Io torus plasma transport under interchange instability and flow shears

The radial transport of plasma in the Io torus has been considered to be controlled by interchange instability, whereas the time scale (≈2080 days) estimated from matching physical chemistry models to spectral observations was much larger than the scale (≤1 day) expected from the process. Previous studies using a magnetospheric convection model suggested that a corotation lag associated with mass ejection from Io can suppress the plasma transport. However, essential processes of fluid dynamics, vortex formation and nonlinear mode interactions were not taken into account but have a critical role in transport and structure development due to plasma instabilities. We performed nonlinear simulations of interchange instability in the Io torus with a reduced magnetohydrodynamic model in two-dimensional dipole magnetic field coordinates. If a single azimuthal perturbation in density is initially given, pure finger-like patterns develop over ≈1 day but are stabilized due to a strong gradient of magnetic fields. If the initial torus density increases, vortex patterns are spontaneously formed through a nonlinear mode coupling, resulting in a collapse of the fingers. In the case of a multiple perturbation, a merging of vortices appears and the torus plasma expands radially to ≈8R _J. The radial transport time scale at the saturation stage of convection energies is estimated to be ≈3 days in this case, while it is ≈72 days in the single perturbation case. By including effects of the corotation lag, a radially striated structure is produced within 5.5-7R _J, but the radial transport is still fast, ≈2 days. We conclude that the radial transport could be controlled by the nonlinear effects of fluids and discuss implications for the ribbon-like structures inside the torus as well as a contribution of the corotation lag.