The Influence of Tracking Rate on Helioseismic Flow Inferences

Traditionally, most local helioseismic studies of subsurface flows have removed the large signal due to the Sun s rotation by tracking the analysis region across the solar surface. In order to work in a uniformly rotating reference frame, the ring-analysis pipeline of the recently launched Helioseismic and Magnetic Imager (HMI) will track all analysis regions at the solid-body Carrington rate. To test this tracking scheme, we compare flow determinations resulting from two different tracking schemes. In one scheme we use the HMI pipeline implementation which tracks at the Carrington rotation rate. In the other, the tiles are tracked at the local differential surface rotation rate as measured by Snodgrass (1984). We observe systematic differences between the flows obtained by the two schemes even after transforming them to a common frame (Snodgrass frame), with the zonal flows measured in the Carrington frame being faster by 5-20 m/s.