Velocities and divergences as a function of supergranule size

Context: The origin of supergranulation is not understood yet and many scenarios, which range from large-scale deep convection to large-scale instabilities of surface granular flows, are possible. Aims: We characterize the velocities and divergences in supergranulation cells as a function of their size. Methods: Using local correlation tracking, we determine the horizontal flow fields from MDI intensity maps and derive the divergences. The smoothed divergences are used to determine the cells for various spatial smoothings, in particular at the supergranular scale. Results: We find evidence of intermittency in the supergranular range and a correlation between the size of supergranules and the strength of the diverging flow. We also show that the relation between rms velocities and scale (the supergranule radius R) can be represented by a law V_rms? R0.66. Conclusions: . The results issued from our data point towards a scenario where supergranulation is a surface phenomenon of the sun, probably the consequence of a large-scale instability triggered by strong positive correlated rising flows