Analysis of Intense Z‐Mode Emission Observed During the Cassini Proximal Orbits

Abstract

The role of Z‐mode emission in the diffusive scattering and resonant acceleration of electrons is believed to be important at Saturn. A survey of the “5 kHz” component of this emission at Saturn earlier reported strong intensity in the lower density regions where the ratio of plasma frequency to cyclotron frequency, fp/fc < 1. At Saturn this occurs along the inner edge of the Enceladus torus near the equator and at higher latitudes. Using the Cassini Radio and Plasma Wave Science instrument observations during the Cassini proximal orbits, we have now identified these emissions extending down to and within the ionosphere. Wave polarization measurements and unique frequency cutoffs are used to positively identify the wave mode. Analogous to the role of whistler mode chorus at Earth, Saturn Z‐mode emissions may interact with electrons contributing to the filling or depleting of Saturn's inner radiation belts.

Extended Survey of Saturn Z‐Mode Wave Intensity Through Cassini's Final Orbits

“…The wave polarization is measured as described earlier, and we use it to calculate the wave normal angle as a function of frequency. The results are presented in Figure 5 for the periapsis of the ring-grazing orbit shown in Figure 1 (a similar figure for one of the proximal orbits is shown in Figure 4 of Menietti et al, 2018). The angles are typically small, less than~30°for most of the frequency range, except perhaps at higher densities and when the wave frequency is close to f c .…”

Saturn's Plasma Density Depletions Along Magnetic Field Lines Connected to the Main Rings

“…Many of the proximal orbits had these dropout features near ±40° along Cassini's path. In some cases, other emissions created in remote locations would mask or obscure these features (see Figures S8 and S9 in the supporting information; Menietti et al, ). However, in the northern sunlit hemisphere, in 15 out of 22 proximal cases, the abrupt dropout feature was observed.…”

“…Geophysical Research Letters supporting information; Menietti et al, 2018). However, in the northern sunlit hemisphere, in 15 out of 22 proximal cases, the abrupt dropout feature was observed.…”

Enceladus Auroral Hiss Emissions During Cassini's Grand Finale

“…This trajectory granted large latitudinal coverage as well as unrivaled proximity to the giant planet over an orbital period of ~6.5 days. First in situ measurements of the ionosphere have revealed intense plasma waves and large variability in electron density of up to 2 orders of magnitude as well as a north‐south asymmetry attributed to the shadow cast by Saturn's A and B rings (Menietti et al, ; Persoon et al, ; Sulaiman et al, , this issue; Wahlund et al, ).…”