Detection of dusty plasma near the E-ring of Saturn

We present several independent in-situ measurements, which provide evidence that charged dust in the E-ring interacts collectively with the dense surrounding plasma disk of Saturn, i.e., form a system of dust-plasma interaction. The results are based on data sampled by the Radio and Plasma Wave Science (RPWS) investigation onboard Cassini, which allows for interferometry of plasma density inhomogeneities (δn/n) with two antenna elements and a Langmuir probe sensor. The interferometer experiment detects two ion populations; one co-rotating with the planetary magnetic field and another moving with near Keplerian speed around Saturn. The full range of RPWS measurements indicates that the Keplerian population consists of colder ions (Ti<few eV), which would interact with the electrical potential cavities associated with the few volts negatively charged E-ring water-rich dust grains. The presence of the charged dust inhibits E×B pick-up of freshly ionized particles by the rotating magnetic field of Saturn, since the local potential gradients near the dust grains are stronger than the large-scale co-rotation electric field. Even so, most ions are eventually energised to energies above the dust potentials of a few volts and become part of a hotter co-rotating ion population. The observed results have direct relevance to the interactions occurring in planet forming accretion disks around young stars, since the physical environment is similar.