A model based on the nonlinear coupling of electrostatic plasma waves is proposed to explain the emission of the narrow-band Jovian kilometric radiation (nKOM) observed by the Voyager spacecraft. It is shown that upper-hybrid branch electrostatic waves propagating through the inhomogeneities in the outer periphery of the Io plasma torus can attain the proper geometry for localized upconversion interactions leading to pump depletion. Plasma waves propagating into a weak density gradient (increasing &ohgr;p/&OHgr;e) and reflected at the critical layer (&ohgr;L=&OHgr;p), interact with the incident waves leading to the electromagnetic emission at 2&ohgr;UH, which is beamed at large angles with respect to the background magnetic field. In general, both L-O and R-X mode waves can be generated. The observed power and net polarization (L-O) are consistent with pump depletion of electrostatic waves at a level of ~10 mV/m. A possible excitation mechanism for the electrostatic waves is also discussed. ¿ American Geophysical Union 1987