The Sun's Global Flows: Differential Rotation

As a subgroup of the COFFIES DRIVE center funded by NASA, we have developed a comprehensive catalog of the variable differential rotation and meridional flow patterns measured near the solar photosphere. This catalog includes measurements of these flows as obtained by several techniques: direct Doppler, granule tracking, magnetic pattern tracking, magnetic feature tracking, as well as both time distance and ring diagram methods of local and global helioseismology. Here, we show a comparison of the differential rotation across these varied techniques for the MDI-HMI-GONG-Mount Wilson overlap period (May-July 2010). Counter-intuitively, near-surface measurements of flows measured using magnetic pattern tracking and sunspot tracking lead to flow measurements from deeper inside the solar convection zone. We calibrate the depth of magnetic pattern tracking to be about 25 Mm by comparing the rotation rate with local and global helioseismic techniques and plan to do the same for sunspot tracking rate as well. We investigate the differences between velocities obtained from different techniques and attempt to explain any discrepancies e.g., due to systematic effects associated with the different measurement techniques. Finally, we show many available measurements of the torsional oscillation from sunspot cycles 23 and 24 and discuss the differences within. This analysis will pave the way toward accurately determining the global patterns of flows and their regular and irregular variations during the solar magnetic cycle.