  Planetary and stellar magnetic fields are thought to be sustained by helical
motions ($\alpha$-effect) and, if present, differential rotation
($\Omega$-effect). In the Sun, the strong differential rotation in the
tachocline is responsible for an efficient $\Omega$-effect creating a strong
axisymmetric azimuthal magnetic field. This is a prerequisite for Parker dynamo
waves that may be responsible for the solar cycle. In the liquid iron cores of
terrestrial planets, the Coriolis force organizes convection into columns with
a strong helical flow component. These likely dominate magnetic field
generation while the $\Omega$-effect is of secondary importance. Here we use
numerical simulations to show that the planetary dynamo scenario may change
when the heat flux through the outer boundary is higher in one hemisphere than
in the other. A hemispherical dynamo is promoted that is dominated by fierce
thermal wind responsible for a strong $\Omega$-effect. As a consequence Parker
dynamo waves are excited equivalent to those predicted for the Sun. They obey
the same dispersion relation and propagation characteristics. We suggest that
Parker waves may therefore also play a role in planetary dynamos for all
scenarios where zonal flows become an important part of convective motions.
