Galileo constraints on the secular variation of the Jovian magnetic field

The variations of the terrestrial magnetic field place important constraints on the behavior of the fluid in the Earth's magnetic dynamo. Jupiter is currently the only other planet for which magnetic measurements exist over a sufficiently long baseline to enable a study of the secular variation of the field to be undertaken. The average magnetic moment during the Galileo epoch was 4.334 +/- 0.010 Gauss R-J(3) or 1.584 +/- 0.004 x 10(20) Tm-3. The tilt angle of the dipole was 9.71 degrees +/- 0.05 degrees. We examine both the change in the magnetic field during the period in which Galileo operated in Jovian orbit and the change in the field between the initial observations with Pioneer 11 and the Galileo epoch. Neither approach definitively identifies secular change in the Jovian field but rather puts a limit on that rate of change. The only significant change is associated with the current imprecision of the International Astronomical Union-defined System III period.