Our results have shown that ambipolar diffusion (AD) provides a useful model for nonlinearity, enabling analytic (or semi-analytic) progress to be made in understanding nonlinear dynamos. There are two key features that are shared both by this model and by the full MHD equations: (i) large scale fields are the result of a nonlocal inverse cascade as described by the -effect, and (ii) after some initial saturation phase the large scale field continues to grow at a rate limited by magnetic diffusion. We reiterate that in astrophysical bodies the presence of open boundaries may relax the helicity constraint. Furthermore, the presence of large scale shear or differential rotation provides a means of amplifying toroidal magnetic fields quite independently of magnetic helicity, but this still requires poloidal fields for which the above conclusions hold.
© European Southern Observatory (ESO) 2000
Online publication: October 2, 2000