太阳风湍流间歇亚区的串级和加热机制

The power spectra of the fluctuations at small scales in the solar wind turbulence were found to have some new characteristics. On the basis of these observations, we proposed a new concept of an intermittency domain (Wang, X, Tu, C.-Y.* et al., ApJL 2014; ApJL 2015; ApJ 2016). The intermittency domain is in the MHD inertial range but just above the dissipation range and consists largely of intermittent structures, which contribute essentially to the scaling behavior of the turbulence. We also found that the anisotropy of the power spectra differs for fluctuations of different amplitudes. For the moderate-amplitude fluctuations, the averaged magnetic spectral indices are close to -2 and -5/3, respectively, for small and large sampling angles, i.e., the angle between the Sun-to-Earth radial direction and the mean magnetic field direction. However, for the low-amplitude fluctuations, these two indices are -1.67 and -1.46, respectively. This new result cannot be explained by existing turbulence theories, like the critical balance theory, and thus opens a new research direction of the solar wind turbulence. In this project, we plan to establish a theoretical cascading model for the intermittency domain. We will also do numerical simulations and then compare with observations. The innovation of this project is to establish a two-component turbulence model which includes Alfvén waves and intermittent structures. One of the previous works of our group (Tu and Marsch, 1993) provides preliminary study for this project. It is expected that this work will reveal the physical process about the influence of intermittent structures on the turbulence cascade, and will explain the new observational results.

我们发现太阳风小尺度湍流功率谱具有新的特征，并提出了“间歇亚区”的概念（Wang,X; Tu C.-Y.*等,ApJL2014,2015,ApJ2016）。间歇亚区在太阳风湍流惯性区内，紧连耗散区,其中包含大量间歇结构，控制振幅和各向异性特征。在中等振幅的湍流中，相应于平行与垂直方向采样的磁场功率谱斜率分别接近-2和-5/3 ，然而，小振幅湍流中相应的斜率分别接近-5/3和-3/2。该观测新结果不能被现有理论（例如临界平衡理论）解释，为太阳风湍流的研究开辟了新的方向。本项目的目标是建立间歇亚区的湍流串级模型，包括理论模型和数值模拟并与观测比较。创新特色是建立包含阿尔芬波与间歇结构的湍流模型。我们以往的工作（Tu and Marsch JGR1993,被SCI总引用 120次，近10年被引63次）为本项目研究奠定了基础。预期揭示间歇结构对串级影响的物理过程，解释上述新结果，发展湍流加热模型。

太阳风湍流间歇亚区处于湍流惯性区的高频端，与耗散区比邻，传承串级能量。该尺度中包含大量间歇结构，控制湍流扰动振幅和各向异性特征，在能量串级和耗散过程中起着关键作用。本项目通过对间歇亚区太阳风湍流本质的观测研究发现太阳风湍流在平稳随机背景下有各向同性特征，不支持当前流行的“临界平衡串级理论”的解释，同时确定了回旋共振耗散机制。具体内容如下：1)太阳风中的高阿尔芬特性湍流不包含内传阿尔芬波，由高速流观测数据计算出的Elsässer变量z-只是误差产生的高频噪音和低频虚假结构，不支持由外传和内传的阿尔芬波的非线性相互作用所产生的能量串级的模型；2)在小尺度惯性区，二维和三维的自相关函数等值线是各向同性的, 不支持临界平衡串级理论关于小尺度湍流结构各向异性的推论；3)平稳随机背景下的太阳风湍流磁场扰动和速度扰动的结构函数指数均表现为各向同性，磁场扰动在平行方向功率谱指数为-2的分析结果是由非平稳起伏导致的，不支持临界平衡理论；4)湍流功率谱在惯性区频率高频端的拐点对应的尺度符合回旋共振耗散理论，不支持动力学阿尔芬波耗散的机制。根据上述新的分析结果，我们提出了新的描述太阳风能量串级过程的理论框架：太阳风湍流能量串级由K41各向同性流体湍流串级机制控制，在“间歇亚区”间歇结构有重要影响；串级能量由回旋共振机制耗散，加热太阳风。