We use the aa index of geomagnetic activity recorded for 130 years to study the strength and phase properties of the solar dynamo. We split the monthly averaged aa into two parts: one part proportional to the sunspot number and the other part proportional to the residual. We argue that the first part is a proxy for the solar dynamo's large-scale mean toroidal magnetic field. The residual has the same periodicity as the sunspot cycle and has closely related amplitude, but is shifted in phase. We relate this residual to the poloidal field generated from the toroidal field in the dynamo process. The changes in both components demonstrate that there was a long-term trend in the strength and relative phases of the toroidal and poloidal components of the mean-field dynamo. The changes in the relative phase of the toroidal and poloidal fields suggest that the distributions of the differential rotation and helicity within the Sun's convection zone have changed over timescales of 50 years. ¿ 2001 American Geophysical Union