  Geomagnetic activity has two main peaks in the 11-year sunspot cycle caused
by two types of solar agents: coronal mass ejections and high speed solar wind
streams, whose variations in number and intensity are related to the variations
in solar toroidal and poloidal fields, respectively. From the long-term
variations in the lag between the two aa-index peaks we derive the long-term
variations in solar meridional circulation. We use these relations to test the
flux-transport solar dynamo theory and to explain some well known
characteristics of the solar cycle. Based on these relations, we give a
prediction for the amplitude (125) and epoch of the maximum (April 2011) of
sunspot cycle 24.
