Jovian S bursts that are V shaped in the frequency-time domain have been reported by several observers. These can arise if an outward moving ~2.5-keV electron beam in the Io flux tube is longitudinally modulated in velocity by ~1 keV peak to peak, at a frequency of several hertz. Such modulation near the ionosphere could also explain the difference between the maximum observed frequencies of S burst and L burst emission. In order for this beam to produce the observed total S burst flux, a large part of the ~5¿106 A current observed in the Io flux tube must participate. In this case the observed duration of individual S bursts is consistent with magnetospheric ion densities greater than ~15 cm-3. This ballistic model for S bursts also suggests an interpretation for the origin of L burst emission.