The enigmatic control of the occurrency frequency of Jupiter's decametric emissions by the satellit Io has been explained theoretically on the basis of its strong electrodynamic interaction with the corotating Jovian magnetosphere leading to field-aligned currents connecting Io with the Jovian ionosphere. Direct measurements of the pertubation magnetic fields due to this current system were obtained by the Goddard Space Flight Center (GSFC) magnetic field experiment on Voyager 1 on March 5, 1979, when it passed within 20,500 km south of Io. An interpretation in the framework of Alfv¿n waves radiated by Io leads to current estimates of 2.8¿106 A. A mass density of 7400-13,600 proton mass units per cm3 is derived, which compares very favorably with independent observations of the torus composition characterized by 7--9 proton mass units per electron for a local electron density of 1050--1500 cm-3. The power dissipated in the current system may be important for heating the Io heavy ion torus, inner magnetosphere, Jovian ionsophere, and possible the ionosphere of even the interior ot Io.