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We present a dynamo mechanism arising from the presence of barotropicallyunstable zonal jet currents in a rotating spherical shell. The shearinstability of the zonal flow develops in the form of a global Rossby mode,whose azimuthal wavenumber depends on the width of the zonal jets. We obtainself-sustained magnetic fields at magnetic Reynolds numbers greater than 1000.We show that the propagation of the Rossby waves is crucial for dynamo action.The amplitude of the axisymmetric poloidal magnetic field depends on thewavenumber of the Rossby mode, and hence on the width of the zonal jets. Wediscuss the plausibility of this dynamo mechanism for generating the magneticfield of the giant planets. Our results suggest a possible link between thetopology of the magnetic field and the profile of the zonal winds observed atthe surface of the giant planets. For narrow Jupiter-like jets, the poloidalmagnetic field is dominated by an axial dipole whereas for wide Neptune-likejets, the axisymmetric poloidal field is weak.

A dynamo driven by zonal jets at the upper surface: Applications to giant planets