The MI-coupling in global simulations of the Jovian and Kronian magnetospheres

A new model of the interactions between the solar wind and the magnetospheres of Jupiter and Saturn is presented. It differs from other models by explicitly introducing the Magnetosphere-Ionosphere coupling via an extended ionospheric region, located inside the simulation domain, where ion-neutral collisions are included in the MHD equations. Since the MI-coupling is included inside the simulation domain (above the inner boundary), the current system is closed inside the simulation domain and the boundary conditions do not interfere with the MI-coupling. This ionospheric region, even though not an accurate representation of the Jovian ionosphere, is characterized through its Pedersen conductance and imposes through its coupling with the magnetosphere good results at magnetospheric locations where the corotation breaks down and further outside. In addition, the mass-loading caused by Io or Enceladus is introduced in an axi-symmetric toroidal region where a ionization source term is added to the MHD equations. With this model, two key parameters of the giant planets magnetospheres can be controlled: namely the ionospheric conductance and the mass-loading associated with Io or Enceladus. To test our model, numerical experiments are performed where the ionospheric conductance and the mass-loading are changed; the results are then compared with measurements and analytical models. For Jupiter's magnetosphere, for example, the position of the corotation break-down and the azimuthal velocity profile are in good agreement with analytical models. We do not observe any spurious supercorotation (as often seen in global MHD simulations of Jupiter's magnetosphere). The density profiles and radial velocity profiles are also compared with observations and theory and give satisfactory results. As expected by theory, in our simulations, the position of the corotation break-down maps to the main auroral emission. Since the current systems are closed inside the ionospheric region above the inner boundary no current is lost or gained through this boundary; the total current flowing through the ionosphere (39.6 MA) agrees with estimates from measurements and analytical models.