定常背景场下太阳风Turbulence的各向同性标度特征
Isotropic Scaling Features Measured Locally in the Solar Wind Turbulence with Stationary Background Field
摘要：
标度各向异性是解释太阳风湍流非线性相互作用的关键。以前的观测提供了不同的结果和结构功能分析也被报道是一种方法来研究基于本地磁场的标度各向异性。然而，相对于局部背景磁场的采样角度的确定要求所观察到的平均值的时间序列是时间稳定的。这是否需要的时间平稳性是兼容的测量尚未调查。本文利用二阶结构函数方法研究了时间平稳背景场的标度各向异性，分析了WIND探测器在2005-2018年期间观测到的88个持续时间大于2天的快速太阳风间隔。我们将局部磁场计算为时间序列B的平均值，该时间序列B的时间平稳性由我们的小于10度的标准φ满足（φ是将B切成两半后两个平均磁场之间的角度）。我们发现的各向同性标度特征的磁迹结构函数的标度指数分别为-0.63 pm 0.08和-0.70 pm 0.04，与当地磁场平行和垂直于太阳风的速度方向。速度迹结构函数的标度也是各向同性的，指数为-0.47 pm 0.10和-0.51 pm 0.09。我们还发现，随着φ阈值增加到90 °，平行方向上的磁迹结构函数的标度指数减小到-0.81，而磁场与太阳风速度之间的瞬时角均方根在时间尺度150 s时增加到45 °，表明在大尺度上垂直测量与平行测量的混合。
英文摘要：
The scaling anisotropy is crucial to interpret the nonlinear interactions in solar wind turbulence. Previous observations provide diverse results and the structure function analyses are also reported to be an approach to investigate the scaling anisotropy based on a local magnetic field. However, the determination of the sampling angle with respect to the local background magnetic field requires that the observed time series for the average are time stationary. Whether or not this required time stationarity is compatible with the measurements has not been investigated. Here we utilize the second-order structure function method to study the scaling anisotropy with a time-stationary background field.We analyze 88 fast solar wind intervals each with time durations larger than 2 days measured by WIND spacecraft in the period 2005-2018. We calculate the local magnetic field as the average of the time series B whose time stationarity is fulfilled by our criterion phi smaller than 10 deg (phi is the angle between the two averaged magnetic fields after cutting B into two halves). We find for the first time the isotropic scaling feature of the magnetic-trace structure functions with scaling indices -0.63 pm 0.08 and -0.70 pm 0.04, respectively, with the local magnetic field parallel and perpendicular to the solar wind velocity directions. The scaling for the velocity-trace structure functions is also isotropic and the indices are -0.47 pm 0.10 and -0.51 pm 0.09. We also find that with increasing phi threshold to 90 deg, the scaling index of the magnetic-trace structure function in the parallel direction decreases to -0.81, while the rms of the instantaneous angle between magnetic field and solar wind velocity increases up to 45deg at the timescale 150 s, indicating a mix of perpendicular measurements into parallel ones at large scales.
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