Time Dependence of Solar Surface High-Latitude Meridional Flows

A recent re-analysis of the digital archive record of Magnetogramand Dopplergram observations of the synoptic program at the 150-foot solartower telescope at the Mt. Wilson Observatory (MWO) has shown that theprocesses known as meridional circulation and torsional oscillations are closelyrelated (Ulrich, Tran, and Boyden 2023). The meridional circulation velocity isalong lines of constant heliographic longitudes while the torsional oscillations arechanges in the rotation rate. Previously an extension of the meridional circulationanalysis by Ulrich (2010) revealed that the high latitude rotation slowdown periodsof the torsional oscillations also coincide with the high latitude meridionalcirculation flow being toward the equator instead of the usual toward the polepattern. We interpret these two high latitude velocity patterns as indicating thatthe material flow during the solar minimum periods has a polar upwelling thatpushes matter toward the equator. It is generally believed that the polar regionmagnetic fields interact with the meridional flow in the following sequence: a)at or before sunspot minimum the meridional flow brings additional magneticflux of the same polar sign to the polar region increasing the high latitude fieldstrength; b) after solar minimum the new cycle field reverses and opposes thepolar field, reversing it. The polar upwelling is not part of this sequence and ourobservations could indicate the need for modification of the models. This paperprovides details about the time dependence of the solar surface high latitudemeridional circulation as well as rotation rates.