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 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.

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

“…We use the following expression: Here H is the plasma scale height, the subscript 0 denotes the reference altitude, and P, D, and C corresponds to the different ionospheric regions. Setting the reference altitude to the 1‐bar surface of Saturn ( h 0 =0km), we find from the following values: H P ∼4,500km (consistent with the scale height estimation of the ionosphere in the northern hemisphere by Persoon et al, ), H D ∼840km, H C ∼260km, and , , . The derived plasma scale heights can be compared with a theoretical estimate: where k is the Boltzmann's constant, T p is the plasma temperature, m i is the mean ion mass, and g is the gravitational acceleration at Saturn.…”

“…When , then a band at f uh is apparent. Hence, f uh can be used as it is related to f p e by and f c e is well known from the magnetometer measurements of | B | since f c e [Hz]=28| B |[nT] (Gurnett et al, ; Persoon et al, , ).…”

“…In this work, the electron densities are estimated from the LP high‐resolution 20‐Hz sampling mode for all the 23 perikrones. At times when LP was partly (Revs 273, 284, 286, and 289) or entirely (Revs 272 and 285) in the wake of the spacecraft, we use the electron densities derived from the upper cutoff frequency of the whistler waves or the f uh line (Persoon et al, ), which give identical results, although at lower time resolution (Wahlund et al, ). The n e profiles of the 23 passes are presented in Figure a as a function of altitude ( h ) and geographic latitude ([[spiphi]]).…”

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

“…This calculation is limited to regions of n e < 5 cm −3 , generally. Within the high density ionosphere, f p can be calculated using the LP “sweep mode” (Wahlund et al, ) or the whistler mode high‐frequency cutoff obtained from the RPWS instrument (Persoon et al, ).…”

The Ion Composition of Saturn's Equatorial Ionosphere as Observed by Cassini

“…H 2 + ions rapidly react with H 2 , producing H 3 + ions, which can rapidly dissociatively recombine with electrons. The RPWS Langmuir probe measured the electron density (N e ) along the spacecraft track (Wahlund et al, 2017), and RPWS upper hybrid wave measurements gave independent determinations of N e (Hadid et al, 2018;Persoon et al, 2019). Among the suggestions made for reducing the predicted electron density was the reaction of H + with water coming from the rings, creating molecular ion species (H 2 O + and then H 3 O + ) with short chemical lifetimes (Connerney & Waite, 1984;Moore et al, 2010).…”