The so-called torsional oscillation is a pattern of migrating zonal flows in the solar convection zone, associated with the solar cycle. Because the flows change in advance of large-scale surface magnetic activity, we can use them to anticapate the evolution of the solar cycle. We study these flows using global and local helioseismology with 22 years of data from the Global Oscillation Network Group, the Michelson Doppler Imager onboard the Solar and Heliospheric Observatory and the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory.
At present, the flows that will be associated with Solar Cycle 25 are becoming apparent at about 35 degrees latitude. The flows are weaker than was seen in previous cycles, but this does not necessarily mean that Cycle 25 will be weak. The position of the new flow bands corresponds to that seen in 1995 and 2006, which suggests that we are within a couple of years of solar minimum and that the new cycle activity might emerge as early as 2019.