Traveling Through Field Line Resonances at Saturn

Using a 3D MHD simulation refined for the analysis of terrestrial ultralow frequency (ULF) wave signals in a dipole background field, we have reproduced multiple features of magnetohydrodynamic waves observed during the final stages of the Saturn Cassini orbiter mission by the Cassini spacecraft magnetometer. These recently published observations show azimuthally polarised magnetic field pulsations which are present on each of the 22 Cassini proximal orbit periapsides, in the region of high plasma density planetward of the D-ring magnetic shells. We model these oscillations as field line resonances (FLRs), driven by the natural global modes of a larger magnetospheric cavity outside of the D-ring. We show that the high density inner region where the waves are observed can be effectively pumped by this global mode of the outer cavity, with the imposed boundary conditions leading to a reservoir of fast mode energy there to drive the observed FLRs. We analyse in detail the mode structure of this coupled system. Furthermore, we reproduce 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. In particular, the complex phase structure of the azimuthal magnetic field component in the vicinity of the FLR is found to provide significant Doppler shifts. This effect leads to a counterintuitive result, that the azimuthal field measured in the spacecraft frame may show a different apparent frequency to that of the meridional components. We will provide examples of this, as well as the effect of north-south asymmetry, with comparisons to the observed signals. We will further discuss the implications of these results for the spacecraft observation of FLRs at Earth and other planets.