Holes in the Nightside Ionosphere of Titan

MODOLO, R.; WAHLUND, J.; KURTH, W. S.; GURNETT, D. A.; BERTUCCI, C.; COATES, A.; SZEGO, K.; CRARY, F.

Congreso; AGU Fall Meeting 2007; 2007

Flybys of Titan by Cassini reveal a highly variable and complex structure of its ionised environment. On July 2, 2006, Cassini flew across the wake and nightside of Titan. Measurements of the electron density and temperature by the Radio and Plasma Waves Science instrument (RPWS) have been employed to investigate the presence of density holes in the antisolar ionosphere of Titan. During this flyby (T15), three depletions of the electron number density by a factor 10 to 50 are observed on the nightside. These holes are large scale structures which extend ~10° in longitude (500-700 km). Furthermore, the magnetic pressure in these holes is larger than the thermal pressure, suggesting that the magnetic pressure is adequate to prevent ionospheric plasma from quickly refilling holes. A larger plasma speed in these structures is also implied by the RPWS observations and supports a change in the ion plasma composition. Presence of magnetospheric plasma (ions and electrons) in these holes is investigated by the Cassini Plasma Spectrometer (CAPS) observations. The magnetic field topology provided by the Cassini magnetometer instrument (MAG) and different scenarios of hole formations are examined. Analogous structures have been observed in the Venusian ionosphere by Pioneer Venus Orbiter and a comparison between Titan and Venus is presented.