The legacy of Cassini RPWS: Radio and plasma waves at Saturn

International audienceIntroduction:The 13-year exploration of Saturn with Cassini provided enormous scientific return from the Radio and Plasma Wave Science (RPWS) investigation. While it is not possible to achieve absolute concensus on the most important results fromthis investigation, here we attempt to show the breadth of contributions to the study of the Saturnian system with this instrument. Saturn Kilometric Radiation: Cassini’s RPWS determined that not only does the SKR rotational modulation vary in time, there are typically two different periods in the northern and southern hemispheres. Cassini crossed the SKR source region, confirming that the cyclotron maser instability (CMI) can drive the auroral radio emissions. These are the first in situ observations of a non-terrestrial CMI radio source. In addition, SKR was shown to be generated on field lines threading the UV auroras and diagnositic of magnetospheric dynamics imposed by solar wind compressions and tail reconnection. Enceladus and the E ring:RPWS observations contributed to the mapping of dust from the plumes of Enceladus and the resulting E ring. The discovery of auroral hiss generated by electron beams accelerated from the moon informed our understanding of the electromagnetic interaction of the moon with the magnetosphere. Auroal hiss also provided evidence of this interaction very close to the planet on field linesthreading the moon. Plasma resulting from the ionoization of material coming from Enceladus was modeled through the determination of the electron density in Saturn’s inner magnetosphere. The depletion of the electron density in the plume led to the realization that charged micron-sized dust grains were a major component of a dusty plasma in the vicinity of the moon. Lightning:High frequency radio emissions initiated in lightning strokes enabled RPWS to characterize the occurrence of lightning, hence, convective storms in Saturn’s atmosphere and tracked the development of a Great White Spot storm beginning in late 2010. Coupled with amateur and ISS images, an extensive compilation of thunderstorm activity in Saturn’s atmosphere was possible.Titan’s Ionosphere: The RPWS Langmuir Probe was the first instrument to confirm the existence of a substantial ionosphere at Titan. The Solar EUV dominates the ionization of the upper atmosphere of Titan, although energetic particles in Saturn’s magnetosphere do contribute substantially at all Solar Zenith Angles. Observations over the orbital mission showed the effect of the vaiation of EUV over the solar cycle in the ionospheric density. One of the more intriguing discoveries was the formation of complex negative organic ions and aerosol pre-cursors in the deep ionosphere of Titan, which may have implications for how pre-biotic chemistry occurred on the early Earth. RPWS observations also helped characterize the interaction of Titan with the solar wind and the development of a compound bowshock encompassing both Titan and Saturn’s magnetosphere.Saturn’s Ionosphere:An obvious result of Cassini’s Grand Finale was the first in situ obervations of electron densities and temperatures in Saturn’s topside ionosphere andthe revelation of strong interactions between the rings and the ionosphere