The Sun’s supergranulation

Abstract

Supergranulation is a fluid-dynamical phenomenon taking place in the solar photosphere, primarily detected in the form of a vigorous cellular flow pattern with a typical horizontal scale of approximately 30-35 megameters, a dynamical evolution time of 24-48 h, a strong 300-400 m/s (rms) horizontal flow component and a much weaker 20-30 m/s vertical component. Supergranulation was discovered more than sixty years ago, however, explaining its physical origin and most important observational characteristics has proven extremely challenging ever since, as a result of the intrinsic multiscale, nonlinear dynamical complexity of the problem concurring with strong observational and computational limitations. Key progress on this problem is now taking place with the advent of 21st-century supercomputing resources and the availability of global observations of the dynamics of the solar surface with high spatial and temporal resolutions. This article provides an exhaustive review of observational, numerical and theoretical research on supergranulation, and discusses the current status of our understanding of its origin and dynamics, most importantly in terms of large-scale nonlinear thermal convection, in the light of a selection of recent findings. Keywords Supergranulation · Convection · Turbulence · MHD 4 François Rincon, Michel Rieutord 6 François Rincon, Michel Rieutord

Supergranular turbulence in the quiet Sun: Lagrangian coherent structures

“…The photosphere is permeated by convection cells, called granules, with a typical spatial scale of ∼1 Mm and a lifetime of a few minutes. On scales larger than granules, there is supergranulation, which is a complex nonlinear dynamical phenomenon taking place in the solar photosphere that exhibits a cellular flow pattern with a typical horizontal scale of ∼30-35 Mm, a lifetime of ∼24-48 hr, a strong horizontal flow velocity of ∼300-400 m/s and a weaker vertical velocity of ∼20-30 m/s (Rincon & Rieutord 2018). The origin of supergranulation can be attributed to thermal magnetoconvection such as magnetised Raleigh-Bénard convection or large-scale instabilities, inverse cascades and collective interactions.…”

Stellar Surface Magnetoconvection as a Source of Astrophysical Noise. III. Sun-as-a-Star Simulations and Optimal Noise Diagnostics

“…Note today, that this often described by a linear combination of Lorentzian and super-Lorentzian functions (as done in Paper II); at least two components are necessary to adequately fit the background signal from the granulation phenomena in empirical power spectra, though we highlight that the physical origin for a component on the mesoscale (not discussed here) remains an open question (e.g. see Matloch et al 2010;Kallinger et al 2014;Corsaro et al 2017;Rincon & Rieutord 2018;Kessar et al 2019, and references therein). Such an approach provides a characteristic timescale, τ , and a corresponding disc-integrated rms for each phenomenon, σ.…”

Structure of the Solar Atmosphere: A Radio Perspective

“…From the above discussion we expect plasma motion to dominate in the QS photosphere and drag the magnetic field lines (for an overview of solar magnetic fields see Wiegelmann et al, 2014 andBellot Rubio andOrozco Suárez, 2019). Indeed, the most prominent photospheric structure is the granulation, with a spatial scale of ∼1.5 ′′ , and a temporal scale of ∼15 min, which is attributed to convection currents (see the classic work by Bray and Loughhead, 1967 for a historic account and the reviews by Nordlund et al, 2009 andRincon andRieutord, 2018). As can be seen in Figure 5, the intensity and velocity images of the photosphere at the center of the solar disk are practically identical, which proves that hot material in the bright granules ascends, while cooler material in the dark inter-granular lanes descends; the same effect produces the zigzag appearance of photospheric absorption lines (see, e.g., Figure 6.20 in Zirin, 1988).…”

“…In the older approach, the three scales of convection would be associated with the ionization zones of HI, HeI, and HeII. However, the existence of mesogranulation as distinct scale of convection has been contested; views have been expressed that it is an extension of granulation or even an artifact produced by the correlation tracking algorithm (see discussion in Nordlund et al, 2009;Rincon and Rieutord, 2018).…”