The rotation period of Jupiter

The period with which radio bursts recur on Jupiter (the System III period) is defined by the IAU to be 9h 55m 29.71s based on early radio astronomical data, and is generally assumed to represent the period of rotation of the Jovian interior. A recent estimate of the System III period from radio burst data is 0.025s shorter than the IAU value. In apparent contradiction to the radio observations, in situ measurements of the rotation period of Jupiter using the orientation of the dipole moment are consistent with the original rotation period defined by the IAU and inconsistent with the suggested period decrease. Thus, the present IAU period should be retained as the best estimate of the rotation rate of the Jovian interior. Since the radio bursts are generated near the base of the Io field line while the dipole field is measured by Galileo near the equatorial plane, the difference between the two rotation periods could be explained if Jupiter's magnetic field is undergoing perceptible secular variation.