Rotation period of Jupiter from the observations of its magnetic field

Unlike the Earth, Jupiter does not have solid surface. Thus we cannot tell its interior rotation period from the rotation of visible features. To determine the rotation rate historically one has used the repetition period of decametric radio waves. Currently, this period with which radio burst recur on Jupiter (the System III period) is defined by the IAU to be 9h 55m 29.71s based on early radio astronomical data. If this period corresponds to the period of the interior of Jupiter the magnetic moment of Jupiter should rotate at this period too. We have used the observed location of the dipole moment determined by the Galileo over the period 1996 -2003 to define the rotation period and we have combined these measurements with those obtained earlier by Pioneer, Voyager and Ulysses. No drift of magnetic dipole longitude can be unambiguously detected using only the Galileo period. However over the past 25 years, a small longitude shift has occurred that is statistically significant and corresponding to a period within the error expected in the original IAU definition. This slightly improved value is 9h 55m 29.703±0.004s.