electrostatic analyzer experiment on Pioneer 10 detected an increase in ion density as the spacecraft crossed the L The newly detected oxygen atmosphere of Europa is modeled by invoking charged particle sputtering with H 2 O and O 2 mole-shell of Europa (Intriligator and Miller 1982). This feature cules as the main ejecta. The magnetospheric corotating ions might be associated with the formation of a Europa plasma could provide the required source strength (ศ3 ุ 10 26 sec ุ1 ) torus as a result of mass injection from this icy satellite. of O 2 molecules if a fraction (ศ20%) of the exospheric ions were This issue was followed up recently by Bagenal (1994), recycled to Europa's surface where they produce additional who showed that the O ฯฉ density observed by Voyager 1 sputtering product. Two exospheric components are expected increases suddenly at radial distances ีฟ7.5 R J .
to form: an extended corona with a size of a few satellite radii
Using a plasma chemical model, Schreier et al. (1993) which is composed of sputtered molecules in ballistic motion, investigated how the local increase of the number density and a thermal population with a surface density of 10 8 -10 9 of the O ฯฉ ions could be related to the ionization of Europa's cm ุ3 and a scale height of about 20 km. The electron impact neutral oxygen atomic cloud. From parameter fittings, ionization of this exosphere would lead to an Io-like interaction with the jovian magnetosphere with a field-aligned Birkeland these authors found that the number density of the neutral current of about 5 ุ 10 5 A.