Supergranulation and multiscale flows in the solar photosphere: Global observations vs. a theory of anisotropic turbulent convection
(Sep 19, 2016)Abstract
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
scaling 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. The theory 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.