A physical model of the radiation source and an excitation mechanism have been suggested for the S component in Jupiter's sporadic radio emission. The model provides a unique explanation for most of the interrelated phenomena observed, allowing a consistent interpretation of the emission cone structure, behavior of the integrated radio spectrum, occurrence probability of S bursts, location and size of the radiation source, and fine structure of the dynamic spectra. The mechanism responsible for the S bursts is also discussed in connection with the L type emission. Relations are traced between parameters of the radio emission and geometry of the Io flux tube. Fluctuations in the current amplitude through the tube are estimated, along with the refractive index value and mass density of the plasma near the radiation source. ¿ American Geophysical Union 1994