $k^{-3/2}$ spectrum, and a statistically marginal branch with a Kolmogorov-like $k^{-5/3}$ spectrum. Utilizing principles of causality and cascade stability, we show that the system selects the faster cascade rate between the two available whenever energy-flux fluctuations become supercritical, preventing intermittent flux accumulation. Consequently, during solar wind expansion, balanced domains (with low cross-helicity) undergo a first-order phase transition from the slow $k^{-3/2}$ cascade to the fast $k^{-5/3}$ cascade. The transition is accelerated by heterogeneous nucleation at switchbacks. Incorporating a forward magnetic helicity cascade slaved to the energy cascade, we show that the large-scale spectra decouple into a flat $k^{-3/4}$ magnetic spectrum and a $k^{-3/2}$ kinetic spectrum. Data from Voyager, Ulysses, Helios, Wind, and PSP confirm these spectral signatures across diverse heliospheric regions.

发表机构

• ICAR（以色列应用物理研究所）

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