Regulation of the centrifugal interchange cycle in Saturn's inner magnetosphere

Abstract

Multifluid modeling of Saturn's magnetosphere produces the first numerical simulation showing the development of hot, tenuous plasma from the plasma sheet interchanging with cold, denser plasma from the inner magnetosphere. Individual injection events are seen regularly by Cassini, but with a single observation it is impossible to determine the global distribution. Multifluid simulations enable us to characterize the growth and development of not merely one injection event but show that it is a global process dependent on both the plasma distribution of ions from Enceladus and forcing by solar wind conditions. Development of the interchange arises in a fashion similar to a Rayleigh‐Taylor instability, except that the heavy ions are being driven outward not by gravity but by centrifugal forces. Interplanetary magnetic field (IMF) parallel to the planetary magnetic field reduces centrifugal forcing, whereas antiparallel IMF increases the forcing, by altering the bowl‐like shape of the plasma sheet. However, the interchange instability also develops under normally quiet parallel IMF conditions when the mass loading of the Enceladus torus is increased. The total number of interchange events is 1–2 higher for the antiparallel case versus the increased mass case. Interchange develops in the vicinity of 7 RS, and once the fingers of cold plasma reach ∼12–14 RS (close to the inner edge of the plasma sheet) they spread in the azimuthal direction, because of the fact that the magnetic field is too weak to keep the fingers solidly locked in rotation. The derived energy characteristics of the interchanging plasma are shown to be consistent with Cassini data.