A two parameter model of time-dependent, flux-tube interchange diffusion is fit to the Voyager 1 plasma data obtained in the Io plasma disk. The interpretation of the parameters required to achieve the fit is that plasma magnitude) at some time prior to the arrival of Voyager 1 (between 1 and 100 days prior). The injection rate was about 2¿1029¿1 ion/sec. At this rate, the centrifugally driven interchange instability dominated outward diffusion, causing the outward diffusion rate to be about a factor of 50 greater than the inward diffusion rate. The material diffusing inward had time to cool by radiation, possibly accounting for the observed temperature drop inside the orbit of Io.