Electron Density Distributions in Saturn’s Ionosphere

Abstract

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 twoâ part exponential scale height model is fitted to the electron density measurements. The model yields a doubleâ layered ionosphere with plasma scale heights of 545/575Â km for the northern/southern hemispheres below 4,500Â km and plasma scale heights of 4,780/2,360Â km for the northern/southern hemispheres above 4,500Â km. The interpretation of these layers involves the interaction between the rings and the ionosphere.Plain Language SummaryFor the final 5Â 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 PointsWe 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 twoâ layered ionospheric electron density distribution up to an altitude of 15,000Â kmWe present a scale height electron density model for a doubleâ layered ionosphere for both the northern and southern hemispheres