Jupiter's magnetosphere does not rigidly rotate with the System III angular rate. There is a corotation lag as a function of distance in the Jovian magnetosphere. The dominant rotational component, aside from System III, is a 3% longer period (System IV). System IV could be interpreted as a sideband of System III being amplitude-modulated by the 14.1-day period. Whether System IV is an independent Jovian coordinate or just a sideband is investigated in this thesis. The 9.64 hour period does not appear in periodogram analyses of EUV data from Voyager 2 and nKOM data from Voyager 1 and 2. Therefore, it is concluded that System IV is not a sideband and it is independent of System III. Through harmonic filtering, a signal with a 12.5-day period in the EUV data is found. It is suggested that the 12.5-day period is a solar-wind effect. (Abstract shortened with permission of autor.)