Neutral Gas Nebula Observed by Cassini via Pickup Ion

 Jupiter's Neutral Gas Nebula Observed by Cassini via Pickup Ions
 A nebula of neutral gases surrounds Jupiter, extending to large 
 distances. It has been investigated for some time via scattered 
 sunlight from sodium atoms. Since these atoms largely originate from 
 Io, sodium is expected to be a trace element compared to oxygen and 
 sulfur, which are much more difficult to observe optically.  The nebula 
 is produced from the Io ion torus, which corotates with the planet at a 
 speed of 75 km/s. Fast neutrals are formed via charge exchange and 
 escape the system with the possibility of traveling large distances 
 before being photo-ionized.

 For several months before Cassini's closest approach to Jupiter on Dec. 
 30, 2000, and for almost a year afterward, the MIMI/CHEMS instrument 
 indirectly detected the neutral nebula via pickup ions that are carried 
 outward by the solar wind after the neutrals are photo-ionized.  The 
 presence of sulfur flagged the Jovian origin. Pickup oxygen was also 
 present, competing with oxygen of interstellar origin near the planet.  
 The +1 charge state of the ions and their anti-sunward streaming 
 indicated they were pickup ions and not escaping magnetospheric ions. 
 I will discuss these observations as well as those of the 
 magnetospheric ion population taken during Cassini's brief entry into 
 Jupiter's outer magnetosphere during the flyby.