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C. T. Russell, Z. J. Yu, M. G. Kivelson, K. K. Khurana (Institute of Geophysics and Planetary Physics, University of California Los Angeles)
The System III (1965.0) rotation period of Jupiter, as defined by the IAU based on early radio astronomical data, is 9h 55m 29.71s. Higgins et al. (JGR, 22033, 1997) have suggested, based on more recent radio data, that this period is too high by perhaps 25 ms. In the 25 years since the Pioneer and Voyager measurements, such an error would cause a 6 degree shift in apparent longitude of features tied to the internal magnetic field. A comparison of the longitude of the projection of the dipole moment obtained over the period 1975-1979 with that obtained by Galileo today shows that the average dipole location has drifted only one degree eastward in System III (1965.0). This one-degree shift is not significant given the statistical errors. A possible resolution to this apparent paradox is that the dipole moment observation is sensitive to the lower order field while the radio measurement is sensitive to the high order field at low altitude. Estimates of the secular variation from the in situ data are being pursued.