太阳风湍流在二维波矢空间分布特征的观测研究

项目名称： 太阳风湍流在二维波矢空间分布特征的观测研究

项目编号： No.41504130

项目类型： 青年科学基金项目

立项/批准年度： 2016

项目学科： 天文学、地球科学

项目作者： 王新

作者单位： 北京大学

项目金额： 20万元

中文摘要： 太阳风湍流的各向异性是指湍流特性随背景磁场方向的变化而变化，它是太阳风湍流研究中的重要课题，被认为可以证实太阳风湍流中存在临界平衡串级。然而，由于背景磁场方向保持与太阳风速度方向平行的时段较少，使得计算平行情况下的谱存在一定困难。为了克服这一困难，近年来人们提出了随时间和尺度变化的局地背景磁场的概念，并按照局地背景磁场的方向把不同时间的小波谱揉和在一起求平均，就得到各向异性的湍流谱。但是这一过程不能保证数据是平稳随机的。本工作计划挑选满足平稳随机的观测时段，重新研究太阳风湍流在二维波矢空间中的分布特征。我们将从WIND卫星20年的数据中，寻找满足局地背景磁场不随时间变化和观测时长>5倍的相关时间这两个条件的时段，对每一时段计算功率谱和相关函数，并从它们在二维波矢空间中的分布看各向异性是否仍然存在。这一工作将为揭示湍流各向异性的起源提供新线索，并帮助人们进一步弄清楚太阳风湍流扰动的组成成分。

中文关键词： 太阳风；湍流；磁场；各向异性

英文摘要： A fundamental issue in the study of solar wind turbulence is to understand the anisotropy of it. The observed power spectrum anisotropy was considered as being consistent with the presence of the “critical balance” type cascade in the solar wind turbulence. However, the duration of the intervals in which the background magnetic field keeps parallel to the solar wind velocity is too short to produce a power spectrum in the inertial region. In order to solve this problem, people introduced a scale- and time-dependent local background magnetic field (local B0) when calculating the turbulent spectra. In each scale, they rearranged the wavelet coefficients according to the local B0, and consider the mean of these coefficients as the average power, and then obtained the anisotropic spectra. But in this process, the observed data cannot be seen as a stationary and random function of time. In our work, the intervals which can satisfy the stationary and random conditions will be selected to further study the distribution of solar wind turbulence in 2-D wavevector space. We will use the magnetic field and plasma data measured by WIND spacecraft to find the time series with constant local B0 and with duration much longer than the correlation time. For each time series, the power spectrum and correlation function will be calculated. Statistically we can obtain the 2-d wavevector-space distribution of the spectrum and correlation function, and find out if the anisotropy still exists. This work will provide new clues on the origin of the turbulence anisotropy. It will also help us further understand the components of solar wind fluctuations.

英文关键词： solar wind;turbulence;magnetic field ;anisotropy