Intense Harmonic Emissions Observed in Saturn's Ionosphere
Résumé
The Cassini spacecraft’s first Grand Finale orbit was carried out in April 2017. This set of 22 orbits
had an inclination of 63° with a periapsis grazing Saturn’s ionosphere, thus providing unprecedented
coverage and proximity to the planet. Cassini’s Radio and Plasma Wave Science instrument repeatedly
detected intense electrostatic waves and their harmonics near closest approach in the dayside equatorial
topside ionosphere. The fundamental modes were found to both scale and trend best with the H^{+} plasma or
lower hybrid frequencies, depending on the plasma composition considered. The fine-structured harmonics
are unlike previous observations, which scale with cyclotron frequencies. We explore their generation
mechanism and show strong evidence of their association with whistler mode waves, consistent with theory.
The possibility of Cassini’s presence in the ionosphere influencing the resonance and harmonics is discussed.
Given their link to the lower hybrid frequency, these emissions may offer clues to constraining Saturn’s
ionospheric properties.