  The frequency spectrum of the hemispheric asymmetry of solar activity shows
enhanced power for the period ranges around 8.5 years and between 30 and 50
years. This can be understood as the sum and beat periods of the superposition
of two dynamo modes: a dipolar mode with a (magnetic) period of about 22 years
and aquadrupolar mode with a period between 13 and 15 years. An updated
Babcock-Leighton-type dynamo model with weak driving as indicated by stellar
observations shows an excited dipole mode and a damped quadrupole mode in the
correct range of periods. Random excitation of the quadrupole by stochastic
fluctuations of the source term for the poloidal field leads to a time
evolution of activity and asymmetry that is consistent with the observational
results.
