Reconciling Subsurface Flow Measurements from GONG, HMI and MDI

The subsurface zonal and meridional flows play important roles in the operation of differential rotation and solar dynamo. There is a rich history of these measurements using different local helioseismic techniques and different instruments. However, investigating variations over longer time scales is a challenge as different instruments provide different flow measurements and require scaling adjustments between different instruments. The existence of simultaneous measurements from three instruments, GONG, HMI and MDI, for about two months provide a unique opportunity to reconcile the subsurface flows and to establish a scaling factor between them. With this objective, we analyze full-disk Dopplergrams from all the three different instruments which were observing the Sun simultaneously during May 7-July 12, 2010. It may be noted that during this period, MDI also operated in high-resolution mode and thus the data can be used for local helioseismic studies. In order to minimize the biases, we compute subsurface flows from all three instruments using the GONG ring-diagram pipeline and verify the consistency of both zonal and meridional flow measurements to a depth of about 16 Mm below the surface.