Auroral Hiss Emissions During Cassini's Grand Finale: Diverse Electrodynamic Interactions Between Saturn and Its Rings

Abstract

The Cassini Grand Finale orbits offered a new view of Saturn and its environment owing to multiple highly inclined orbits with unprecedented proximity to the planet during closest approach. The Radio and Plasma Wave Science instrument detected striking signatures of plasma waves in the southern hemisphere. These all propagate in the whistler mode and are classified as (1) a filled funnel‐shaped emission, commonly known as auroral hiss. Here however, our analysis indicates that they are likely associated with currents connected to the rings. (2) First observations of very low frequency saucers directly linked to the planet on field lines also connected to the rings. The latter observations are unique to low altitude orbits, and their presence at the Earth and Saturn alike shows that they are fundamental plasma waves in planetary ionospheres. Our results give an insight, from a unique perspective, into the dynamic and diverse nature of Saturn's environment.

Survey of Saturn Whistler Mode Hiss Intensity

“…See Gurnett and Bhattacharjee () for details. Hiss observed at Saturn is observed with the same funnel‐shaped frequency limits (Kopf et al, ; Sulaiman et al, ; Xin et al, ), as well as less descriptive signatures. Saturn auroral hiss has also been associated with ~20 KeV < E < 1 MeV upward or bidirectional electron beams (Mitchell et al, ).…”

“…An example of intense, high‐latitude hiss at Saturn is shown in Figure , displaying approximately 1‐hr periodic enhancements. Funnel‐shaped emission (as observed on a frequency‐time spectrogram) associated with Saturn's B‐ring (Xin et al, ), the Enceladus magnetic field line (Kopf et al, ), and with field lines connecting to the inner edge of the Saturn D‐ring (Sulaiman et al, ) has also been reported.…”

Saturn's Ionosphere: Electron Density Altitude Profiles and D‐Ring Interaction From The Cassini Grand Finale

“…Moreover, since at those L values, the ionosphere is mainly dominated by H + (Wahlund et al, ); it implies the dominance of electrodynamical processes via flux tubes, which traverse the electrically conductive D‐ring over chemical processes (Waite et al, ). In fact, using the RPWS radio measurements, Sulaiman, Kurth, Hospodarsky, et al () show the presence of clear signatures of whistler waves (classified as auroral hiss and very low frequency saucers) in the southern hemisphere at similar altitudes, directly linked to the planet on field lines connected to the D‐ring, hence suggesting as well an electrodynamic coupling with the ring. This highlights the effect of the D‐ring interaction on structuring and increasing the variability of the electron density profiles in the topside southern hemisphere.…”

Enceladus Auroral Hiss Emissions During Cassini's Grand Finale

“…They are characterized by their funnel or V‐shaped features in the electric field frequency‐time spectrogram (hence sometimes called “saucers”), and irrespective of their source, they are all believed to be propagating in the whistler mode and are understood to be generated via Landau resonance in the presence of electron beams (Gurnett et al, ; James, ). Saturn's environment is known for the ubiquity of auroral hiss emissions associated with diverse sources, namely, the planet (Kopf et al, ), the rings (Xin, Gurnett, Santolík, et al, ; Sulaiman et al, , this issue), and most notably Enceladus (Gurnett et al, ; Leisner et al, ). Using ray‐tracing analyses, the latter authors concluded that the emissions are generated by electron beams accelerated close to Enceladus.…”