The aim of these proposed observations is to explore the hemispheric dependence (or lack of) in vorticity of intergranular flows.
Many details of turbulent dynamo operating at/near photosphere remain largely unknown. One of these critical details is helicity of turbulent dynamo flows and the corresponding helicity of magnetic fields generated by these flows. In this observing investigation, we propose to study the vorticity and (inferred) helicity of flows employed by the surface turbulent dynamo. Pevtsov and Longcope (2001, 2007) had proposed that helicity can be used as an ultimate test for turbulent dynamo action, i.e. if the surface (intergranular) magnetic fields are generated by surface dynamo, no hemispheric preference in sign of helicity should be observed. However, if the surface dynamo merely recycles magnetic fields generated by subphotospheric helical dynamo, one should still see a weak hemispheric sign-preference in helicity of these fields. Pevtsov’s and Longcope’s conclusions are based on the Rossby number argument: Ro = ν/2LΩ, where v and L are a typical velocity and a typical length of a process and Ω is angular velocity of rotation. In the upper convection zone (at/near photosphere), where Ro>>1, the Coriolis force will not significantly influence the turbulence, and the sign of votrical flows in and outside granules will be determined by random interaction of granular cells. When averaged over large area, one should see no hemispheric preference in sign of helicity of these flows. A weak hemispheric preference for sign of vorticity can indicate that for example flux concenrations survive long enough to be affected by solar rotation (Coriolis force), or there are other mechanisms injecting preferred sign of vorticity for a given hemisphere (e.g. latitudinal gradient of the differential rotation).
To verify and explore these predictions, one would normally require simultaneous observations of photospheric (granular) flows and vector magnetograms. However, since no such observations are currently available, we propose using movies of granular flows taken in G-band as proxy to infer both vorticity of flows and helicity of magnetic flux concentrations. Our preliminary analysis of several existing G-band movies shows the presence of rotational motions of G-band bright features situated at the boundaries of granular cells. From visual, or feature tracking analysis we are able to determine sense of rotation (clockwise or counterclockwise). Since G-band features of interest are located at the boundary of granular cells, it is reasonably to assume that they are located at areas of downflows. Combining this reasonable assumption with derived sense of rotation, one can infer sign of kinetic helicity in such areas.
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