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D. J. Stevenson (Caltech)
Jupiter's magnetic field is spectrally rich, including substantial field strength at non-spin axisymmetric harmonics (the non-zero m coefficients in the usual spherical harmonic expansion, the largest of which represents the dipole tilt). After allowance for a shallower dynamo generating region than Earth, Jupiter exhibits Earthlike characteristics that are plausibly typical of all planetary dynamos with thick shells . By contrast, the data for Saturn (primarily Pioneer 11) can be well described by a field which has little or no power in non-axisymmetric terms (e.g. no dipole tilt is required). In a modification of ideas I first put forward in 1980, and taking in to account recent shock wave data for electrical conductivity of dense hydrogen, I argue that the main difference between Jupiter and Saturn lies in the differences of their deep seated zonal winds and the hydromagnetic effect of these winds. Galileo probe data permit the idea that the observed winds extend deep into the planet interior on cylindrical surfaces. Jupiters' winds are weaker and confined to a narrower latitudinal range, centered on the equator, leading to only a moderate reduction in magnetic power at non-zero m, primarily detectable near the equator. Saturn has stronger winds with broader latitudinal extent, leading to a much stronger attenuation of the non-zero m coefficients. There are three testable consequences of the ideas put forward here: (1) The Saturnian field is less spin-axisymmetric near the poles than at the equator where Pioneer 11 did its most sensitive measurements. (2) The Jovian field should exhibit a "dip" in power at non-zero m values near the equator, detectable by the proposed INSIDE Jupiter mission. (3) The time variability of the fields may be less than is typical of dynamos because of the screening due to the conductivity of hydrogen in non-dynamo regions.