Comparative ENA imaging of Earth, Saturn, Titan and Mars  

P.C. Brandt, D.G. Mitchell, E.C. Roelof, J. Saur, I. Dandouras, S. Barabash, 
S.M. Krimigis, and B.H. Mauk


Remote imaging of space plasma by detection of energetic neutral atoms (ENA)
have now been in use for ten years. The High Energy Neutral Atom (HENA) imager
on board IMAGE mission is imaging the Terrestrial ring current and plasmasheet.
The Ion Neutral Camera (INCA) on board Cassini is returning ENA images of
SaturnÕs magnetosphere and its interaction with the moons. The Neutral Particle
Imager (NPI) and Detector (NPD) on board Mars Express images the interaction
between the solar wind and the martian atmosphere. These missions are providing
unique global comparisons of the interactions between energetic ions and neutral
gas around magnetized (Earth, Saturn) and weakly magnetized (Mars, Titan)
bodies. While the EarthÕs magnetosphere is largely a convection-dominated system
where plasma-neutral interactions take place at low altitudes, INCA showed a
corotating saturnian magnetosphere where energetic ions (10-200 keV) interacting
strongly with neutral gas confined to the equatorial plane with no apparent
energetic ions reaching the upper atmosphere of Saturn. Both the terrestrial and
saturnian magnetospheres see frequent injections of energetic ions in the
nightside region. At Earth these injections end up in the ring current region
and magnetically drift around the Earth. At Saturn the injections end up in a
corotation region where the energetic ions drift in the corotation electric
field with only modest dispersion due to magnetic drifts. We discuss the
relative importances of charge-exchange loss and transport of the energetic ions
in the two systems. Orbiting Saturn in about 16 (Earth) days at a distance of
about 20 RS, Titan sees energetic ion clouds corotating over it with a speed of
about 150km/s. INCA images show how a large region around Titan ÒlightsÓ up in
ENAs every time a corotating plasma cloud sweeps past it. The extension of the
ENA emissions around Titan are consistent with an H2 exosphere extending to
several 10,000 km altitude above Titan. At lower altitudes strongly asymmetric
and directional ENA emissions indicate a complex interaction that requires
treatment of the magnetic field geometry and single-particle trajectories to be
explained. We will discuss and compare the plasma interactions at Titan and Mars
based on recent findings by Cassini and the NPI and NPD imagers on board Mars
Express.

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Presentation, 2nd Annual Asia Oceanic Geosciences Society Meeting, Singapore 
20-24 June 2005