4.1 Mesogranulation

Mesogranulation was first detected by Larry November and coworkers (November, 1980November et al., 19811982), and has since been the subject of a large number of observational papers; (e.g., Oda, 1984Simon et al., 1988Brandt et al., 1991Muller et al., 1992Jump To The Next Citation PointRoudier et al., 1998Shine et al., 2000Jump To The Next Citation PointRoudier and Muller, 2004Jump To The Next Citation PointLeitzinger et al., 2005Jump To The Next Citation Point). There has also been a heated debate as to the nature of mesogranulation, and about whether mesogranulation is a distinct scale of convection or not; (e.g., Wang, 1989Deubner, 1989Stein and Nordlund, 1989Jump To The Next Citation PointGinet and Simon, 1992Straus et al., 1992Ploner et al., 2000Cattaneo et al., 2001Rast, 2003). A recent example of the techniques commonly used to observe mesogranulation is Leitzinger et al. (2005), who used correlation tracking and a running (1 h) mean in time to map the horizontal component of the mesogranulation velocity field, and to study the drifts of mesogranules in the supergranular velocity field. Others (e.g., Brandt et al., 1988Title et al., 1989DeRosa and Toomre, 1998Jump To The Next Citation PointRoudier et al., 1999Shine et al., 2000Jump To The Next Citation PointDeRosa and Toomre, 2004Jump To The Next Citation Point) have used similar techniques to study flows on scales ranging from meso- to supergranulation. Inherent in these techniques is that, because of the spatial and temporal averaging, smaller spatial scales and shorter time scales cannot be adequately resolved. In a situation where the velocity amplitudes increase with decreasing motion scale, such a cut-off automatically (but incorrectly) brings out the smallest scale that is adequately resolved as a dominant, distinct scale.

Another inherent limitation of the tracking techniques is the assumption that the granule-scale fragments and features used in the tracking process are actually good tracers of the solar surface velocity field. This is not necessarily the case, since granules are active flow features, whose horizontal motions presumably also sample the conditions over a range of depths. Rieutord et al. (2001) compared the velocity field derived from tracking granule motions in numerical simulations to the actual meso-scale velocity field of the plasma in the simulations. They conclude that neither velocity fields at scales less than 2500 km nor time evolution at scales shorter than 30 min can be faithfully measured by granule tracking, but that at larger scales and over longer time scales granule tracking works well.

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