Dust-acoustic waves in collisional dusty plasmas of planetary rings

Aims. The effects of ion and neutral drag forces on stability of low-frequency wave modes is investigated from the point of view of parameters relevant to the plasma environment in the vicinity of the main Saturn rings. The electrons and ions are considered to be magnetized and corotate with the planet, while the dust grains are not magnetized and move around the planet on Keplerian orbits. Methods. The wave modes of low-frequencies in such dusty plasma, propagating along the ring in the azimuthal direction are analyzed based on the susceptibilities derived from the standard fluid approach and model presentation of the drag forces valid in the parameter regime of the dusty plasma of planetary rings. Results. It is found that the ion drag force can crucially change the stability of dust-acoustic perturbations. Inside the co-rotation distance the ion drag force can be responsible for the excitation of dust-acoustic waves, while for the region outside the synchronous orbit this mode can hardy be excited ( at radial distances corresponding to the main rings). The instability due to the ion drag force could be also of some importance for perturbations in remote rings, where the relative ion-dust velocities exceed their thermal speed.