Saturn’s near-equatorial ionospheric conductivities from in situ measurements

Abstract

Cassini's Grand Finale orbits provided for the first time in-situ measurements of Saturn's topside ionosphere. We present the pedersen and Hall conductivities of the top near-equatorial dayside ionosphere, derived from the in-situ measurements by the cassini Radio and Wave plasma Science Langmuir Probe, the Ion and Neutral Mass Spectrometer and the fluxgate magnetometer. The Pedersen and Hall conductivities are constrained to at least 10 −5-10 −4 S/m at (or close to) the ionospheric peak, a factor 10-100 higher than estimated previously. We show that this is due to the presence of dusty plasma in the near-equatorial ionosphere. We also show the conductive ionospheric region to be extensive, with thickness of 300-800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities from orbit 288 to 292.

Planetary Period Oscillations of Saturn’s Dayside Equatorial Ionospheric Electron Density Observed on Cassini’s Proximal Passes

“…Investigation of the properties of this predominantly heavy ion layer by Shebanits et al. (2020) has shown that its electrical conductivity is increased by more than an order of magnitude compared with previous estimates, forming an extensive conducting layer ∼300–800 km thick.…”

“…These particles consist of water, silicate, and organic (carbon-containing) grains, that were also directly detected on the proximal passes (Hsu et al, 2018;Moore et al, 2018;Mitchell et al, 2018;Perry et al, 2018;Waite et al, 2018). Investigation of the properties of this predominantly heavy ion layer by Shebanits et al (2020) has shown that its electrical conductivity is increased by more than an order of magnitude compared with previous estimates, forming an extensive conducting layer ∼300-800 km thick.…”

Constraining the Temporal Variability of Neutral Winds in Saturn's Low‐Latitude Ionosphere Using Magnetic Field Measurements

“…Our analysis shows that the ionospheric currents must be in steady‐state on timescales of ∼ 10 min and azimuthally symmetric between 11 and 13 h in LT between −5° and −10° in latitude in the southern hemisphere, and between 15° and 19° in latitude in the northern hemisphere. We constrain the temporal variability of the peak zonal flows in the southern hemisphere to ∼350 ms −1 over an interval of ∼5 months, which is subject to being scaled by variability in Pedersen conductances, which could be variable by up to a factor of ∼10 at near equatorial latitudes (Shebanits et al., 2020).…”

“…In the absence of charged dust, Pedersen conductivities scale linearly with the ion densities, which are equal to the electron densities in a quasi-neutral ionosphere. Radio occultations of the mid-and low-latitude ionosphere at Saturn have shown temporally variable electron densities on week-long timescales (Hadid et al, 2018b;Kliore et al, 2009;Nagy et al, 2006), and recent in-situ measurements from the Cassini Grand Finale in the near-equatorial upper atmosphere (between − 15° and 5°) have shown the Pedersen conductivities at ∼1,000 km above the peak conducting layer of the ionosphere to be variable by up to an order of magnitude between the ∼6.5 days periapsis passes (Shebanits et al, 2020). However, there are no observations of the magnitudes or extent of variability in Σ P in the peak conducting layer of the low-latitude ionosphere during Revs 271-292.…”

“…The conductances are variable by ∼1.5S between Revs 271 and 292, which on the D-ring field lines corresponds to a maximum uncertainty in the modeled values of ∼4 nT which is at the limit of the B ϕ resolution during the periapsis passes of the Grand Finale orbits, and thus negligible. Shebanits et al (2020) determined the Pedersen conductivity close to the peak conducting layer between − 15° and 10° in latitude to be 10 −4 − 10 −5 Sm −1 from in situ measurements taken during Revs 288-293 (with Rev 293 being Cassini's final orbit), which integrated over an ionospheric layer of ∼350 km corresponds to a Pedersen conductance of 3.5 − 35S. These conductances are significantly larger than previous observations have suggested, and this is primarily due to the presence of charged dust in the near-equatorial (±5°) ionosphere, with some dust still being present at slightly higher (between 10° and −15°) latitudes.…”