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Abstract

Gravity signatures observed by the Juno and Cassini missions that are associated with the strong zonal winds in Jupiter's and Saturn's outer envelopes suggest that these flows extend for several thousand kilometers into the interior. It has been noted that the winds seem to abate at a depth where electrical conductivity becomes significant, suggesting that electromagnetic effects play a key role for confining the winds to the outer weakly conducting region. Here, we explore the possible mechanisms for braking the zonal flow at depth in two model setups with depth-dependent conductivity and forced jet flow, i.e., in axisymmetric shell models and in more simple linearized box models that allow the exploration of a wide parameter range. Braking of the winds directly by Lorentz forces does not reduce their speed in the conducting region enough to be compatible with the inferred secular variation of Jupiter's field. Stable stratification above the depth where conductivity becomes significant can solve the problem. Electromagnetic forces drive a weak meridional circulation that perturbs the density distribution in the stable region such that the wind speed decreases strongly with depth, due to a thermal wind balance. For this mechanism to be effective, the stable layer must extend upward into a region of low conductivity. Applying the results of the linearized calculations to Jupiter suggests that the dissipation associated with the zonal winds can be limited to a fraction of the internal heat flow and that the jets may drop off over a depth range of 150–300 km.