3.1 Granulation

The solar granulation is an intensity pattern with a contrast around 15%, which displays cellular convective motions with length scales ranging from ∼ 0.5 Mm to 2 Mm. From recent observations with the Solar Optical Telescope on board the Hinode satellite (Ichimoto et al., 2004Suematsu et al., 2008), Rieutord et al. (2010Jump To The Next Citation Point) showed that power spectra of both intensity fluctuations and vertical velocities have a maximum amplitude at a scale around 1.7 Mm, with a weak dependence on photospheric height for Vz. Previous work had shown that the typical lifetime of granules is 10 min and that the associated velocities range from 0.5 to 1.5 km s–1(Title et al., 1989Jump To The Next Citation Point). Hundreds of observations of the solar granulation have been done, and several reviews are dedicated to this subject. We notably refer the reader to Spruit et al. (1990Jump To The Next Citation Point) and Nordlund et al. (2009Jump To The Next Citation Point).

The granulation pattern is certainly the best understood feature of solar convection. Most notably, it is well reproduced by numerical simulations (Section 6). A remarkable property is that the advection of heat by the velocity field and the radiation of heat proceed on comparable timescales in a granule, so the corresponding Péclet number is order unity. As already discussed in Section 2.2, this means that the scale of granulation is comparable to the thermal dissipation scale. Physically, the granulation pattern corresponds to a thermal boundary layer formed in the strongly non-adiabatic surface region of the SCZ where the solar plasma becomes optically thin (see Figure 1View Image).

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