Titan's ionosphere in the magnetosheath: Cassini RPWS results during the T32 flyby

Abstract

Abstract. The Cassini mission has provided much information about the Titan environment, with numerous low altitude encounters with the moon being always inside the magnetosphere. The only encounter taking place outside the magnetopause, in the magnetosheath, occurred the 13 June 2007 (T32 flyby). This paper is dedicated to the analysis of the Radio and Plasma Wave investigation data during this specific encounter, in particular with the Langmuir probe, providing a detailed picture of the cold plasma environment and of Titan's ionosphere with these unique plasma conditions. The various pressure terms were also calculated during the flyby. The comparison with the T30 flyby, whose geometry was very similar to the T32 encounter but where Titan was immersed in the kronian magnetosphere, reveals that the evolution of the incident plasma has a significant influence on the structure of the ionosphere, with in particular a change of the exo-ionospheric shape. The electrical conductivities are given along the trajectory of the spacecraft and the discovery of a polar plasma cavity is reported.

The Induced Magnetospheres of Mars, Venus, and Titan

“…During the T32 flyby, both and Garnier (2009) report a gradual increase in the electron density in coincidence with the magnetic field strength drops observed in the collisional ionosphere. However, it is still unclear if pressure balance is satisfied across the field drop.…”

Electron density and temperature measurements in the cold plasma environment of Titan: Implications for atmospheric escape

“…We have not considered the upstream energetic particle pressure which would add even more to the upstream pressure [ Garnier et al , 2009]. In the topside ionosphere, since no pressure balance is reached, plasma will be set in motion due to the forced increase of the dynamic pressure.…”

Statistical analysis of the energetic ion and ENA data for the Titan environment

“…The thermal pressure alone in the Titan exo-ionosphere may easily reach hundreds of eV/cm 3 (Garnier et al, 2009), which is larger than typical solar wind pressures as established by Achilleos et al (2008), with $ 0:02 nPa (or 125 eV/cm 3 ) for a magnetopause near Titan's orbit at L¼20R S . As a result, the comparison with Fig.…”

“…The magnetosheath plasma is indeed a solar wind-like plasma, heated after the bow shock, but still much colder than the magnetospheric plasma, with an electron temperature around, respectively, 50 and 200 eV (Garnier et al, 2009;Bertucci et al, 2008). Nevertheless, the first orbit crossing in the magnetosheath (day 87 in 2005, at 5-6 UT) exhibited large fluxes -twice the mean magnetospheric values -which is a known possibility near the magnetopause, with a similar observation during SOI (Saturn Orbit Insertion) discussed by Krupp et al (2005).…”

“…The largest satellite of the Saturnian system, Titan, is most often located in the outer kronian magnetosphere, even when it is around noon in magnetospheric local time, where it may be sometimes in the magnetosheath or even (theoretically, for extreme solar conditions) in the solar wind (Bertucci et al, 2008;Garnier et al, 2009;Achilleos et al, 2006).…”