Io-Jupiter interaction, millisecond bursts and field-aligned potentials

Jovian millisecond (or S-) bursts are intense impulsive decametric radio bursts drifting in frequency in tens of milliseconds. Most of the theories about their origin comprise an interpretation of their frequency drift. Previous analyses suggest that S-bursts are cyclotronmaser emission in the flux tubes connecting Io or Io's wake to Jupiter. Electrons are thought to be accelerated from Io to Jupiter. Near Jupiter, a loss cone appears in the magnetically mirrored electron population, which is able to amplify extraordinary (X) mode radio waves. Here, we perform an automated analysis of 230 high-resolution dynamic spectra of S-bursts, providing 5 x 10(6) frequency drift measurements. Our data are consistent with the above scenario. In addition, we confirm over a large number of measurements that the frequency drift df/dt(f) is in average negative and decreases (in absolute value) at high frequencies, as predicted by the adiabatic theory. We find a typical energy of 4keV for the emitting electrons. In 15% of the cases (out of 230), we find for the first time evidence of localized similar to 1 keV electric potential jumps at high latitudes along the field lines connecting Io or Io's wake to Jupiter. These potential jumps appear stable over tens of minutes. Finally, a statistical analysis suggests the existence of a distributed parallel acceleration of the emitting electrons along the same field lines. (c) 2006 Elsevier Ltd. All rights reserved.