Survey of low‐energy plasma electrons in Saturn's magnetosphere: Voyagers 1 and 2

This paper is a survey of the low‐energy plasma electron environment within Saturn's magnetosphere made by the plasma science experiment (PLS) during the Voyager encounters with Saturn. Over the full energy range of the PLS instrument (10 eV to 6 keV) the electron distribution functions are clearly non‐Maxwellian in character; they are composed of a cold (thermal) component with Maxwellian shape and a hot (suprathermal) non‐Maxwellian component. A large‐scale positive radial gradient in electron temperature is observed, increasing from less than 1 eV in the inner magnetosphere to as high as 800 eV in the outer magnetosphere. This increase in electron temperature explains the observed order of magnitude increase in plasma sheet thickness with increasing radial distance from Saturn. Scale heights of the cold heavy ion plasma can be as small as 0.2 R s in the inner magnetosphere and as much as 3 R s in the outer magnetosphere. Many of the observed density variations can be attributed to changes in density scale height without a change in plasma flux tube content. Three fundamentally different plasma regimes have been identified from the measurements: (1) the hot outer magnetosphere, (2) the extended plasma sheet, and (3) the inner plasma torus. The hot outer magnetosphere is a region within which the suprathermal electrons are the dominant contributors to the electron pressure, and at times to the electron density. Near the noon meridian, the electrons display a highly time dependent behavior with order of magnitude changes in density and temperature, which can occur in less than 96 s. Sudden density enhancements of cold plasma occur, which are thought to be either “plumes” associated with Titan (Eviatar et al., 1982) or plasma “blobs”, ( Goertz , 1983). The extended plasma sheet, with mean inner and outer boundaries of 7 and 15 R s , respectively, has enhanced levels of cold plasma relative to that in the hot outer magnetosphere, although the suprathermals continue to dominate the electron pressure. Plasma with energy less than 6 keV from this region is an important contributor to the ring current, and a significant current system is probably present between 6 and 8 R s . The inner plasma torus is a region of reduced electron temperature (as low as 1 eV), enhanced equatorial densities (as high as 100/cm³), and reduced scale height (as small as 0.2 R s ). Localized reductions in electron temperature are observed near the L shells of Tethys, Dione, and possibly Rhea. The suprathermal electrons are observed to be severely depleted within the inner plasma torus, relative to that in the extended plasma sheet and hot outer magnetosphere. The energy dependence of the depletions indicate an interaction with dust or plasma waves at times; interactions with neutral gas or plasma ions may also contribute to the depletions. The data also indicate an association between the appearance of suprathermal electrons and the observed emission of whistler mode waves reported by Gurnett et al. (1981).