  We investigate the nature of the dynamo bifurcation in a configuration
applicable to the Earth's liquid outer core, i.e. in a rotating spherical shell
with thermally driven motions. We show that the nature of the bifurcation,
which can be either supercritical or subcritical or even take the form of isola
(or detached lobes) strongly depends on the parameters. This dependence is
described in a range of parameters numerically accessible (which unfortunately
remains remote from geophysical application), and we show how the magnetic
Prandtl number and the Ekman number control these transitions.
