Dust-magnetosonic modes in planetary rings

Dust particles on near-Keplerian orbits in planetary rings drift relative to the co-rotating background plasma. Such relative streaming may drive dust-magnetosonic instabilities. Using a magnetofluid model, we revisit this situation, and include the possibility of polydisperse dust. In particular, we show that the growth rate is lower than anticipated from earlier work and that the instabilities cannot occur close to synchronous distance from the planet.