Coupled low‐energy ‐ ring current plasma diffusion in the Jovian magnetosphere

Abstract

We derive a functional form of the diffusion coefficient appropriate to fully developed, steady state, centrifugally driven turbulence in Jupiter's magnetosphere, incorporating in the diffusion coefficient the effects of the pressure gradient of the ring current. We set up a steady state, coupled diffusion model to describe the simultaneous outwardly diffusing low‐energy (Iogenic) plasma and the inwardly diffusing ring current plasma. The model is a nonlinear, third‐order, ordinary differential system containing four parameters. In order to test the hypothesis that the pronounced reduction in the radial diffusion coefficient (implied by the existence of the Io plasma ramp) is caused by the pressure gradient of the ring current, we seek solutions to the model under the condition that a reduced form of the diffusion coefficient takes on a minimum value at some point within the spatial extent of the ramp, i.e., within the range 7 ≤ L ≤ 8, where L is radial distance in units of Jovian radius RJ. This condition leads to several necessary conditions on the model parameters and the values of the ring current density and density gradient at the minimum point. We find that high‐energy (0.5–1.05 MeV) ring current particles, which have been previously observed and whose density decreases inwardly by several orders of magnitude over the range 6 ≤ L ≤ 10, cannot cause a sufficient inhibition of Iogenic plasma diffusion to cause the ramp. Further, we show that for ring current pressure to cause the ramp there must exist a component of the ring current particle population with sufficiently low thermal energy and with sufficiently high density, e.g., corresponding to an Iogenic plasma flux of 2×1028 s−1 the bulk of the ring current particles must have thermal energies of less than 3.9 keV and corresponding ring current particle densities at, say, L = 7.5 must exceed by two orders of magnitude those observed for the high‐energy component. We infer the existence of this dense, low‐energy component of the ring current from an independent energy calculation. Our model predicts that the density of the dense, low‐energy ring current particles decreases inwardly only by about an order of magnitude over the range 6 ≤ L ≤ 10. We provide representative numerical solutions for the density profiles of the Iogenic and ring current plasmas.