The Vorticity of Solar Photospheric Flows on the Scale of Granulation

TL;DR

This study investigates the vorticity and helicity of granular flows in the solar photosphere, finding weak hemispheric preferences and suggesting a non-helical photospheric dynamo at granular scales.

Contribution

It provides the first analysis of vorticity and helicity of granular flows using Hinode data, revealing weak hemispheric preferences and implications for the solar dynamo.

Findings

Weak hemispheric preference in kinetic helicity of granular flows

No hemispheric preference in magnetic field helicity density

Photospheric dynamo likely operates in a non-helical manner

Abstract

We employ time sequences of images observed with a G-band filter (4305{\AA}) by the Solar Optical Telescope (SOT) on board of Hinode spacecraft at different latitude along solar central me-ridian to study vorticity of granular flows in quiet Sun areas during deep minimum of solar activity. Using a feature correlation tracking (FCT) technique, we calculate the vorticity of granular-scale flows. Assuming the known pattern of vertical flows (upward in granules and downward in inter-granular lanes), we infer the sign of kinetic helicity of these flows. We show that the kinetic helicity of granular flows and intergranular vortices exhibits a weak hemispheric preference, which is in agreement with the action of the Coriolis force. This slight hemispheric sign asymmetry, however, is not statistically significant given large scatter in the average vorticity. The sign of the current he-licity density of network magnetic fields computed using full disk vector magnetograms from the Synoptic Optical Long-term Investigations of the Sun (SOLIS) does not show any hemispheric preference. The combination of these two findings suggests that the photospheric dynamo operating on the scale of granular flows is non-helical in nature.