The Structure of the warped Io Plasma Torus constrained by the Io Footprint

Standard models of force balance along Jovian field lines predict the location of the Io plasma torus to be the centrifugal equator of Jupiter's magnetosphere, i.e. the position along the magnetic field lines farthest away from Jupiter's rotational axis. In many models, the centrifugal equator is assumed to lay on a plane, calculated from a (shifted) dipole magnetic field, rather than on a warped surface which incorporates Jupiter's higher magnetic field moments. In this work, we use Hubble Space Telescope observations of the Io Main Footprint to constrain density, scale height and lateral position of the Io Plasma Torus. Therefore, we employ the leading angle of the footprints to calculate expected travel times of Alfven waves and carry out an inversion of the observations. For the magnetic field we use the JRM33 magnetic field model. The inversion results show peak densities between 1830 and 2032 particles per cubic centimenter and scale heights between 0.92 and 0.97 Jupiter radii consistent with current literature values. Using a warped multipole centrifugal equator instead of a planar dipole increases the quality of the fit by about twenty-five percent. We additionally develop two tests to confirm that the multipole centrifugal equator from the JRM33 model fits explains the applied data set better than the dipole centrifugal equator. The quadropole moments alter Io's relative position to the torus, which changes the plasma density around Io by up to twenty percent.