(3–4 μm—H3+ emissions) parts of the spectrum. Saturn kilometric radiation Saturn is the source of rather strong low frequency radio emissions called Saturn kilometric radiation (SKR). The frequency of SKR lies in the range 10–1300 kHz (wavelength of a few kilometers) with the maximum around 400 kHz. The power of these emissions is strongly modulated by the rotation of the planet and is correlated with changes in the solar wind pressure. For instance, when Saturn was immersed into the giant magnetotail of Jupiter during Voyager 2 flyby in 1981, the SKR power decreased greatly or even ceased completely. The kilometric radiation is thought to be generated by the Cyclotron Maser Instability of the electrons moving along magnetic field lines related to the auroral regions of Saturn. Thus the SKR is related to the auroras around the poles of the planet. The radiation itself comprises spectrally diffuse emissions as well as narrowband tones with bandwidths as narrow as 200 Hz. In the frequency–time plane arc like features are often observed, much like in the case of the Jovian kilometric radiation. The total power of the SKR is around 1 GW. The modulation of the radio emissions by planetary rotation is traditionally used to determine the rotation period of the interiors of fluid giant planets. In the case of Saturn, however, this appears to be impossible, as the period varies at the timescale of tens years. In 1980–1981 the periodicity in the radio emissions as measured by Voyager 1 and 2 was , which was then adopted as the rotational period of Saturn. Scientists were surprised when Galileo and then Cassini returned a different value—. Further observation indicated that the modulation period changes by as much as 1% on the characteristic timescale of 20–30 days with an additional long-term trend. There is a correlation between the period and solar wind speed, however, the causes of this change remain a mystery. One reason may be that the saturnian perfectly axially symmetric magnetic field fails to impose a strict corotation on the magnetospheric plasma making it slip relative to the planet. The lack of a precise correlation between the variation period of SKR and planetary rotation makes it all but impossible to determine the true rotational period of Saturn. Radiation belts Saturn has relatively weak radiation belts, because energetic particles are absorbed by the moons and particulate material orbiting the planet. The densest (main)