太阳风湍流间歇对谱各向异性的调制及其产生机制

Solar wind turbulence is considered to be the energy source of the solar wind heating, and it is also the interplanetray medium of energetic particles travelling that can cause the radiation damage to the spacecraft, instruments and astronauts in space. A fundamental issue in the study of solar wind turbulence is to understand the turbulent cascading process. The observed spectral anisotropy of the solar wind turbulence is currently believed to be consistent with the presence of a ''critical balance'' style turbulent cascade. However, in our recent work (Wang, X., Tu, C.-Y. et al., 2014, ApJL), it is found that the spectral anisotropy in the inertial range could result from turbulence intermittency rather than critical balance cascade. This novel finding proposes a new research direction for the study of the turbulece cascading. Along the new direction, this project plan to work on the nature of the solar wind turbulence. The magnetic field data and plasma data measured by WIND spacecraft and the data from other spacecrafts will be used, together with the new method we developed to identify the intermittent structures and to investigate the influence of intermittency on the spectral anisotropy. We will further study the generation mechanism of the spectral anisotropy, and the nature along with the formation process of the intermittent structures. It is expected that this work will reveal the cascading mechanism related to the intermittency and the spectral anisotropy, and then help to build a theoretical model to explain the relation between the intermittency and the spectral anisotropy. Hopefully, this project will improve our understanding about the turbulence cascading process.

太阳风湍流被认为是太阳风加热的能源，是影响对空间飞行有伤害作用的日球层高能粒子传播的媒介。然而，太阳风湍流的本质仍不清楚，其核心的研究课题是能量串级机制。太阳风湍流谱各向异性现象目前被认为表征太阳风湍流存在"临界平衡串级"。可是，本研究组的近期工作(Wang, X., Tu, C.-Y. et al., 2014, ApJL)发现，间歇强度对谱各向异性程度有控制作用。这一发现为谱各向异性的起源提出了新的可能性，开辟了研究太阳风湍流串级机制的新方向。本申请计划沿着这一新方向，研究太阳风湍流的本质。我们将用WIND等卫星的观测数据，结合已有的关于谱各向异性和间歇结构的研究方法，以及本研究组创造的确定间歇强度影响各向异性程度的新方法，进一步研究谱各向异性的产生机制，间歇结构特性和起源，揭示控制两现象相关特征的能量串级机制，建立描述谱各向异性及其与间歇关系的理论模型，揭示太阳风湍流的能量串级特性。

太阳风小尺度湍流的两个重要特性是其间歇性和谱各向异性。学术界主流观点认为太阳风湍流功率谱的各向异性的产生机制是由“临界平衡串级理论”描述的。本项目通过对观测数据的详细分析发现谱各向异性是由湍流间歇产生的，不适合用“临界平衡串级理论”的描述，具体内容如下：1)太阳风小振幅扰动呈现出的较弱的谱各向异性 2)太阳风湍流不符合临界平衡串级理论假设条件3)太阳风湍流间歇结构表现为各向异性，其垂直尺度小于平行尺度 4)具有幂律谱的上传阿尔芬扰动表现为各向异性。数值模拟研究给出：1)发现去除间歇后垂直谱变陡，各向异性减弱 2)强湍流下湍流间歇结构各向异性消失 3)湍流间歇结构旋转间断面是由于阿尔芬波的陡化产生，而湍流中磁力线管的挤压及分离导致切向间断面的形成和瓦解；切向间断面可引起平行垂直温度增加，旋转间断面没有。这些研究成果表明，没有理论能描述太阳风湍流的间歇性与谱各向异性的关系，需要建立新的磁流体湍流模型，为太阳风湍流的研究开辟了新的研究方向。