Helioseismic Measurements of the Torsional Oscillation

The zonal flows known as the torsional oscillation have by now been observed for more than 15 years using observations from the Michelson Doppler Imager (MDI) and Global Oscillation Network Group (GONG). Even with this limited set of data it is clear that there are significant differences between the cycles, as also evidenced by the prolonged recent solar minimum. Here we discuss some of these differences and their significance. In particular we discuss the slower development of the current solar cycle and the lack of a polar branch in the torsional oscillation. We will also compare results from MDI and the Helioseismic Magnetic Imager (HMI) and discuss the prospects for generating longer consistent sets of observations.