宁静太阳中的湍流磁场：Hinode观测和小尺度发电机模拟的含意
Turbulent magnetic fields in the quiet Sun: implications of Hinode observations and small-scale dynamo simulations
摘要：
利用湍流MHD模拟（磁雷诺数高达8000）和日结观测，我们研究了湍流对测量活动区外太阳磁场的影响。首先，从合成斯托克斯V配置文件的FeI线在630.1和630.2 nm，我们表明，一个峰值概率分布函数（PDF）的观测得出的字段估计是一致的单调PDF的实际垂直场强度。因此，弱场的普遍性比从观测中天真地推断出来的要大。其次，我们采用的分形自相似几何的湍流太阳磁场得到两个估计（数值和观测）的真正平均垂直无符号通量密度。我们还发现观测证据表明，在光球中的磁结构的规模至少延伸到小于200公里的数量级：自相似幂律标度在有符号的措施从日出磁图范围（超过二十年的长度尺度，包括颗粒尺度）下降到200公里的分辨率限制。从自相似标度，我们确定了一个下限的真正安静的太阳平均垂直无符号通量密度的~50 G。这与我们基于数值的估计是一致的，即80%或更多的垂直无符号通量应该是不可见的斯托克斯-V观测在200公里的分辨率，由于从相反的磁极信号的取消。我们的估计显着减少了塞曼和汉勒为基础的估计之间的数量级差异。
英文摘要：
Using turbulent MHD simulations (magnetic Reynolds numbers up to 8000) and Hinode observations, we study effects of turbulence on measuring the solar magnetic field outside active regions. Firstly, from synthetic Stokes V profiles for the FeI lines at 630.1 and 630.2 nm, we show that a peaked probability distribution function (PDF) for observationally-derived field estimates is consistent with a monotonic PDF for actual vertical field strengths. Hence, the prevalence of weak fields is greater than would be naively inferred from observations. Secondly, we employ the fractal self-similar geometry of the turbulent solar magnetic field to derive two estimates (numerical and observational) of the true mean vertical unsigned flux density. We also find observational evidence that the scales of magnetic structuring in the photosphere extend at least down to an order of magnitude smaller than 200 km: the self-similar power-law scaling in the signed measure from a Hinode magnetogram ranges (over two decades in length scales and including the granulation scale) down to the 200 km resolution limit. From the self-similar scaling, we determine a lower bound for the true quiet-Sun mean vertical unsigned flux density of ~50 G. This is consistent with our numerically-based estimates that 80% or more of the vertical unsigned flux should be invisible to Stokes-V observations at a resolution of 200 km owing to the cancellation of signal from opposite magnetic polarities. Our estimates significantly reduce the order-of-magnitude discrepancy between Zeeman- and Hanle-based estimates.
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