The cause of the sunspot cycle remains as one of the major mysteries of nature. While the material motions of the electrically charged particles of the solar plasma generate the basic magnetic field, just how this becomes organized into a quasi-periodic phenomenon has resisted satisfactory explanation. Since sunspots are at the root of the space weather that is an increasing concern as humanity becomes more dependent on technology, it is of considerable societal as well as scientific importance to understand the dynamo mechanism.

All dynamo theories incorporate some concept of the bulk large-scale motions of the plasma inside the Sun. Prior to the advent of helioseismology; these concepts were naturally restricted to extrapolations of the motions as seen in the photosphere, guided by fluid dynamic laws. For example, the internal rotation of the Sun was thought to be “constant on cylinders” throughout the entire Sun, a logical conclusion given the surface differential rotation and the Taylor-Proudman theorem. Helioseismology showed that this was incorrect and that the surface differential rotation was actually almost constant along radii, turning into solid-body rotation at the base of the convection zone. This forced a revision to virtually all dynamo theories of the mid 1980s.

With the modern helioseismology observing programs from SOHO, SDO, and GONG, we have been able to measure flows other than rotation that are related to the dynamo. The most important of these are the meridional flows in the North-South direction, and the zonal flows that are oriented East-West. The meridional flows and their temporal variation could play an important role in the timing and strength of the solar cycle if modern flux-transport dynamo models are correct. The zonal flows, also known as the torsional oscillation, are apparently good indicators of the timing of the cycle and may be useful for long-term forecasts. This colloquium will present the latest helioseismic observations of the meridional and zonal flows. Particular emphasis will be placed on the nature of deep meridional flows, and the implications of the zonal flow observations for the behavior of the future Cycle 25.