  To understand the generation of the Earth's and planetary magnetic fields, we
investigate numerically the combined effect of precession and convection on the
dynamo action in a spherical shell. The convection alone, the precession alone
and the combined effect of convection and precession are studied at the low
Ekman number at which the precessing flow is already unstable. The key result
is that although the precession or convection alone is not strong to support
the dynamo action the combined effect of precession and convection can support
the dynamo action because of the resonance of precessional and convective
instabilities. This result may interpret why the geodynamo maintains for such a
long history compared to the Martian dynamo.
