How astrophysical mean field dynamos can circumvent existing quenching constraints

Mean field dynamo theory is a leading candidate to explain the large scale magnetic fields of galaxies and stars. However, controversy arises over the extent of quenching by the backreaction of the growing field. Here boundar y conditions and magnetic helicity flow are shown to play a role in determi ning whether the mean field dynamo action is fast, as required by astrophys ical systems, or resistively limited (slow). Existing work suggesting that mean field dynamos are resistively limited include restrictive approximatio ns such as stationarity and periodic boundary conditions that suppress magn etic helicity flow. Thus even though the backreaction is present, such stud ies cannot unambiguously reveal whether real astrophysical mean field dynam os are dynamically suppressed when the helicity flow is allowed. If the dyn amo is sustained by an outflow of helicity from the system, then a magnetic ally active corona is expected. Open boundaries alone may not be sufficient for rapid dynamo action and the additional physics of buoyancy and outflow s may be required. Possible simulation approaches to test some of the princ iples are briefly discussed. Some limitations of the "Zeldovich relation" a re also addressed. (C) 2001 American Institute of Physics.