ALBERT

Description: The interaction of planetary bodies with their surrounding magnetized plasma can often be described with the magneto-hydrodynamic (MHD) equations, which are commonly solved by numerical models. For these models it is necessary to define physically correctboundary conditions for the plasma mass and energy density, the plasma velocity and the magnetic field. Many planetary bodies have electrically non-conductive surfaces, which do not allow electric current to penetrate their surfaces. Magnetic boundary conditions, which consider that the associated radial electric current at the planetary surface is zero are difficult to implement because they include the curl of the magnetic field. Here we derive new boundary conditions by a decomposition of the magnetic field in poloidal and toroidal parts. We find that the toroidal part of the magnetic field needs to vanish at the surface of the insulator. For the spherical harmonics coefficients of the poloidal part we derive a Cauchy boundary condition, whichalso matches a possible intrinsic field by including its Gauss coefficients. Thus we can additionally include planetary dynamo fields as well as time-variable induction fields within electrically conductive subsurface layers. We implement the non-conducting boundary condition in the MHD simulation code ZEUS-MP using spherical geometry and provide a numerical implementation in Fortran 90 as auxiliary-material on the JGR website. We apply it to a model for Ganymede's plasma environment. Our model also includes a consistent set of boundary conditions for the other MHD variables density, velocity and energy. With this model we can describe Galileo spacecraft observations in and around Ganymede's mini-magnetosphere very well.