Dust-plasma interaction through the magnetosphere-ionosphere coupling in Saturn's inner magnetosphere

A few hundred eV ion observations by the particle detectors on the Cassini spacecraft showed that the plasma speeds in the Saturn's inner magnetosphere are close to the ideal co-rotation speed around 5 Rs and gradually become 70--80% of the ideal co-rotation speed at 7 Rs. On the other hand, the Cassini observation using the Langmuir Probe (LP) showed that the ion bulk speeds are close to Keplerian in the E ring. The E ring of Saturn consists of small (micron- and nano- meter sized) dust grains. These dusts are negatively charged inside 7 Rs and expected to contribute to the electro dynamics in the plasma disk. Near Enceladus, which is a major source of the E ring dusts, the electron densities are significantly smaller than the ion densities and the ion speeds are near Keplerian. Recently the Cassini Plasma Spectrometer (CAPS) has also observed that the sub-co-rotation speed of the ions slows down to 50-80% of the ideal co-rotation speed. We investigated statistically the ion bulk speeds in the equatorial region of the inner magnetosphere using the LP onboard the Cassini spacecraft from February 2005 to October 2008 (Rev003--087). The LP observations showed that the ion speeds are about 60% of the ideal co-rotation speed at 5 Rs. Beyond 7 Rs the ion speed values are spread toward the ideal co-rotation speed. This may come from that the sub-micron sized negatively charged E ring dust contributes to the plasma dynamics in the plasma disk. We have also calculated the ion speeds using the multi-component MHD equations including dust to investigate the effect of the ion-dust coulomb collision and the mass loading. Our model shows that the ion-dust collision can reduce the ion acceleration by the co-rotation electric field when the ion-dust collision frequency is comparable to the ion cyclotron frequency, and that the ions significantly slow down from co-rotation speed by mass loading. The ion-dust collision and the mass loading generate currents and magnetopheric electric fields in the inner magnetosphere, i.e. a magnetosphere-ionosphere (M-I) coupling. The magnetosperic electric fields based on the M-I coupling reduce the co-rotation electric fields in the inner magnetosphere and can slow the ion speeds. We attribute the ion speeds with about 60% of the ideal co-rotation speed at 5 Rs to coming from the M-I coupling through the dust-plasma interaction in the inner magnetosphere.