Observational characterisation of large-scale transport and horizontal turbulent diffusivity in the quiet Sun

The Sun is a magnetic star, and the only spatio-temporally resolved astrophysical system displaying turbulent magnetohydrodynamic thermal convection. This makes it a privileged object of study to understand fluid turbulence in extreme regimes and its interactions with magnetic fields. Global analyses of high-resolution solar observations provided by the NASA Solar Dynamics Observatory (SDO) can shed light on the physical processes underlying large-scale emergent phenomena such as the solar dynamo cycle. Combining a coherent structure tracking reconstruction of photospheric flows, based on photometric data, and a statistical analysis of virtual passive tracers trajectories advected by these flows, we characterise one of the most important such processes, turbulent diffusion, over an unprecedentedly long monitoring period of six consecutive days of a significant fraction of the solar disc. We first confirm, and provide a new global view of the emergence of a remarkable dynamical pattern of Lagrangian coherent structures tiling the entire surface. These structures act as transport barriers on the time and spatial scale of supergranulation and, by transiently accumulating particles and magnetic fields, appear to regulate large-scale turbulent surface diffusion. We further statistically characterise the turbulent transport regime using two different methods, resulting in an estimated range D=2-4 m s for the effective long-time horizontal turbulent diffusivity. This estimate is consistent with the transport coefficients required in large-scale mean-field solar dynamo models, and is in broad agreement with the results of global simulations. Beyond the solar dynamo, our analysis may have implications for understanding the structural connections between solar-surface, coronal and solar-wind dynamics, and it also provides valuable lessons to characterise turbulent transport in other, unresolved turbulent astrophysical systems.