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

Abstract

Intense Z‐mode emission is observed in the lower density region near the inner edge of the Enceladus torus at Saturn, where these waves may resonate with MeV electrons. The source mechanism of this emission, which is narrow‐banded and most intense near 5 kHz, is not well understood. We survey the Cassini Radio and Plasma Wave Science data to isolate several probable source regions near the inner edge of the Enceladus density torus. Electron phase space distributions are obtained from the Cassini Electron Spectrometer, part of the Cassini Plasma Spectrometer investigation. We perform a plasma wave growth analysis to conclude that an electron temperature anisotropy and possibly a weak loss cone can drive the Z mode as observed. Electrostatic electron acoustic waves and perhaps weak beam modes are also found to be unstable coincident with the Z mode. Quasi‐steady conditions near the Enceladus density torus may result in the observations of narrowband Z‐mode emission at Saturn.

Formation of electron radiation belts at Saturn by Z-mode wave acceleration

“…A case study of Z-mode waves at Saturn 16 assumed that the direction of wave propagation with respect to the magnetic field (known as the wave normal angle (WNA)) was highly oblique. However, Z-mode waves can also propagate along the magnetic field direction 17,18 .…”

“…1b shows that there is a mixture of strong and weak left-hand circular polarization with components of linear polarization (not shown) indicating that there is a distribution of WNAs which includes parallel propagation. Instability analysis shows that Z-mode waves can be generated in the field-aligned direction by an unstable distribution of electrons 17,18 . An analysis of the polarization data for this case shows that the WNAs are mostly close to the magnetic field with a peak at ~22° (Fig.…”

Cassini Observation of Relativistic Electron Butterfly Distributions in Saturn’s Inner Radiation Belts: Evidence for Acceleration by Local Processes

“…At L < 4 R S in Saturn's magnetosphere, Woodfield et al (2019) proposed that the acceleration of electrons to relativistic energies by whistler waves may lead to the butterfly PADs observed at L ∼ 3 R S (Paranicas, Mitchell, Krimigis, et al, 2010). It has also been shown that there are intense Z-mode waves in this area (Menietti et al, 2015(Menietti et al, , 2016Ye et al, 2010). The intensity of Z-mode waves gradually increase toward smaller L-shells, which may fit to the increasing α eq range of near-equatorial depletion presented in Figure 3.…”

Statistical Study on Spatial Distribution and Polarization of Saturn Narrowband Emissions

“…Recently, Menietti et al (2016Menietti et al ( , 2019 studied the generation mechanism of Z mode emissions for both 5 and 20 kHz and the relation of upper hybrid resonance with NB emissions by analyzing the growth rates of the waves based on the electron phase space distribution measured by the Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS; Young et al 2004). Case studies show the wave normal angel of the Z mode to be usually smaller than 30° (Menietti et al 2018a(Menietti et al , 2018b, and DF results show two possible source regions: one at high latitude on the same magnetic field lines as SKR and another one at the equatorial region interior to the plasma torus (Menietti et al 2018b).…”