7.1 Observations

The Sun’s supergranulation is a large-scale coherent pattern detected in the surface layers of the quiet Sun. The impression given by observations is that it is simply superimposed on a stochastic, highly nonlinear background smaller-scale flow pattern, the granulation. Characterising the supergranulation velocity pattern requires monitoring solar surface flows over long times, over wide fields of views, or over a large set of independent observations. The properties of the supergranulation velocity field can be summarised as follows.

Note that the foregoing determinations are not independent of each other, because velocity scales can be derived from the combination of length and time scales. Namely, 30 Mm divided by 1.7 d gives 205 m s–1, which is in reasonable agreement with direct measurements of supergranulation-scale velocities. In our view, the computation of the power spectra of solar surface flows provides one of the most robust methods to make progress on the determination of these various quantities in the future. Most notably, an accurate determination of the vertical velocity spectrum of vertical velocities in the supergranulation range is still lacking.

Besides this set of typical scales associated with the supergranulation velocity pattern, several other observational signatures and properties of supergranulation have been studied.

The bounds on intensity variations seem to be well established now. The proper rotation of supergranules, as measured by their local mean vertical vorticity, is also well constrained by local helioseismology. On the other hand, we believe that more work is required to quantitatively constrain the interaction of supergranules with magnetic fields. A determination of the magnetic energy spectrum of the quiet Sun over a wide range of scales would be extremely useful to put constraints on the physical processes at the origin of the network and internetwork fields and on their interactions with supergranulation (see Section 8 below).

Figure 14View Image is an attempt to depict the standard view of the supergranulation phenomenon, as constrained by the observations summarised above.

View Image

Figure 14: A schematic view of the supergranulation phenomenon, as constrained by observations. λ is the scale where the horizontal kinetic energy spectral density is maximum. d is the diameter of “coherent structures” (supergranules). The red and blue patches depict the warm and cold regions of the flow. I.N.B denotes the internetwork magnetic field. Note that the indicated internetwork and network fields geometries roughly correspond to the standard historical picture of quiet Sun magnetic fields and their relation to supergranulation (Section 4.6). As discussed in Sections 4.6.2 and 8, this picture must be significantly nuanced in reality, as the dichotomy between network and internetwork fields is probably not quite as clear as indicated in this drawing.

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