Ulysses assessment of the Jovian planetary field

The magnetic field measurements made by the Ulysses dual fluxgate/vector helium magnetometer during the day of closest approach to Jupiter in February 1992 are reported. The data are compared to a model which describes the field in terms of an internal spherical harmonic component together with an external current sheet field. The sensitivity of the magnetic field model parameters is examined. It is concluded that the dipole term which best fits the Ulysses closest approach data is lower than that used in most recent models and is closer in value to that found for the Pioneer 11 epoch models. Our favored value of the dipole term is 4.059 G. The contribution of the external ring current is weaker than that observed during the Voyager epoch, although the sheet is in fact thicker. The lower dipole value is also found to produce a good fit to the Pioneer 11 and Voyager 1 closest approach data sets, and hence there is no evidence for any secular change in the planetary field at Jupiter.