Polar field puzzle: Solutions from flux-transport dynamo and surface-transport models

Polar fields in solar cycle 23 were about 50% weaker than those in cycle 22. The only theoretical models which have addressed this puzzle are surface-transport models and flux-transport dynamo models. Comparing polar fields obtained from numerical simulations using surface-flux-transport models and flux-transport dynamo models, we show that both classes of models can explain the polar field features within the scope of the physics included in the respective models. In both models, how polar fields change as a result of changes in meridional circulation depends on the details of meridional circulation profile used. Using physical reasoning and schematics as well as numerical solutions from a flux-transport dynamo model, we demonstrate that polar fields are determined mostly by the strength of a surface poloidal source provided by the decay of tilted, bipolar active regions. The profile of a meridional flow with the latitude and its changes with time have much less effect in flux-transport dynamo models than in surface-transport models.