A REDEFINITION OF JUPITERS ROTATION PERIOD

We measured the rotation period of Jupiter's inner magnetosphere with precision previously unattainable, using 35 years of observations of t he Jovian decametric radiation at the University of Florida Radio Obse rvatory at frequencies between 18 and 22.2 MHz. The new rotation perio d is the weighted mean of 13 independent 24-year average determination s. Each of these was found by measuring the drift of the histogram of occurrence probability versus System III (1965) central meridian longi tude over an interval of approximately 24 years. The measured drift wa s used to correct the System III (1965) period to obtain the new value . Our weighted mean is 9 hours 55 min 29.6854 s, with a standard devia tion of the weighted mean (sigma) of 0.0035 s. This new rotation perio d is 7.4 sigma shorter than that of the System III (1965), indicating that the latter is in need of revision. Our measurements indicate an u pper limit of about 4 ms/year on any possible Jovian rotation period d rift.