Sub-hour modulation of non-Io Jovian decametric emission

The variability of Jovian decametric emission (DAM), which is not controlled by the Io satellite (non-Io), was studied of timescales of between 1 min and 1 h with DAM records of 1991-2007 from the archive of Nancay Radio Observatory. We found that the internal structure of the non-Io radio storms has the dominating periodicity on of average 21 +/- 2 min. This estimate is in close agreement with the periodicity of Io-related emission and with fundamental eigenoscillations of transversal magnetic pulsations in the Io's plasma torus. The average duration of an arc in non-Io DAM dynamic spectra was estimated to be 7.0 +/- 0.3 min, although the most probable value was 5.3 +/- 0.7 min, which corresponds to Io-related arcs and 3rd and 4th harmonics of torus proper-oscillations. Our results could be interpreted in terms of electron acceleration in parallel electric fields of standing Alfven waves generated by Io and trapped in the Io torus. This interpretation agrees with previous arguments that the location of some sources of non-Io emission are in the magnetic shell of the Io plasma torus. However, the aurora component of non-Io DAM is not excluded, since the suggested DAM modulation by the fundamental mode of eigenoscillations inherent in aurora magnetic-lines is undetectable in our analysis because of the short duration of non-Io radio storms.