Saturn chorus intensity variations

Abstract

[1] Whistler mode chorus plasma wave emissions have been observed at Saturn near the magnetic equator and the source region. During crossings of the magnetic equator along nearly constant L shells, the Cassini Radio and Plasma Wave Science Investigation often observes a local decrease in whistler mode intensity and bandwidth closest to the magnetic equator, where linear growth appears to dominate, with nonlinear structures appearing at higher latitudes and higher frequencies. We investigate linear growth rate using the Waves in a Homogeneous, Anisotropic, Multi-component Plasma dispersion solver and locally observed electron phase space density measurements from the Electron Spectrometer sensor of the Cassini Plasma Spectrometer Investigation to determine the parameters responsible for the variation in chorus intensity and bandwidth. We find that a temperature anisotropy (T ⊥ /T ∥~1 .3) can account for linear spatiotemporal growth rate of whistler mode waves, which provides a majority of the observed frequency-integrated power. At the highest frequencies, intense, nonlinear, frequency-drifting structures (drift rates~200 Hz/s) are observed a few degrees away from the equator and can account for a significant fraction of the total power. Chorus emission at higher frequencies is distinct from lower frequency whistler mode emission and is sometimes correlated with simultaneously observed low-frequency electromagnetic ion cyclotron waves. These electromagnetic ion cyclotron waves appear to modulate a slow frequency drift (~15 Hz/s) which develops into nonlinear growth with much larger frequency drift associated only with the higher-frequency chorus.

“Zipper‐like” periodic magnetosonic waves: Van Allen Probes, THEMIS, and magnetospheric multiscale observations

“…It is possible that there may still be an external driver that modulates the wave excitation, which has not been identified or observed yet, or that the periodicity of the MS waves arises due to an internal instability, analogous to what has been suggested for whistler mode chorus waves [e.g., Bespalov et al , ; Trakhtengerts , ]. The observation of zipper‐like MS waves exhibits additional complexity but may provide a new clue in understanding the causative mechanism of the modulation, which is important since periodic wave structures have been observed in a variety of waves [ Fu et al ., ], including chorus [e.g., Hayosh et al , ; Němec et al , ; Manninen et al , ] and EMIC waves [e.g., Pickett et al , ; Grison et al , ; Sakaguchi et al , ], and they have been observed at other magnetized planets as well [e.g., Menietti et al, ]. Periodic emissions appear to be a universal plasma physical process and are still poorly understood.…”

The brightness ratio of H Lyman‐α/H2 bands in FUV auroral emissions: A diagnosis for the energy of precipitating electrons and associated magnetospheric acceleration processes applied to Saturn

“…The precipitating electron energy spectrum can also be affected by the efficiency of pitch angle scattering in the magnetosphere. Whistler‐mode chorus waves have been detected by previous observations and can resonate with electrons with energies from a few 100 s eV to a few keV [ Hospodarsky et al ., ; Menietti et al ., ]. Whistler‐mode hiss waves are associated with the FA electron beam with energy of a few 100 eV [e.g., Kopf et al ., ; Gurnett et al ., ].…”

Source region and growth analysis of narrowband Z‐mode emission at Saturn

“…A source of temperature anisotropy near the Enceladus torus may be damping of ion cyclotron waves which have been identified and studied by many researchers [cf. Leisner et al, 2006;Menietti et al, 2013]. The narrow size of the source location may be due to the steep gradient in the density in this inner region of the Saturn magnetosphere.…”