The electric potential of the charged dust near the E ring

Recent Cassini observations revealed that the moon Enceladus expels water vapour and ice grains from its south pole and forms a plume that becomes the major source for the E ring and the surrounding neutral gas. The Enceladus' plume and dispersed neutral gas produces a large amount of ionized gas, which become a dominant plasma source for Saturn's magnetosphere. A striking feature is that small grains in the plume and in the E ring are negatively charged and electrically couples to the ambient plasma electrically. Cassini Radio and Plasma Wave Science (RPWS) measurements of the cold plasma properties observed a large ion and electron density difference (Ne/Ni < 0.01-0.5) that is associated with the micro-meter sized dust grains near Enceladus plume and the E ring region. At the same time the ion bulk speed is slowed down to near to the Keplerian speed. The conditions are such that rd << dg << D, where rd, dg and D are the typical grain radius, the inter-grain distance and the plasma Debye length, respectively, a so called "dusty plasma". Large ion and electron density differences (Ne/Ni < 1) can be actually found not only near the Enceladus plume but also in a wide area around Saturn's E ring and the plasma disc. To investigate if the charged dust is collectively coupled to the background plasma (i.e., the dusty plasma state), the electric potential of the dust is an important parameter. We use the RPWS/LP data to statistically investigate the dust electrical potential near the E ring. We will present the characteristics of the dust electric potential and discuss the dust-plasma interaction in the E ring and the plasma disc.