Meridional Flow Variations: Implications for Flux Transport Models
Observations of the transport of magnetic elements across the Sun's surface indicate that, in general, the meridional flow extends all the way to the poles. Furthermore, the speed of this flow varies systematically over each solar cycle faster at minimum and slower at maximum. The flow speed on the approach to this (Cycle 24) minimum was substantially faster than it was at the last minimum. This increased flow speed should have produced a short Cycle 23 with strong polar fields in the flux transport dynamos used to predict Cycle 24. This is contrary to what we have seen. The increased flow speed does produce weak polar fields in the surface flux transport models like those used to estimate past irradiance variations. However, these surface flux transport models use meridional flow profiles that do not agree with the observations. Both types of flux transport models are missing identifiable components. We note that the observed changes in the structure of the meridional flow profile are consistent with Spruit's model for the torsional oscillations - suggesting that there may be associated irradiance variations.