在马耳他十字架之外：turbulence的几何形状在0.2和1之间AU
Beyond the Maltese cross: geometry of turbulence between 0.2 and 1 AU
摘要：
自1990年以来，在太阳风中测量了湍流结构的谱各向异性，依赖于关于平均磁场B0的轴对称假设。然而，一些作品表明，这一假设可能是部分错误的，从而提出了两个问题：（i）是正确的解释测量在1 AU（所谓的马耳他十字架）的总和板和二维湍流？(ii)马耳他十字架到底包含了什么信息我们解决直接数值模拟的MHD方程，包括横向拉伸施加的太阳风流动和研究真正的三维各向异性的湍流，以及一个由轴对称的假设所产生的关于B0。我们发现，从0.2到1 AU的湍流谱的演变强烈依赖于它的初始各向异性。相对于B0的轴对称谱保持其轴对称性，即，抵抗垂直于径向的拉伸，而各向同性光谱相对于径向方向变得基本上轴对称。我们的结论是，靠近太阳，慢风湍流有一个频谱，是轴对称的B0和测量的二维组件在1 AU描述的湍流结构的真实形状。相反，快风湍流在源处具有更各向同性的谱，并且在1 AU处变得径向对称。这种结构被应用于数据的对称化所隐藏，而不是返回板几何。
英文摘要：
The spectral anisotropy of turbulent structures has been measured in the solar wind since 1990, relying on the assumption of axisymmetry about the mean magnetic field, B0. However, several works indicate that this hypothesis might be partially wrong, thus raising two questions: (i) is it correct to interpret measurements at 1 AU (the so-called Maltese cross) in term of a sum of slab and 2D turbulence? (ii) what information is really contained in the Maltese cross? We solve direct numerical simulations of the MHD equations including the transverse stretching exerted by the solar wind flow and study the genuine 3D anisotropy of turbulence as well as that one resulting from the assumption of axisymmetry about B0. We show that the evolution of the turbulent spectrum from 0.2 to 1 AU depends strongly on its initial anisotropy. An axisymmetric spectrum with respect to B0 keeps its axisymmetry, i.e., resists stretching perpendicular to radial, while an isotropic spectrum becomes essentially axisymmetric with respect to the radial direction. We conclude that close to the Sun, slow-wind turbulence has a spectrum that is axisymmetric around B0 and the measured 2D component at 1 AU describes the real shape of turbulent structures. On the contrary, fast-wind turbulence has a more isotropic spectrum at the source and becomes radially symmetric at 1 AU. Such structure is hidden by the symmetrization applied to the data that instead returns a slab geometry.
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