Saturn's Dusty Ionosphere

M. W. Morooka (1) , J.‐ E. Wahlund (1) , Lina Hadid (1) , A. I. Eriksson (2) , N. J. T. Edberg (1) , E. Vigren (1) , D. J. Andrews (1) , A. M. Persoon (3) , W. S. Kurth (3) , D. A Gurnett (3) , W. M. Farrell (4) , J. H. Waite Jr. (5) , R. S. Perryman (5) , M. Perry (6)

1 IRF - Swedish Institute of Space Physics [Uppsala / Kiruna]
2 IRF - Swedish Institute of Space Physics [Kiruna]
3 University of Iowa [Iowa City]
4 GSFC - NASA Goddard Space Flight Center
5 SwRI - Southwest Research Institute [San Antonio]
6 APL - Johns Hopkins University Applied Physics Laboratory [Laurel, MD]

Measurements of electrons and ions in Saturn's ionosphere down to 1,500-km altitudes as well as the ring crossing region above the ionosphere obtained by the Langmuir probe onboard the Cassini spacecraft are presented. Five nearly identical deep ionosphere flybys during the Grand Finale orbits and the Final plunge orbit revealed a rapid increase in the plasma densities and discrepancies between the electrons and ions densities (Ne and Ni) near the closest approach. The small Ne/Ni ratio indicates the presence of a dusty plasma, a plasma which charge carrier is dominated by negatively charged heavy particles. Comparison of the Langmuir probe obtained density with the light ion density obtained by the Ion and Neutral Mass Spectrometer confirmed the presence of heavy ions. An unexpected positive floating potential of the probe was also observed when Ne/Ni ≪ 1. This suggests that Saturn's ionosphere near the density peak is in a dusty plasma state consisting of negatively and positively charged heavy cluster ions. The electron temperature (Te) characteristics in the ionosphere are also investigated and unexpectedly high electron temperature value, up to 5000 K, has been observed below 2,500-km altitude in a region where electron-neutral collisions should be prominent. A well-defined relationship between Te and Ne/Ni ratio was found, implying that the electron heating at low altitudes is related to the dusty plasma state of the ionosphere.

Domaines: Planétologie et astrophysique de la terre [astro-ph.EP]; Physique des plasmas [physics.plasm-ph]

M. W. Morooka, J.‐ E. Wahlund, Lina Hadid, A. I. Eriksson, N. J. T. Edberg, et al.. Saturn's Dusty Ionosphere. Journal of Geophysical Research Space Physics, 2019, 124 (3), pp.1679-1697. ⟨10.1029/2018JA026154⟩. ⟨hal-02640891⟩