A plausible explanation for the strong correlation between the occurrence of Jupiter's sporadic decametric radio emission and the orbital position of Io is that the emission is either generated in discrete radio sources located along the instantaneous magnetic flux tube connecting Io with Jupiter or in multiple discrete radio sources associated with other flux tubes excited by ionospheric Alfv¿n wave reflections downstream from Io. On the assumption that a periodic increase in the probability of receiving radio emission should be observable as the directional emission beams from these multiple discrete radio sources are swept past the observer by Io's orbital motion, a search has been made for periodic modulation of the Jupiter radio emission using a statistical analysis of the data from a total of 34 Io-B radio noise storms recorded at a wave frequency of 20 MHz during the 1984 and 1996 apparitions of Jupiter. This analysis shows that a weak periodic modulation of the radio emission with a repetition interval near 15 min is present in this source. Assuming that this modulation results from the action of multiple sweeping radio beams, calculations using the O4 model of Jupiter's magnetic field and recently measured emission cone half-angles give an angular separation between successive Alfv¿n wave reflections in the Io plasma torus of 7.8¿1¿ (or possibly half this value, depending on assumptions). This estimate is in approximate agreement with theoretical predictions. ¿ 1998 American Geophysical Union