太阳风湍流惯性区中两个子区间的标度与各向异性研究
On the scaling and anisotropy of two subranges in the inertial range of solar wind turbulence
AI总结：
本研究通过分析Ulysses和PSP的太阳风观测数据，发现湍流惯性区存在两个具有不同间歇性、各向异性与标度特征的子区间，揭示了强弱湍流转变的可能性，为太阳风湍流理论提供了观测约束。
摘要：
间歇性与各向异性是等离子体湍流的两个重要特性，太阳风为研究这一特性提供了天然实验室。然而，它们的形式与本质仍存在争议，导致理论解释难以达成共识。本文对Ulysses（尤利西斯号）在1.48天文单位处观测到的快太阳风、以及Parker Solar Probe (PSP，帕克太阳探测器)在0.17天文单位处观测到的慢太阳风开展了高阶统计分析。研究发现，惯性区内明确存在两个子区间，二者在间歇性和各向异性方面呈现出截然不同的特征。尺度较小的子区间1具有多重分形标度特性，其二阶标度指数$ξ(2) \sim 2/3$；尺度较大的子区间2同样为多重分形，但二阶标度指数$ξ(2) \sim 1/2$。在Ulysses与PSP的观测数据中，两个子区间的分界点均位于相同的空间尺度上。子区间1在垂直于局地磁场的方向上为多重分形，垂直方向二阶标度指数$ξ_{\perp}(2) \sim 2/3$，在平行于局地磁场的方向上则表现为单标度，平行方向二阶标度指数$ξ_{\parallel}(2) \sim 1$。子区间2在平行与垂直方向上均为多重分形，其中垂直方向二阶标度指数$ξ_{\perp}(2) \sim 1/2$，平行方向二阶标度指数$ξ_{\parallel}(2) \sim 2/3$。两个子区间均表现出功率各向异性与波矢各向异性。两个子区间的差异化特征表明，湍流可能发生了从弱到强的转变，且分界点的空间尺度可能不会随太阳风膨胀而演化。这些新结果更新了我们对惯性区的认知，为理解太阳风湍流的间歇性与各向异性提供了有力的观测约束。
英文摘要：
Intermittency and anisotropy are two important aspects of plasma turbulence, which the solar wind provides a natural laboratory to investigate. However, their forms and nature are still under debate, making it difficult to achieve a consensus in the theoretical interpretation. Here, we perform higher-order statistics for the observations in the fast solar wind at 1.48 au obtained by Ulysses and in the slow solar wind at 0.17 au obtained by Parker Solar Probe (PSP). We find that two subranges clearly exist in the inertial range and they present distinct features with regard to the intermittency and anisotropy. The subrange 1 with smaller scale has a multifractal scaling with the second index $ξ(2) \sim 2/3$ and the subrange 2 with larger scale is also multifractal but with $ξ(2) \sim 1/2$. The break between two subranges locates at the same spatial scale for both Ulysses and PSP observations. Subrange 1 is multifractal in the direction perpendicular to the local magnetic field with $ξ_{\perp}(2) \sim 2/3$ and seems to be monoscaling in the parallel direction with $ξ_{\parallel}(2) \sim 1$. Subrange 2 is multifractal in both parallel and perpendicular directions with $ξ_{\perp}(2) \sim 1/2$ and $ξ_{\parallel}(2) \sim 2/3$. Both subrange 1 and subrange 2 present power and wavevector anisotropies. The distinct features of two subranges suggest that a transition from weak to strong turbulence may occur and the spatial scale of the break may not evolve with the solar wind expansion. These new results update our knowledge of the inertial range and provide strong observational constraints on the understanding of intermittency and anisotropy in solar wind turbulence.
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