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Abstract

Gravity measurements by the Juno and Cassini spacecrafts provide valuable information about the depth dependence of the zonal winds observed in the cloud decks. However, the inversion based on gravity data alone is non-unique and magnetic measurements can provide important additional constraints. We simulate the dynamo action of the zonal winds in Jupiter and Saturn, concentrating on the outer layers where they provide the dominant dynamics. Zonal wind induction provides an efficient means to dampen non-axisymmetric field contributions produced deeper in the planet. The effect increases with the maximum magnetic Reynolds number Rm reached along the zonal wind depth profile. Rm increases with electrical conductivity and with zonal wind speed. Since Jupiter’s field is highly non-axisymmetric, Rm has to remain below one to guarantee only a mild damping. For a given conductivity profile, this provides an independent constraint on the depth dependence of the zonal winds. Saturn’s field, however, seems to be perfectly axisymmetric. Using a new conductivity profile, base on ab initio calculations, in combination with zonal winds profiles suggested by gravity data yields Rm values significantly larger than one. This means that the zonal winds could contribute to making Saturn’s field axisymmetric. For both gas giants, the secular variation caused by zonal winds tends to be very small. We also discuss energy and entropy constraints for the depth of the zonal winds.