Subsurface Meridional Flow from HMI Using the Ring-Diagram Pipeline

Abstract

We have determined the meridional flows in subsurface layers for 18 Carrington rotations (CR 2097 to 2114) analyzing high-resolution Dopplergrams obtained with the Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO). We are especially interested in flows at high latitudes up to 75∘ in order to address the question whether the meridional flow remains poleward or reverses direction (so-called counter cells). The flows have been determined in depth from near-surface layers to about 16 Mm using the HMI ring-diagram pipeline. The measured meridional flows show systematic effects, such as a variation with the B 0-angle and a variation with central meridian distance (CMD). These variations have been taken into account to lead to more reliable flow estimates at high latitudes. The corrected average meridional flow is poleward at most depths and latitudes with a maximum amplitude of about \(20~\mathrm{m\,s}^{-1}\) near 37.5∘ latitude. The flows are more poleward on the equatorward side of the mean latitude of magnetic activity at 22∘ and less poleward on the poleward side, which can be interpreted as convergent flows near the mean latitude of activity. The corrected meridional flow is poleward at all depths within ± 67.5∘ latitude. The corrected flow is equatorward only at 75∘ latitude in the southern hemisphere at depths between about 4 and 8 Mm and at 75∘ latitude in the northern hemisphere only when the B 0 angle is barely large enough to measure flows at this latitude. These counter cells are most likely the remains of an insufficiently corrected B 0-angle variation and not of solar origin. Flow measurements and B 0-angle corrections are difficult at the highest latitude because these flows are only determined during limited periods when the B 0 angle is sufficiently large.