The motion of thermal ions is surprisingly unresolved at Saturn. Due to Saturn’s strong magnetic field it is expected that plasma close to planet will co-rotate with the magnetic field, but where does this break down? The two Voyager fly-bys suggest this happens around 6 Saturn radii from the planet, although the passes were non- equatorial. One exciting result from the Cassini orbiter was the discovery that Enceladus (at 4 Saturn radii) is a primary source of magnetospheric plasma, but does the subsequent mass loading of field lines by the fresh plasma inhibit rigid co-rotation?
Cassini’s Plasma Spectrometer (CAPS) ion counting data provides the ability to calculate thermal ion moments, such as ion density, temperature, flow speed and composition, however due to various features this is not a simple matter. Two techniques are described that allow moments values to be calculated, allowing an equatorial velocity profile to be calculated from in-situ measurements on Saturn’s day-side for the first time.