Signatures of solar cycle 25 in subsurface zonal flows

The pattern of migrating zonal flow bands associated with the solar cycle, known as the torsional oscillation, has been monitored with continuous global helioseismic observations by the Global Oscillations Network Group (GONG), together with those made by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) and its successor, the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO), since 1995, giving us nearly two full solar cycles of observations.

We report that the flows now show traces of the mid-latitude acceleration that is expected to become the main equatorwardmoving branch of the zonal flow pattern for Cycle 25. Based on the current position of this branch, we speculate that the onset of widespread activity for Cycle 25 is unlikely to be earlier than the middle of 2019.