Traditionally, larger scale flows at the solar surface have been classified into mesogranulation, with horizontal scales of the order of 5 – 10 Mm, supergranulation, with horizontal scales of the order of 20 – 50 Mm, and giant cells, with horizontal scales of the order of 100 Mm or larger. However, as discussed in more detail below, this distinction is largely of historical origin, and current evidence indicates that there is a continuous spectrum of motions, on all scales from global to sub-granular.
The larger flow scales are related to, and unavoidable consequences of, the larger scale convective flows that exist in the subsurface layers of the solar convection zone. Since the scales of these subsurface flows increase continuously with depth it is also hard to imagine a physical reason that would cause distinct flow scales at the solar surface. Already the fact that there is only a factor 2 gap between the scales traditionally assigned to granulation, mesogranulation and supergranulation makes it very difficult to imagine a true scale separation.
Below, we first summarize the observational evidence categorized by the traditional flow scale labels, ‘mesogranulation’, ‘supergranulation’, and ‘giant cells’. We then consider multi-scale observations and show that unbiased multi-scale observations, as are available, e.g., from SOHO/MDI, show a smoothly increasing spectrum of velocity amplitudes with increasing wave number (decreasing size), incorporating in a continuous manner the scales with the traditional labels.
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