太阳风湍流中局地背景磁场下的结构函数指数特征

Scaling features measured locally in the precise parallel and precise perpendicular directions in the solar wind turbulence

太阳风是天然的磁流体湍流实验室.当前流行的描述磁流体湍流的临界平衡串级模型,预测平行局地背景磁场的谱指数为−2.小波变换分析和结构函数分析是得到相对于局地背景磁场的谱指数各向异性的两个主要方法.前人的工作得到了太阳风中平行局地背景磁场的谱指数为−2的观测结果.然而,这一结果被认为是受到了太阳风中的间歇或结构的影响.一方面,去除间歇后,小波变换分析得到的平行谱指数为−1.63.去除结构后,结构函数分析得到的平行谱指数为−1.63.两个方法得到的结果都不支持临界平衡串级模型在太阳风中的应用.另一方面,前人利用小波变换分析,考虑平行和垂直磁场条件的精确性,发现当要求严格平行局地背景磁场条件下的谱指数为−1.75,也不支持临界平衡串级.我们采用结构函数分析方法,利用WIND卫星在拉格朗日1点观测的高速太阳风数据,分析了磁场和速度的结构函数指数的各向异性.研究发现,基于更严格的局地平行条件,平行局地背景磁场的磁场结构函数指数为−0.67,平行局地背景磁场的速度结构函数指数为−0.55,在误差范围内,两者均与垂直局地背景磁场的指数接近.结果显示,中等振幅扰动的指数也是各向同性的.这一结果不支持临界平衡串级应用于描述太阳风湍流,为发展新的理论模型提供了观测依据.

The solar wind turbulence is now widely believed as describable by a critical balance cascade based on-2 scaling index measured in the direction parallel to the local magnetic field reported by many papers.However,previous studies have shown that both wavelet analyses and structure function analyses may be influenced by the intermittency and structures in the solar wind.One way to reliably obtain the true spectral index is to remove the intermittency and structures and another way is to put strict criteria on the parallel selection.Previous results show that the parallel scaling index is around−5/3 after removing intermittency or under strict criteria for low-amplitude fluctuations.Here we present results of the scaling with precise local magnetic field directions.We first select the parallel observations with a small(<10o)θVB(the angle between the average of two-time-instant magnetic field(Bl)and the average of the same two-time-instant velocity).We define precise parallel direction as the parallel observations with a small(<10o)ϕ(the angle between Bl and another local magnetic field B0 deter-mined as the average of the time series observed between the corresponding two time instants.)The precise perpen-dicular direction is defined in a similar way.The condition whether or not Bl and B0 are parallel could help to de-termine the precise magnetic direction.The lowerϕ,the more precise the direction is.We find that the scaling in-dex of the magnetic-trace structure function is strongly affected by the value ofϕ.Asϕdecreases,in other words,the preciseness of direction increases,the anisotropy gradually weakens and eventually almost disappears.This could be due to the fact that there are more structures at the larger scale and constrainingϕto a low value leads to more power removed at the larger scale.This effect causes the change of the scaling index:the steep parallel index becomes flatter,especially in the parallel direction.We find that the scaling index of the magnetic(velocity)-trace structure functions are−0.67±0.11(−0.55±0.18)and−0.55±0.10(−0.46±0.09)respectively in the precise parallel and perpendicular directions.These new results are not consistent with the−2 scaling index and may not favor the application of the critical balance framework in the solar wind fluctuations.The fluctuation amplitudes of the mag-netic field and the velocity are both moderate,which expands the determination of isotropy from the low-amplitude condition in previous work to the moderate-amplitude condition.Our results help to distinguish the true feature of the scaling index anisotropy in the solar wind and will initiate new theoretical study on solar wind turbulence.