Scale variability in convection-driven MHD dynamos at low Ekman number

We have undertaken a numerical study of convection-driven MHD dynamos in a rapidly rotating spherical shell with the Ekman number, E, down to 2 × 1 0 − 6 and the magnetic Prandtl number, Pm, down to 0.2. We focus on the characteristic scales of the flow and the magnetic field. Smaller-scale convection vortices responsible for generating the magnetic field appear at lower Ekman numbers, while the scale of the magnetic field shows less variation compared with the flow. As a result, scale separation between the flow and the magnetic field occurs as the Ekman number is decreased. Scale separation helps dynamos to maintain the magnetic field at P m < 1 through increase in the effective value of the magnetic Reynolds number.