Plasma convection in Saturn's outer magnetosphere determined from ions detected by the Cassini INCA experiment

The Ion and Neutral Camera (INCA), one of three sensors comprising the Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft, measures intensities of hydrogen and oxygen ions and neutral atoms in the Saturnian magnetosphere. The measured intensity spectrum and anisotropy of hot hydrogen and oxygen ions may be used to deduce the spectral parameters and the velocity of the ion population. The anisotropies are frequently convective in nature, allowing for the determination of a bulk velocity. Under the frozen in'' assumption, this is also the velocity of the cold plasma of magnetospheric ions. Initial analysis of selected measurements of nightside ion populations with strong anisotropies indicates nearly rigid corotation of magnetospheric plasma interior to Titan's orbit. Beyond this distance, these measurements infer that the plasma maintains at best a constant rotation velocity, falling farther behind the rigid corotation rate at increasing distance.