Characterizing the spatial pattern of solar supergranulation using the bispectrum
arXiv:2002.08262 · doi:10.1051/0004-6361/201937331
Abstract
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.
arXiv comment16 pages, 12 figures, accepted for publication by A&A
References in corpus (17)
- Acoustic Signatures in the Primary Microwave Background Bispectrum
- The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence
- Planck 2015 results. XVII. Constraints on primordial non-Gaussianity
- Cascades and transitions in turbulent flows
- Anomalously Weak Solar Convection
- Solar supergranulation as a wavelike phenomenon
- N-body simulations with generic non-Gaussian initial conditions I: Power Spectrum and halo mass function
- The Emergence of Solar Supergranulation as a Natural Consequence of Rotationally-Constrained Interior Convection
- A fast estimator for the bispectrum and beyond - A practical method for measuring non-Gaussianity in 21-cm maps
- Spatially resolved vertical vorticity in solar supergranulation using helioseismology and local correlation tracking
- Universality of the Small-Scale Dynamo Mechanism
- Supergranulation as the largest buoyantly driven convective scale of the Sun
- Supergranulation and multiscale flows in the solar photosphere: Global observations vs. a theory of anisotropic turbulent convection
- Generating non-Gaussian maps with a given power spectrum and bispectrum
- Additional Evidence Supporting a Model of Shallow, High-Speed Supergranulation
- Evolution and wave-like properties of the average solar supergranule
- Magnetic energy cascade in spherical geometry: I. The stellar convective dynamo case