Jupiter's magnetic field, like that of the Sun, and perhaps Saturn, exhibits a clear, persistent dual periodicity, the two Jovian periods differing by almost exactly 3%. We offer a provisional definition of a new Jovian longitude system (which we call system IV) to organize magnetospheric data that are not stationary in system III. We show that available, independent data sets, covering a time interval of 4 years, which either drift in system III or show no particular organization in system III, fit mutually consistent patterns in system IV. All of the data sets covering several rotations of the planet that are presently available to us, including Voyager observations of ultraviolet and narrow-band kilometric emissions and ground-based optical observations, are organized in either system III, system IV, or both. Using these data, we derive provisional values for a transformation between systems III and IV: &lgr;IV=&lgr;III +338-25.486(t-244387.5) where t is the Julian day and fractional day of the observation. There are pronounced 14.1-day variations in a number of Jovian magnetospheric phenomena. One possible interpretation of the system IV modulation is that it is a sideband resulting from the 14.1-day amplitude modulation of system III phenomena. Alternately, the 14.1-day period could be explained if we assume the existence of an active sector that is fixed in system IV but drifts approximately 25.5¿/d relative to the active sector in system III. When the system III and system IV activity maxima are aligned, magnetospheric activity is subdued. The interval between alignments (or anti-alignments) is 14.1 days. Finally, we note that, in developing system IV, we have utilized only a small number of data sets. System IV needs to be tested against additional data before its durability is assured. ¿ American Geophysical Union 1988