Jupiter's and Saturn's fine-scale magnetic fields

Abstract

Jupiter's field is strongly dipolar but with relatively large high order moments compared to the Earth's. In situ magnetic field data allow us to interpret most of the Earth-based microwave observations of Jupiter, with the exception of Branson's hot spot. Decametric emissions have a complex rotational pattern which has been stable since 1950; their agreement with the spacecraft magnetic fields is much less satisfactory than that of the microwaves. We conclude that the extrapolation of magnetic fields from the spacecraft to the surface of Jupiter is in error by 40% in the Southern Hemisphere. Saturn's radio emissions show complexities similar to Jupiter's. They are strongly asymmetric about the rotational axis, although Saturn's Field is nearly axisymmetric. Their strong asymmetry suggests strong longitudinal variations in the magnetic field a few thousand kilometers from the cloud tops, in conflict with the field measured aboard Pioneer 11. The magnetic fields within a few thousand kilometers of either Jupiter's or Saturn's cloud tops are probably unknown. It is discouraging that more is not known about the fields after a total of 7 encounters. Perhaps the Galileo probe can test usefully models of the Jupiter field, even if its measurements refer to just one trajectory through the clouds. An arguable case can be made that the giant planets exhibit complexity of magnetic structure similar to the Sun.