Applicability of Taylor’s hypothesis during Parker Solar Probe perihelia
Applicability of Taylor’s hypothesis during Parker Solar Probe perihelia
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DOI：
10.1051/0004-6361/202039879
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发表时间：
2021-03
影响因子：
6.5
通讯作者：
J. C. Perez;S. Bourouaine;C. Chen;N. Raouafi
中科院分区：
文献类型：
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作者：
J. C. Perez;S. Bourouaine;C. Chen;N. Raouafi
利用帕克太阳探测器（PSP）观测到的Alfvénic型太阳风流场，研究了泰勒假说（TH）在分析前四次观测中的速度和磁场涨落时的有效性。分析是基于最近的模型的时空相关性的磁流体动力学（MHD）湍流，这已被验证在高分辨率的数值模拟强减少MHD湍流。我们使用PSP的速度和磁场测量从24小时的时间间隔，从每一个前四次相遇。TH的适用性是通过测量参数δ u = δ u 0/δ 2 V θ来研究的，该参数量化了大尺度波动的典型速度δ u 0与太阳风框架中的局部垂直PSP速度V θ之间的比率。当PSP几乎垂直于等离子体框架中的局部磁场运动时，TH预计适用于0.5 °，而与阿尔文马赫数MA = VSW scinVA无关，其中VSW和VA分别是局部太阳风和阿尔文速度。对于四个选定的太阳风间隔，我们发现，在10%和60%的时间，参数θ小于0.2和采样角（航天器在等离子体框架和当地磁场之间的速度）大于30°。对于大于30°的角度，采样方向足够倾斜，以允许从其测量的频谱重建磁波动的约化能谱E（k）。从幂律拟合测得的频谱确定的光谱指数准确地表示与等离子体帧中的湍流波动的基本空间谱相关联的光谱指数。除了由于需要仔细考虑的大尺度扫描而导致的频率展宽之外，可以恢复空间谱以获得等离子体框架中跨尺度的波动能量分布。
We investigate the validity of Taylor’s hypothesis (TH) in the analysis of velocity and magnetic field fluctuations in Alfvénic solar wind streams measured by Parker Solar Probe (PSP) during the first four encounters. The analysis is based on a recent model of the spacetime correlation of magnetohydrodynamic (MHD) turbulence, which has been validated in high-resolution numerical simulations of strong reduced MHD turbulence. We use PSP velocity and magnetic field measurements from 24 h intervals selected from each of the first four encounters. The applicability of TH is investigated by measuring the parameter ϵ = δu0/√2V⊥, which quantifies the ratio between the typical speed of large-scale fluctuations, δu0, and the local perpendicular PSP speed in the solar wind frame, V⊥. TH is expected to be applicable for ϵ ≲ 0.5 when PSP is moving nearly perpendicular to the local magnetic field in the plasma frame, irrespective of the Alfvén Mach number MA = VSW∕VA, where VSW and VA are the local solar wind and Alfvén speed, respectively. For the four selected solar wind intervals, we find that between 10 and 60% of the time, the parameter ϵ is below 0.2 and the sampling angle (between the spacecraft velocity in the plasma frame and the local magnetic field) is greater than 30°. For angles above 30°, the sampling direction is sufficiently oblique to allow one to reconstruct the reduced energy spectrum E(k⊥) of magnetic fluctuations from its measured frequency spectra. The spectral indices determined from power-law fits of the measured frequency spectrum accurately represent the spectral indices associated with the underlying spatial spectrum of turbulent fluctuations in the plasma frame. Aside from a frequency broadening due to large-scale sweeping that requires careful consideration, the spatial spectrum can be recovered to obtain the distribution of fluctuation’s energy across scales in the plasma frame.