Jupiter's radio emissions are dominated in intensity by decametric radio emissions due to the Io-Jupiter interaction. A significant part of these emissions consists of short radio bursts (so-called S-bursts) drifting in time and frequency. Previous analyses suggest that these emissions are cyclotron-maser emissions in the flux tube connecting Io or Io's wake to Jupiter. We present simulations of these electrons under the assumption of acceleration by Alfvén waves in the Io flux tube. Near Jupiter, a loss cone and a ring distribution appear in the magnetically mirrored electron population, which can then amplify extraordinary (X) mode radio waves. The X-mode growth rate is computed, which allows us to build theoretical dynamic spectra of the resulting Jovian radio emissions. Additional potential structures are assumed in the Jovian auroral region. We reconstruct their impact on the morphology of the emission. They match some of the time-frequency patterns observed with Jovian S-bursts. This provides the first evidence of bipolar electrostatic structures in the Jovian auroral region.