Flux Tube Entropy and Specific Entropy in Saturn's Magnetosphere

摘要

The motivation of this paper is to discuss the dynamical processes in Saturn's magnetosphere from the plasma entropy perspective. Saturn's magnetosphere is stabilized by a radially increasing profile of flux tube entropy and destabilized by a radially decreasing profile of flux tube content. The traditional radial transport scenario suggested that the magnetic flux with heavy flux tube content moves from the inner magnetosphere to the outer magnetosphere, stretching the magnetic field into a magnetodisc configuration. Subsequently, magnetic flux with low flux tube entropy generated by magnetodisc reconnection circulates back to the inner magnetosphere. However, the low-specific entropy plasma with a narrow distribution in Saturn's inner magnetosphere suggests a significant nonadiabatic cooling process during the inward motion. The flux tube entropy analysis suggests that energetic particles dominate the total flux tube entropy in the magnetodisc region, and newly closed field lines generated by magnetodisc reconnection are likely to be transported into the inner magnetosphere. Based on the flux tube entropy constraint, this study demonstrates that the radial transport process in Saturn's magnetosphere can also be achieved via middle-latitude double reconnection, driven by a low-latitude interchange instability. This process does not involve significant latitudinal convection of magnetic flux in the ionosphere nor does it significantly modify the radial flux tube entropy profile.