利用双谱表征太阳超颗粒的空间分布
Characterizing the spatial pattern of solar supergranulation using the bispectrum
摘要：
上下文超颗粒的空间功率谱并不能完全表征湍流对流的基本物理特性。例如，它没有描述水平流发散中的非高斯性。目标。我们的目标是统计特征的空间格局的太阳超颗粒超出功率谱。下一阶统计量是双谱。它测量三个傅立叶分量的相关性，并与基础物理中的非线性有关。方法.我们估计的双谱的超颗粒水平表面的发散图，获得使用本地相关跟踪（LCT）和时间-距离日震学（TD）从一年的数据从日震和磁成像仪上的太阳动力学天文台从2010年5月开始。结果我们发现显着的非零和一致的估计的双谱。最强的非线性是目前的三个耦合波矢在超颗粒尺度。这些波矢量与正六边形中的波矢量相同，正六边形用于太阳对流的分析研究。在这些傅立叶分量，双谱是积极的，符合数据中的正偏度和超颗粒优先组成的流出包围的流入网络。我们使用的双谱生成合成发散图是非常相似的数据由一个模型，由一个高斯项和一个较弱的二次非线性分量。因此，我们估计的分数的方差在发散图的非线性组件的顺序为4- 6%。结论.我们建议，双谱分析是有用的理解太阳湍流对流，例如比较观测和数值模型的超颗粒流。该分析也可用于生成合成流场。
英文摘要：
Context. The spatial power spectrum of supergranulation does not fully characterize the underlying physics of turbulent convection. For example, it does not describe the non-Gaussianity in the horizontal flow divergence. Aims. Our aim is to statistically characterize the spatial pattern of solar supergranulation beyond the power spectrum. The next-order statistic is the bispectrum. It measures correlations of three Fourier components and is related to the nonlinearities in the underlying physics. Methods. We estimated the bispectrum of supergranular horizontal surface divergence maps that were obtained using local correlation tracking (LCT) and time-distance helioseismology (TD) from one year of data from the Helioseismic and Magnetic Imager on-board the Solar Dynamics Observatory starting in May 2010. Results. We find significantly nonzero and consistent estimates for the bispectrum. The strongest nonlinearity is present when the three coupling wave vectors are at the supergranular scale. These are the same wave vectors that are present in regular hexagons, which were used in analytical studies of solar convection. At these Fourier components, the bispectrum is positive, consistent with the positive skewness in the data and with supergranules preferentially consisting of outflows surrounded by a network of inflows. We use the bispectrum to generate synthetic divergence maps that are very similar to the data by a model that consists of a Gaussian term and a weaker quadratic nonlinear component. Thereby, we estimate the fraction of the variance in the divergence maps from the nonlinear component to be of the order of 4-6%. Conclusions. We propose that bispectral analysis is useful for understanding solar turbulent convection, for example for comparing observations and numerical models of supergranular flows. This analysis may also be useful to generate synthetic flow fields.
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