Tethys and Dione as sources of outward-flowing plasma in Saturn’s magnetosphere

Abstract

Rotating at over twice the angular speed of Earth, Saturn imposes a rapid spin on its magnetosphere. As a result, cold, dense plasma is believed to be flung outward from the inner magnetosphere by centrifugal force and replaced by hotter, more tenuous plasma from the outer magnetosphere. The centrifugal interchange¹ of plasmas in rotating magnetospheres was predicted many years ago^2,3,4 and was conclusively demonstrated by observations in Jupiter’s magnetosphere^5,6,7, which—like that of Saturn (but unlike that of Earth)—is rotationally dominated. Recent observations in Saturn’s magnetosphere^8,9,10 have revealed narrow injections of hot, tenuous plasma believed to be the inward-moving portion of the centrifugal interchange cycle. Here we report observations of the distribution of the angle between the electron velocity vector and the magnetic field vector (‘pitch angle’) obtained in the cold, dense plasma adjacent to these inward injection regions. The observed pitch-angle distributions are indicative of outward plasma flow and consistent with centrifugal interchange in Saturn’s magnetosphere. Further, we conclude that the observed double-peaked (‘butterfly’) pitch-angle distributions result from the transport of plasma from regions near the orbits of Dione and Tethys, supporting the idea of distinct plasma tori associated with these moons^11,12,13.