  It is known that the tilt angles of active regions increase with their
latitude (Joy's law). It has never been checked before, however, whether the
average tilt angles change from one cycle to another. Flux transport models
show the importance of tilt angles for the reversal and build up of magnetic
flux at the poles which is, in turn, correlated with the strength of the next
cycle. Here we analyse time series of tilt angle measurements and look for a
possible relationship of the tilt angles with other solar cycle parameters, in
order to glean information on the solar dynamo and to estimate their potential
for predictions of solar activity. We employ tilt angle data from Mount Wilson
and Kodaikanal observatories covering solar cycles 15 to 21. We analyse the
latitudinal distribution of the tilt angles (Joy's law), their variation from
cycle to cycle and their relationship to other solar cycle parameters, such as
the strength, amplitude and length. The two main results are: 1. An
anti-correlation between the mean normalized tilt angle of a given cycle and
the strength (or amplitude) of that cycle, with a correlation coefficient of
r=-0.95 and r=-0.93 for Mount Wilson and Kodaikanal data, respectively. 2. The
product of the cycle averaged tilt angle and the strength of the same cycle
displays a significant correlation with the strength of the next cycle (r=0.65
and r=0.70 for Mount Wilson and Kodaikanal data, respectively). An even better
correlation is obtained between the source term of the poloidal flux in
Babcock-Leighton-type dynamos (which contains the tilt angle) and the amplitude
of the next cycle. The results of this study indicate that in combination with
the cycle strength, the active region tilt angles play an important role in
building up the polar fields at cycle minimum.
