Observational Constraints on Solar Dynamo Models: Helioseismic Inferences and Magnetic Properties

For many years the solar dynamo could only be constrained by observations at and above the visible layers. Some observations thought to be important include the variation of the number of sunspots with the 11-year cycle, the magnetic polarity of the sunspots, the systematic variation in latitude of the spots during the cycle, the faster rotation at the equator, the poleward flow in both hemispheres (meridional circulation), the apparent diffusion of magnetic fields as sunspot regions age, and the variation of the polar magnetic field over the cycle. With the advent of helioseismology, it has become possible to significantly add to these constraining observations. Using the splitting of global mode frequencies, rotation has been measured reliably over the outer half of the solar interior. Two regions of rotational shear have been revealed, one encompassing the base of the convection zone at r/R=0.7 and the other in the outer 5% of the radius. It is commonly thought that the lower shear region is the site of the main dynamo generation. In addition to rotation, the meridional circulation has also been measured below the surface. Near the minimum of the 11-year cycle, the poleward flow down to r/R=0.9 appears similar to the surface flow. There are indications that the meridional circulation may be changing over the cycle. In addition to these observations whose significance are thought to be understood, there are large-scale flows whose ultimate connection to the dynamo is unknown. In this group are the zonal rotation bands, weak flows first observed at the surface and now below that migrate towards the equator during the cycle and are periodic with the cycle period. Also, the recently observed time variations of rotation near the convection zone bottom could also be important.