The centrifugal flute instability and the generation of Saturnian kilometric radiation

Abstract

We show that the source region for Saturnian kilometric radiation (SKR) which originates in the high latitude near‐noon dayside ionospheric can be mapped via the Saturn magnetic field to the outer edge of the dayside equatorial plasmasheet. The plasmasheet is known to be unstable at this boundary due to the outward centrifugal forces generated by the heavily mass‐loaded equatorial Saturnian magnetosphere. Also previous work has shown that Voyager 1 observations of MHD waves near the plasmasheet are consistent with their being driven by the centrifugal flute instability. The resulting field‐aligned MHD waves at higher latitudes can trap and accelerate electrons. For the plasma environment measured by Voyager, field‐aligned accelerated electrons up to energies of several keV are expected to be deposited into the SKR ionospheric source region. A necessary condition for the acceleration of the electrons to several keV is a population of hot electrons inside of the plasmasheet, near the unstable boundary. The hot electrons are produced as a byproduct of the pick‐up of exospheric atoms from Saturn's moons. The solar wind dependence of the SKR is thus a consequence of the elongation of the Saturn nightside magnetosphere with greater solar wind pressure leading to greater centrifugal pressures on the nightside plasmasheet. Although the theory presented correctly gives the local time dependence of the emissions, we suggest that the strong emission peak at a fixed Saturnian longitude is a consequence of locally reduced electron plasma frequency to electron gyrofrequency ratio. The lack of a nightside source of SKR is consistent with the disruption of the plasmasheet due to loss of plasma down the Saturn magnetotail.