The structure of the magnetic field varies strongly with $P$ in that dynamos with a strong axial dipole field are found for high values of $P$ while the energy of this component is exceeded by that of the axisymmetric toroidal field and by that of the non-axisymmetric components at low values of $P$. Dynamos with a minimum value of $P_m$ seem to be most readily realizable in the presence of convection columns at moderately low values of $P$. The magnetostrophic approximation often used in dynamo theory appears to be valid only for relatively high values of $P$ and $P_m$. Low Prandtl numbers promote dynamo action through the shear provided by differential rotation, while the generation of magnetic fields is more difficult to sustain in high-Prandtl-number fluids where higher values of the magnetic Prandtl number $P_m$ are required. The value of the Prandtl number $P$ exerts a strong influence on convection-driven dynamos in rotating spherical shells filled with electrically conducting fluids.
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