Quasiperiodic 1-hour Alfvén Wave Resonances in Saturn’s Magnetosphere: Theory for a Realistic Plasma/Field Model

Abstract:

In this study, we model the widely known, but not fully understood, quasi-periodic 60-minute perturbations in the magnetic field and particle data in Saturn’s outer magnetosphere. Many observations of these waves were recorded and analyzed during Cassini’s 13-year mission at Saturn, [Roussos et al., 2016; Palmaerts et al., 2016] but the mechanism that produces these waves has not been identified. The waves share the Alfvénic nature of field line resonances in the Earth’s magnetosphere [Radoski, 1971; Southwood & Kivelson, 1986] but at Saturn the wave frequency is independent of invariant latitude. The waves have been modeled previously assuming a plasma density concentrated near the equator [Yates et al., 2016]. However, the analysis assumed a uniform-field magnetospheric box model, and a box-function plasma density independent of distance from Saturn. In this study, we model the field line resonances adopting a more realistic Khurana magnetic field model and a measurement-based plasma density distribution [Bagenal and Delamere, 2011], using Alfvén wave resonance theory for arbitrary field geometries [Singer et al., 1981]. The model eigenfrequencies for the second and higher modes are roughly independent of invariant latitude between 72 and 76 degrees on the dayside, and between 72 and 74 degrees on the nightside, the third and fourth harmonic modes having close to 1-hour eigenperiod. These higher-mode field lines resonances have an eigenperiod roughly consistent with the observed period at distances of 10-20 R_S in the outer magnetosphere. We suggest they correspond to the mysterious quasi-periodic 60-minute pulsations at Saturn.