000008007 001__ 8007
000008007 005__ 20260218175043.0
000008007 0247_ $$2DOI$$a10.1016/j.icarus.2025.116647
000008007 037__ $$aSCART-2026-0124
000008007 100__ $$aWahlund, J.-E.
000008007 245__ $$aOn the equatorial dayside ionosphere of Saturn – In-situ observations give evidence for a dynamic and layered structure in disequilibrium
000008007 260__ $$c2025
000008007 520__ $$aThe Cassini observations of Saturn's ionosphere during the proximal orbits 288–293 in the altitude range 1450–4000 km (above 1-bar level) are revisited. A thorough re-analysis is made of all 159 available Langmuir probe sweeps of the Radio & Plasma Wave Science (RPWS) measurements. We relate them to the RPWS plasma wave inferred electron number densities and compare them with the available Ion Neutral Mass Spectrometer (INMS) measurements of the H+ and H3+ number densities. Different analysis methods are used by RPWS to provide consistent electron number density values for the whole measured altitude interval. Consistent RPWS electron number density (ne) and INMS positively charged ion number density (ni+) profiles are derived for altitudes above ~2200 km. Below this altitude the inability of INMS to measure ions above 8 amu at the 34 km/s flyby speed lead us to infer the presence of heavy ions (> 8 amu) and a negatively charged ion component, presumably related to infalling material from the D-ring of Saturn with its associated local ion-molecule-aerosol chemistry. This lower altitude region shows a highly time variable layered structure. The Langmuir probe data in this region are strongly affected by secondaries emitted from the spacecraft and sensor surfaces when traversing a molecule-rich atmosphere at 34 km/s. There are clear signatures of secondary electron and ion emissions from the spacecraft and sensor surfaces in the data. In the Langmuir probe sweep analysis, we correct for the effect of such impact-generated products. This gives corrected total ion number densities that can be compared to the INMS ion number densities and the electron number densities. From this analysis the number of negative ions and/or nm-sized aerosol/dust particles can be constrained. A clear ionospheric peak is not identified, not even at the lowest observed altitude of approximately 1450 km. There are clear latitudinal variations and temporal evolving structures, which we infer are representative of the difference in infalling material from different regions of the D-ring. In addition, there are indications of a strong heating source for the ambient electrons that are well above expected thermal equilibrium levels (up to 4000 K). The cause of this heating is unknown but may be linked to collisional deacceleration of infalling ring material. The observational profiles presented here can be used for ionosphere theory/model comparisons in the future.
000008007 594__ $$aNO
000008007 6531_ $$aCassini
000008007 6531_ $$aSaturn
000008007 6531_ $$aIonosphere
000008007 6531_ $$aRings
000008007 6531_ $$aAerosol
000008007 700__ $$aEriksson, A. I.
000008007 700__ $$aMorooka, M. W.
000008007 700__ $$aBuchert, S.
000008007 700__ $$aPersson, M.
000008007 700__ $$aVigren, E.
000008007 700__ $$aDreyer, J.
000008007 700__ $$aKurth, W. S.
000008007 700__ $$aWaite Jr., J. H. 
000008007 700__ $$aLebreton, J.-P.
000008007 700__ $$aFarrell, W. M.
000008007 700__ $$aMüller-Wodarg, I.
000008007 773__ $$c116647$$n15 November 2025$$pIcarus$$v441$$y2025
000008007 8560_ $$fjoshua.dreyer@ksb-orb.be
000008007 85642 $$ahttps://doi.org/10.1016/j.icarus.2025.116647
000008007 905__ $$apublished in
000008007 980__ $$aREFERD