  Magnetic helicity is a fundamental constraint in both ideal and resistive
magnetohydrodynamics. Measurements of magnetic helicity density on the Sun and
other stars are used to interpret the internal behaviour of the dynamo
generating the global magnetic field. In this note, we study the behaviour of
the global relative magnetic helicity in three self-consistent spherical dynamo
solutions of increasing complexity. Magnetic helicity describes the global
linkage of the poloidal and toroidal magnetic fields (weighted by magnetic
flux), and our results indicate that there are preferred states of this
linkage. This leads us to propose that global magnetic reversals are, perhaps,
a means of preserving this linkage, since, when only one of the poloidal or
toroidal fields reverses, the preferred state of linkage is lost. It is shown
that magnetic helicity indicates the onset of reversals and that this signature
may be observed at the outer surface.
