Generation mechanism of a dipole field by a magnetohydrodynamic dynamo

Computer simulation of a magnetohydrodynamic dynamo in a rotating spherical shell is performed. A strong magnetic field is generated by thermal convection motion of an electrically conducting fluid. The generated magnetic field has a well-organized structure. The field is expanded by the spherical harmonics ${\mathrm{Y}}_{\mathrm{\ell }}^{\mathrm{m}}$. The leading moment is the dipole (ℓ=1,m=0) which is 1.3 times larger than the second moment, octapole (ℓ=3,m=0). These two moments occupy more than half of the total power of the magnetic field. Structure and the generation mechanism of the magnetic field are examined in detail. It is found that the well-known α-ω dynamo scenario gives a good explanation of the whole dynamo cycle.