Rate of radial transport of plasma in Saturn's inner magnetosphere

Abstract

In the inner part of a rapidly rotating magnetosphere such as that of Saturn, the major observable signature of radial plasma convection is a series of longitudinally localized injections and simultaneous drift dispersions of hot tenuous plasma from the outer magnetosphere. The Cassini Plasma Spectrometer (CAPS) and the Cassini Magnetospheric Imaging Instrument (MIMI) have observed signatures of these processes frequently, thus providing direct evidence for Saturn's magnetospheric convective motions, in which the radial transport of plasma comprises hot, tenuous plasma moving inward and cooler, denser plasma moving outward. On the basis of an extended statistical sample of these injection/dispersion events, we find that the inflow channels occupy only a small fraction (∼7%) of the total available longitudinal space, indicating that the inflow speed is much larger than the outflow speed. We assume that the plasma is largely confined to a thin equatorial sheet and calculate its thickness by deriving the centrifugal scale height profile based on the CAPS observations. We also present the radial and longitudinal dependences of flux tube mass content as well as the total ion mass between 5 and 10 Saturn radii. Combining these results, we estimate a global plasma mass outflow rate ∼280 kg/s.