4.3 Intensity variations in supergranules

Next, one may wonder if supergranulation-scale motions are associated with any temperature or intensity fluctuations, as this information may give an important clue to understand the origin of supergranulation. As shown by Worden (1975), the thermal signature of supergranulation, if any, must be very faint. The intensity contrast between the border and the centre of supergranules probably does not exceed a few percents, at least in the infrared. For comparison, rms intensity variations at the granulation scale have been shown to be up to 27% by Wedemeyer-Böhm and Rouppe van der Voort (2009) using the recent data of Hinode. Several studies (Beckers, 1968Frazier, 1970Jump To The Next Citation PointFoukal and Fowler, 1984Lin and Kuhn, 1992) found an increase of intensity at the edge of supergranulation cells, corresponding to a negative correlation between the supergranulation horizontal divergence maps and intensity maps. These results, which tend to rule out a convective origin for supergranulation, are however subject to caution because supergranulation vertices are strongly correlated with magnetic bright points (see Section 4.6 below). To circumvent this difficulty, Rast (2003a) considered only areas with low magnetic fields and found a small decrease of intensity at the edge of supergranules. The problem was reconsidered in detail by Meunier et al. (2007bJump To The Next Citation Point2008Jump To The Next Citation Point) using MDI intensity maps. There too, the influence of the magnetic network was carefully eliminated. In contrast to most previous studies, they report a very small but significant intensity decrease from the centre to the edge of supergranulation cells (in the range 0.8 – 2.8 K). Goldbaum et al. (2009Jump To The Next Citation Point) recently came to the same conclusion using a different methodology. In addition, Meunier et al. (2008Jump To The Next Citation Point) noticed that the radial temperature profile at the surface of a supergranule is very similar to that of a simulated granule. These observations are consistent with a driving of supergranulation by buoyancy.
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