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

In brief

1-hour waves have been observed around Saturn for a long time, but their origin remains poorly understood. We study them as resonances of Saturn's magnetic field — like a plucked guitar string vibrating at its natural frequency. Approximating Saturn's field as a bar magnet is not enough to reproduce 1-hour waves across the large regions far outside the planet; using a realistic plasma mass density along the field, the higher harmonics of these vibrations match the observed 1-hour period.

Abstract

We model the omnipresent quasiperiodic 60 min waves in Saturn's outer magnetosphere as field line resonances adopting a realistic magnetic field model and a measurement-based plasma density distribution using Alfvén wave resonance theory for arbitrary field geometries (Singer et al., 1981). The modeled eigenfrequencies for the second and higher modes are roughly independent of invariant latitude, mapping into large regions of the magnetosphere up to at least 20 R_S, and the third and fourth harmonic modes having close to 1 h eigenperiod. The model predicts the normalized amplitudes of these higher modes of the magnetic field perturbations at high latitudes to exceed the amplitudes at the plasma sheet, in agreement with the observations of more frequent occurrences at mid-to-high latitudes. The periods of the higher order field line resonances in the outer magnetosphere differ considerably from those in a dipole model, illustrating the importance of a realistic field model.

Key findings

• Saturn's quasiperiodic 60-minute Alfvén waves can be modeled as field line resonances — the third and fourth harmonics.
• Realistic plasma density and field models are critical for accurate eigenfrequencies, especially in the outer magnetosphere.
• Neither a hydromagnetic box model nor a dipole approximation is adequate in the outer magnetosphere.
• Eigenfrequencies of the second and higher modes are roughly independent of invariant latitude, mapping out to ~20 R_S.
• Higher-order modes have larger amplitudes at mid-to-high latitudes than at the plasma sheet — consistent with Cassini observations.
• Resonance periods in a realistic field differ substantially from dipole estimates, validating the use of actual field geometry.

Figures

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Figures reproduced from Rusaitis et al. (2021), Geophysical Research Letters, doi:10.1029/2020GL090967.

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