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Between 26 April and 15 September 2017, Cassini executed 23 highly inclined Grand Finale orbits through a new frontier for space exploration, the narrow region between Saturn and the D Ring, providing the first opportunity for obtaining in situ ionospheric measurements. During the Grand Finale orbits, the Radio and Plasma Wave Science instrument observed broadband whistler mode emissions and narrowband upper hybrid frequency emissions. Using known wave propagation characteristics of these two plasma wave modes, the electron density is derived over a broad range of ionospheric latitudes and altitudes. A two‐part exponential scale height model is fitted to the electron density measurements. The model yields a double‐layered ionosphere with plasma scale heights of 545/575 km for the northern/southern hemispheres below 4,500 km and plasma scale heights of 4,780/2,360 km for the northern/southern hemispheres above 4,500 km. The interpretation of these layers involves the interaction between the rings and the ionosphere. Plain Language Summary: For the final 5 months of the Cassini mission in 2017, the spacecraft executed 23 orbits through a new frontier for space exploration, the narrow region between Saturn and the innermost of Saturn's main rings, the D Ring. For the first time in the history of space exploration, the Cassini instruments were able to take measurements inside Saturn's ionosphere. This paper provides the density distribution of Saturn's ionospheric electrons, derived from plasma waves detected by the Radio and Plasma Wave Science instrument. The electron density distributions with altitude and latitude show that the ionospheric electron densities peak at 10,000 particles per cubic centimeter at low altitudes in the equatorial region and drop below 100 particles per cubic centimeter at higher altitudes and latitudes. Two simple ionospheric scale height density models for the northern and southern hemispheres are presented. Key Points: We present the first in situ measurements of the electron density in the low to middle latitudes of Saturn's ionosphereThe distribution of electron density measurements with altitude shows evidence of a two‐layered ionospheric electron density distribution up to an altitude of 15,000 kmWe present a scale height electron density model for a double‐layered ionosphere for both the northern and southern hemispheres [ABSTRACT FROM AUTHOR]

Saturn auroral hiss is intense whistler mode emission similar in morphology to terrestrial auroral hiss, and is observed at high latitude very often in quasiperiodic episodes with a period of approximately 1 hr. Bader et al. (2019) report auroral pulsations that may be due to duskside magnetodisk reconnection. The source of the 1‐hr period is not definitively known but has been purported to be due to second harmonic Alfven waves standing along near planet magnetic field lines (Yates et al., 2016). Observations of auroral hiss at high latitude along Cassini proximal orbits are often excellent, and we have focused on an event for which we have concurrent ultraviolet auroral images as well as electron flux data. A series of repeating auroral hiss episodes is observed to initiate near the magnetic field line that traverses a Saturn kilometric radiation source region in each hemisphere, with periodic episodes of hiss recurring at higher L‐shells. Magnetic field lines centered on individual hiss episodes have auroral footprints that lie near and within a region of intense auroral ultraviolet emissions. These observations have a parallel in terrestrial return current electron beam‐generated auroral hiss seen near magnetic field lines supporting auroral kilometric radiation source regions. Recent findings link periodic plasma injections with Saturn reconnection sites observed preferentially on the duskside. These injections may spawn Saturn kilometric radiation source regions and periodic auroral hiss emission in nearby return current regions. Key Points: Northern and southern hemisphere observations of Saturn periodic auroral hiss during proximal orbit are studiedPeriodic hiss is observed adjacent to Saturn kilometric radiation source region field lines in each hemispherePeriodic hiss may occur in return current regions adjacent to radiation source regions spawned by periodic reconnection/injections [ABSTRACT FROM AUTHOR]

We show that the plasma and magnetic fields in the inner region of Saturn's plasma disk rotate in synchronism with the time-variable modulation period of Saturn's kilometric radio emission. This relation suggests that the radio modulation has its origins in the inner region of the plasma disk, most likely from a centrifugally driven convective instability and an associated plasma outflow that slowly slips in phase relative to Saturn's internal rotation. The slippage rate is determined by the electrodynamic coupling of the plasma disk to Saturn and by the drag force exerted by its interaction with the Enceladus neutral gas torus. [ABSTRACT FROM AUTHOR]