Variations of Saturn's radio rotation period measured at kilometer wavelengths

The Unified Radio and Plasma Wave (URAP) experiment on the interplanetary s pacecraft Ulysses is able to detect the Saturnian kilometric radiation (SKR ) thanks to the high sensitivity of the receiver and in spite of the remote ness of the planet (8-13 AU). Our knowledge al,out Saturn comes essentially from the observations by the two Voyager spacecraft. Ulysses allows us to reassess the main properties of the SKR as they had been observed by Voyage r 1 and 2: average flux density, spectrum, periodicity, and polarization. A striking difference between the results obtained from Voyager and those ob tained from Ulysses is the periodicity of the radio emission linked to the planetary rotation (10 hours 39 min 24 s): The period deduced from Ulysses' observations is not constant and may differ by 1% from that of Voyager. Th e northern and southern sources of SKR are distributed along magnetic field lines which are fixed in local time. We interpret the source location by a Kelvin-Helmholtz instability developing on the flanks of Saturn's magnetop ause, in the morningside. This instability could be at the origin of the ac celeration of the particles responsible for the radio emission. Because of the solar wind fluctuations, the position of the magnetopause and the magne tohydrodynamic flow around it are modified so that the zone of Kelvin-Helmh oltz instability slowly moves, implying a drift of the radio sources in loc al time. We examine and discuss the possibility for this drift to be the ca use of the variations observed for the SKR period.