Vorticity and Divergence in the Solar Photosphere

We have studied an outstanding sequence of continuum images of the solar granulation from Pic du MidiObservatory. We have calculated the horizontal vector flow field using a correlation tracking algorithm, andfrom this determined three scalar fields: the vertical component of the curl, the horizontal divergence, and thehorizontal flow speed. The divergence field has substantially longer coherence time and more power than doesthe curl field. Statistically, curl is better correlated with regions of negative divergence—that is, the verticalvorticity is higher in downflow regions, suggesting excess vorticity in intergranular lanes. The average value ofthe divergence is largest (i.e., outflow is largest) where the horizontal speed is large; we associate these regionswith exploding granules. A numerical simulation of general convection also shows similar statistical differencesbetween curl and divergence. Some individual small bright points in the granulation pattern show large localvorticities.Subject headings: hydrodynamics—Sun: atmosphere — Sun: granulation