Volume 599, March 2017
|Number of page(s)||10|
|Published online||02 March 2017|
Supergranulation and multiscale flows in the solar photosphere
Global observations vs. a theory of anisotropic turbulent convection
1 Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
2 CNRS; IRAP; 14 avenue Édouard Belin, 31400 Toulouse, France
3 The Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, Oxford, OX1 3NP, UK
4 Merton College, Oxford OX1 4JD, UK
Received: 19 September 2016
Accepted: 24 December 2016
The Sun provides us with the only spatially well-resolved astrophysical example of turbulent thermal convection. While various aspects of solar photospheric turbulence, such as granulation (one-Megameter horizontal scale), are well understood, the questions of the physical origin and dynamical organization of larger-scale flows, such as the 30-Megameters supergranulation and flows deep in the solar convection zone, remain largely open in spite of their importance for solar dynamics and magnetism. Here, we present a new critical global observational characterization of multiscale photospheric flows and subsequently formulate an anisotropic extension of the Bolgiano-Obukhov theory of hydrodynamic stratified turbulence that may explain several of their distinctive dynamical properties. Our combined analysis suggests that photospheric flows in the horizontal range of scales between supergranulation and granulation have a typical vertical correlation scale of 2.5 to 4 Megameters and operate in a strongly anisotropic, self-similar, nonlinear, buoyant dynamical regime. While the theory remains speculative at this stage, it lends itself to quantitative comparisons with future high-resolution acoustic tomography of subsurface layers and advanced numerical models. Such a validation exercise may also lead to new insights into the asymptotic dynamical regimes in which other, unresolved turbulent anisotropic astrophysical fluid systems supporting waves or instabilities operate.
Key words: Sun: photosphere / Sun: interior / convection / turbulence / instabilities
© ESO, 2017
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