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It is known that the tilt angles of active regions increase with theirlatitude (Joy's law). It has never been checked before, however, whether theaverage tilt angles change from one cycle to another. Flux transport modelsshow the importance of tilt angles for the reversal and build up of magneticflux at the poles which is, in turn, correlated with the strength of the nextcycle. Here we analyse time series of tilt angle measurements and look for apossible relationship of the tilt angles with other solar cycle parameters, inorder to glean information on the solar dynamo and to estimate their potentialfor predictions of solar activity. We employ tilt angle data from Mount Wilsonand Kodaikanal observatories covering solar cycles 15 to 21. We analyse thelatitudinal distribution of the tilt angles (Joy's law), their variation fromcycle to cycle and their relationship to other solar cycle parameters, such asthe strength, amplitude and length. The two main results are: 1. Ananti-correlation between the mean normalized tilt angle of a given cycle andthe strength (or amplitude) of that cycle, with a correlation coefficient ofr=-0.95 and r=-0.93 for Mount Wilson and Kodaikanal data, respectively. 2. Theproduct of the cycle averaged tilt angle and the strength of the same cycledisplays a significant correlation with the strength of the next cycle (r=0.65and r=0.70 for Mount Wilson and Kodaikanal data, respectively). An even bettercorrelation is obtained between the source term of the poloidal flux inBabcock-Leighton-type dynamos (which contains the tilt angle) and the amplitudeof the next cycle. The results of this study indicate that in combination withthe cycle strength, the active region tilt angles play an important role inbuilding up the polar fields at cycle minimum.

Sunspot group tilt angles and the strength of the solar cycle