Plasma wave measurements taken during the Voyager 1 and 2 Jupiter encounters are used as a basis for synoptic studies of Jovian kilometric radiation. The studies reveal the existence of a shadow zone near the magnetic equator within which kilometric radiation is seldom or weakly observed. This shadow is presumably cast by the Io plasma torus whose density is high enough to refract electromagnetic waves generated either near the torus or closer to Jupiter. At 56.2 kHz this shadow zone has a half width of about 10 ¿. In addition to a latitudinal shadow zone, there occasionally appear to be long-lived longitudinal variations of kilometric radiation intensities. That is, individual features may reappear on successive rotations at nearly constant System III longitudes. This may imply longitudinally asymmetric structures in the kilometric source region or intervening magnetosphere such as the Io torus which corotate and have lifetimes of up to several days. Some of the features appear to drift to higher longitudes with increasing time suggesting deviations from corotation ranging up to a few percent. Gross changes in the character of Jovian kilometric radiation on time scales of a few days imply dramatic changes in the inner Jovian magnetosphere on the same time scale and may be indicative of magnetospheric substorms or changes in plasma injection rates from Io.