Observations from the Voyager 1 pass by the Io flux tube strongly indicate that large amplitude standing Alfv¿n waves are generated as Jovian plasma flows past Io. Recently Gurnett and Goertz (1981) have proposed that this Io generated standing Alfv¿n wave system extends an appreciable way around the Io L-shell. In this paper, we present observations of magnetic perturbations found in the Pioneer 10 record as it crossed the Io L-shell. The field perturbations are qualitatively consistent with the passage of the spacecraft through a standing Alfv¿n wave pattern. The observations imply an Alfv¿n Mach number of 0.03, which is 1/5 of the value inferred from Voyager 1 observations. This implies a lower plasma density at the time of the Pioneer 10 flyby.