Cartesian convection driven dynamos at low Ekman number.

The Cartesian dynamo model of Childress and Soward [Phys. Rev. Lett. 29, 837 (1972)] is studied numerically in the regime of low viscosity. Dynamos with Ekman numbers E in the range 10(-4) > or =E> or =5 x 10(-7) are discussed and compared with the corresponding nonmagnetic states and with results obtained for imposed magnetic fields. We find that in the range of investigated Ekman numbers, a transition occurs from a flow regime where the planform of convection is only weakly affected by the dynamo generated field to a regime where the typical length scales of the flow are largely controlled by the Lorentz forces. The magnetic field acts to facilitate convection and leads to an increase in both the heat transport and in the amplitude of the flow. We demonstrate that this convection promoting effect allows for dynamo action even for Rayleigh numbers below the critical Rayleigh number for the onset of nonmagnetic convection.