Ambient magnetic fields and dynamos: Pre-main sequence stars and elsewhere

The effects of an ambient (i.e. external) poloidal magnetic field on mean field dynamo action in a sphere are investigated. It is found that an ambient field of strength comparable to that of the poloidal field of the isolated dynamo will inhibit dynamo action. This ambient field will normally be considerably weaker than the typical total field resulting from alpha(2)omega dynamo action. Given earlier similar results for disc dynamos, this appears to be a general result. Applications to pre-main sequence stars and the history of magnetic white dwarfs are discussed.