Harmonic growth of ion‐cyclotron waves in Saturn's magnetosphere

Abstract

[1] Ion-cyclotron waves have been observed by the Cassini spacecraft in Saturn's middle magnetosphere. Waves have frequencies near the gyrofrequencies of water-group ions, they are left handed, and in most regions they propagate at small angles to the ambient magnetic field. Their origin is explained in terms of the ion-cyclotron instability generated by water-group ions picked up from Saturn's neutral cloud. Additionally, in some regions dynamic spectra reveal the existence of fluctuations at approximately twice the value of the ion gyrofrequency. These waves have a significant compressional component and propagate at oblique angles to the magnetic field. Their characteristics suggest that these fluctuations can be identified as a harmonic mode (n = 2) generated by the ring distributions of pickup ions. In this work we study in detail the characteristics of these harmonic mode waves. We also evaluate growth rates using a kinetic dispersion analysis.

Electromagnetic ion cyclotron wave generation by planetary pickup ions: One‐dimensional hybrid simulations at sub‐Alfvénic pickup velocities

“…It is important to note that the Extended Neutral Cloud region is very magnetically quiet compared to the environment at Io, such that the typical wave amplitudes in the region would have been too low to be observed at Io. The magnetically quiet nature of the Extended Neutral Cloud region also revealed the presence of weak waves at the twice the H 2 O + ‐group cyclotron frequency [ Rodriguez ‐ Martinez et al , 2008]. These waves propagated at 20° from the magnetic field and had roughly half the transverse power of and more compressional power than the fundamental mode.…”

“…In our simulations, where the spatial axis is aligned with the magnetic field, we do not model obliquely propagating waves or the generation of ion cyclotron harmonics waves, although such waves would certainly be possible in the system. Dispersion solutions for the waves generated by water‐group pickup ions near Enceladus showed that the ion cyclotron ring instability is the dominant modes [ Leisner et al , 2006; Russell et al , 2006] and that weaker growth at the cyclotron harmonics is also possible [ Rodriguez ‐ Martinez et al , 2008]. The growth rate decreases as the harmonic is increased, but dispersion solutions for the n = 2 harmonic at 50° propagation has a maximum growth rate less than one‐fifth that of the fundamental frequency at parallel propagation.…”

Hybrid Simulations of Positively and Negatively Charged Pickup Ions and Cyclotron Wave Generation at Europa

“…Warm plasma dispersion theory and hybrid simulations have previously constrained the behavior of the Alfvén‐cyclotron instability generated by heavy SO ion rings in the Io plasma torus (Cowee et al, ; Huddleston et al, , ) and also for water group pickup ion rings within Saturn's extended neutral cloud (Cowee et al, ; Leisner et al, ; Rodríguez‐Martínez et al, ). This study focuses on the Europan plasma environment and draws comparisons to these studies.…”

1. Transport of Mass, Momentum and Energy in Planetary Magnetodisc Regions

“…The work by supports the conclusion that these ions are a consistent, but minor, plasma species in the magnetosphere. There were a few intervals where the dominant waves had a frequency near the value Ω(O + 2 ) (Rodríguez-Martínez et al 2010). The variation of peak amplitudes indicates that the pickup ion composition changes across the extent of the neutral cloud.…”

“…Ring distributions have more energy to give to the waves as these two values increase, and this is reflected in enhanced values of growth rate γ . Cassini data also has revealed the presence of weak waves at twice the cyclotron frequency of the water group ions (Rodriguez-Martinez et al 2008). These waves propagate at angles 20…”