Modeling Features of Field Line Resonance Observable by a Single Spacecraft at Saturn.

The observations of Southwood et al. (2021, https://doi.org/10.1029/2020JA028473), using data from the Cassini magnetometer from the final (proximal) orbits of the mission at Saturn, show large scale azimuthally polarized magnetic signals are always present near periapsis. The signals were attributed to standing Alfvén waves excited on the magnetic shells planetward of the Saturn D‐ring. The apparent absence of any systematic variation in frequency as the spacecraft crossed magnetic shells, implied that the signals were not simply locally excited standing Alfvén modes, but were pumped by coupling to global compressional eigenmodes excited in a cavity formed in the dayside magnetosphere. In this study, we use a numerical magnetohydrodynamic (MHD) model to test such theoretical explanations for the observations, by examining in detail the MHD wave coupling and large scale spatial structure of the signals. The modeling not only shows good agreement with the data but further provides new insight into features previously overlooked in the data. In particular, we show how the apparent frequency of a single spacecraft observation is affected by the phase variation present in a local field line resonance. Plain Language Summary: On the final orbits of the Cassini Saturn Orbiter, one surprise was the discovery of large magnetic oscillations perpendicular to the background planetary field. The disturbances were present each time the spacecraft passed planetward of the innermost rings. This study models what a spacecraft would see if the signals were large scale magnetohydrodynamic waves excited within a high‐density plasma. The strong alignment of the magnetic oscillations transverse to the background led to the original proposal that the signals are highly localized Alfvén waves in what is known as field line resonance. The simulations, however, reveal subtle aspects of the overall picture and how the system is being excited. In particular, it is shown that on the quasi‐polar orbits such as Cassini followed one expects some unexpected features, such as the signal frequency detected in the largest field component differs between hemispheres. Moreover, the frequency detected on the spacecraft might substantially differ in field components in different directions. The conclusion that the signals are resonantly excited Alfvén waves hold up but the simulations show that such excitation must have a complicated spatial amplitude and phase structure. A re‐examination of the Cassini data has revealed some of the effects discovered in the computer simulations. Key Points: Simulation results are used to test and validate the interpretation by Southwood et al. (2021, https://doi.org/10.1029/2020JA028473) wave observations at SaturnThe spatial phase structure in field line resonances can shift the observed frequency on a moving spacecraft between in/outbound passesWe highlight how such frequency changes can be accounted for in both the spacecraft and simulation data [ABSTRACT FROM AUTHOR]