The roles of flux-tube entropy and effective gravity in the inward plasma transport at Saturn

The inward plasma transport at Saturnian magnetosphere is examined using the flux tube interchange stability formalism developed by Southwood and Kivelson (1987).  Seven events are selected.  Three cases are considered: (1) the injected flux-tube and ambient plasmas are nonisotropic; (2) the injected flux-tube and ambient plasmas are isotropic and (3) the injected flux-tube plasma is isotropic but the ambient plasma is nonisotropic.  Case (1) may be relevant for fresh injections while case (3) may be relevant for old injections.  For cases (1) and (2), all but one events have negative stability condition, suggesting that the flux-tube should be moving inward.  For case (3), the injections located at L > 11 have negative stability condition, while 4 out of 5 of the injections at L < 9 have positive stability condition.  The positive stability condition for small L suggests that the injection may be near its equilibrium position and possibly oscillating thereabouts---hence the outward transport if the flux tube overshot the equilibrium position.  The flux-tube entropy plays an important role in braking the plasma inward transport.  When the stability condition is positive, it is because the entropy term, which is positive, counters and dominates the effective gravity term, which is negative for all the events.  The ambient plasma and drift out from adjacent injections can affect the stability and the inward motion of the injected flux tube. The results have implications to inward plasma transport in Jovian magnetosphere as well as other fast rotating planetary magnetospheres.