Monday

Using a 3D MHD simulation refined for analysis of terrestrial ULF signals in a dipole background field, we have reproduced multiple features of MHD waves observed during the final stages of the Saturn Cassini orbiter mission by the Cassini spacecraft magnetometer. We do this by reproducing what a spacecraft would see by analysing fields in the simulation as a time series along a curve through the simulation volume, representing the actual spacecraft orbit. Despite needing to allow for amplitude variation with latitude and magnetic shell, the Cassini measurements can be well modelled in this way. In particular, the complex phase structure of the azimuthal component in the vicinity of field line resonance is found to provide significant Doppler shifts. This effect leads to a counterintuitive result, that the resonant structure means the azimuthal field measured in the spacecraft frame may show a different apparent frequency to that of the meridional components. The signals detected by Cassini are always present and appear in two separate bands with the same (even) north-south symmetry in magnetic field. The fit between model and observation implies not only that the wave energy originates probably from buffeting of the dayside magnetosphere cavity but also that local resonant energy absorption is rapid enough to preclude development of large scale phase mixing. Ionospheric damping must thus be effective and wave energy is deposited in the high altitude low latitude atmosphere.