3.2 Mesogranulation

Mesogranulation refers to flows at scales between the granulation and the supergranulation scales. It was first reported by November et al. (1981Jump To The Next Citation Point), who identified a pattern of prominent vertical motions at scales of the order of 8 Mm by time-averaging Doppler images. This was thought as a significant finding because it seemed to provide the missing piece in the theory of multiscale convection at the solar surface (see Section 5) formulated by Simon and Leighton (1964Jump To The Next Citation Point): granulation was associated with the ionisation of Hydrogen, while mesogranulation and supergranulation were associated to the first and second ionisation of Helium, respectively (viz. November et al., 1981Jump To The Next Citation Point). Subsequent ground-based observations (November and Simon, 1988Jump To The Next Citation Point) and space-based observations using the SOUP instrument on SpaceLab 2 (Title et al., 1989) gave some extra weight to this result.

However, the very existence of mesogranulation as a specific distinguishable convection scale at the solar surface remains a source of debate in the solar community. Wang (1989), Chou et al. (1991), and Straus et al. (1992) did not find any local maximum in the power spectrum (the scale-by-scale distribution of energy) of solar convection that would correspond to mesogranulation. Ginet and Simon (1992) and Chou et al. (1992) came to an opposite conclusion. Power spectra computed from MDI observations by Hathaway et al. (2000Jump To The Next Citation Point) only revealed two peaks at granulation and supergranulation scales.

November (1994Jump To The Next Citation Point) wrote that the term “mesogranulation” was misleading and instead suggested to interpret this feature as “the vertical component of the supergranular convection”. Then, Straus and Bonaccini (1997) argued that mesogranulation was a mere powerful extension of granulation at large scales and Roudier et al. (1999b) and Rieutord et al. (2000Jump To The Next Citation Point) suggested that mesogranulation was likely an artefact produced by the correlation tracking algorithm. This view was disputed by Shine et al. (2000) because they found mesogranules in the range of 4 – 7 Mm, with a lifetime of 3 to 6 h (they used local correlation tracking on a 45 h MDI record of wide field images). Soon after, Lawrence et al. (2001), using a new technique based on wavelets applied to MDI images, found what they called a mesogranulation peak, but at 4 Mm, somewhat shorter compared to previous values.

The very recent work of Matloch et al. (2009Jump To The Next Citation Point) and Rieutord et al. (2010Jump To The Next Citation Point) may finally bring this debate to a conclusion. Indeed, Matloch et al. (2009) devised a simple model of granulation which mimics very well the fusion and splitting of granules. A conclusion of this work is that the statistical properties and behaviour of mesogranulation structures are consistent with the results of spatial and temporal averaging of random data. This conclusion underlines the fact that previous detection of mesogranulation were very likely mislead by the weird consequences of averaging procedures. On the other hand, using Doppler measurements of vertical velocities from Hinode/SOT, Rieutord et al. (2010Jump To The Next Citation Point) did not find any spectral signature of a distinguishable scale in between granulation and supergranulation.

To conclude, it is very likely that mesogranulation is a ghost feature of surface convection generated by averaging procedures. In our opinion, the most recent observational results strongly argue against the existence of a genuine surface feature similar to granulation or supergranulation. To avoid any misunderstanding, we shall hereafter refer to the scales in the range of 4 to 12 Mm as the mesoscales. These length scales are between the smallest scale of supergranulation (12 Mm, see Section 4.2 below) and the largest scale of granulation (4 Mm).


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