Abstract
Flow vorticity is a fundamental property of turbulent convection in rotating systems. Solar supergranules exhibit a preferred sense of rotation, which depends on the hemisphere. This is due to the Coriolis force acting on the diverging horizontal flows. We aim to spatially resolve the vertical flow vorticity of the average supergranule at different latitudes, both for outflow and inflow regions. To measure the vertical vorticity, we use two independent techniques: time-distance helioseismology (TD) and local correlation tracking of granules in intensity images (LCT) using data from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Both maps are corrected for center-to-limb systematic errors. We find that 8-h TD and LCT maps of vertical vorticity are highly correlated at large spatial scales. Associated with the average supergranule outflow, we find tangential (vortical) flows that reach about 10 m/s in the clockwise direction at 40{\deg} latitude. In average inflow regions, the tangential flow reaches the same magnitude, but in the anti-clockwise direction. These tangential velocities are much smaller than the radial (diverging) flow component (300 m/s for the average outflow and 200 m/s for the average inflow). The results for TD and LCT as measured from HMI are in excellent agreement for latitudes between $-$60{\deg} and 60{\deg}. From HMI LCT, we measure the vorticity peak of the average supergranule to have a full width at half maximum of about 13 Mm for outflows and 8 Mm for inflows. This is larger than the spatial resolution of the LCT measurements (about 3 Mm). On the other hand, the vorticity peak in outflows is about half the value measured at inflows (e.g. 4/(10^6 s) clockwise compared to 8/(10^6 s) anti-clockwise at 40{\deg} latitude). Results from MDI/SOHO obtained in 2010 are biased compared to the HMI/SDO results for the same period.
Highlights
Duvall & Gizon (2000) and Gizon & Duvall (2003) revealed that supergranules possess a statistically preferred sense of rotation that depends on solar latitude
We have successfully measured the horizontal divergence and vertical vorticity of near-surface flows in the Sun using different techniques (TD and local correlation tracking (LCT)), as well as different instruments (HMI and Michelson Doppler Imager (MDI))
Horizontal divergence maps from time-distance helioseismology (TD) and LCT are in excellent agreement for 8 h averaging
Summary
Duvall & Gizon (2000) and Gizon & Duvall (2003) revealed that supergranules (see Rieutord & Rincon 2010, for a review) possess a statistically preferred sense of rotation that depends on solar latitude. In the northern hemisphere supergranules tend to rotate clockwise, in the southern hemisphere anticlockwise This is due to the Coriolis force acting on the divergent horizontal flows of supergranules. The vorticity induced by the Coriolis force should be measurable by averaging the vorticity of many realizations of supergranules at a particular latitude. Hindman et al (2009) resolved the circular flow component associated with inflows into active regions; the spatial structure of such vortical flows has not yet been studied for many realizations. We aim to spatially resolve the vertical component of flow vorticity associated with the average supergranule. We investigate both outflows from supergranule centers and inflows into the supergranular network. We use the TD method from Langfellner et al (2014), where a measurement geometry that is sensitive to the vertical component of flow vorticity was defined
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