Galileo Radio-Occultation Measurements of the Ionospheres of Jupiter and Its Satellites
F. M. Flasar (NASA/GSFC), D. P. Hinson (Stanford University), A. J. Kliore (JPL), P. J. Schinder (NASA/GSFC, Cornell University), J. D. Twicken (Stanford University)
Measurements obtained during the first five occultations of Galileo by
Jupiter indicate an ionosphere that is very heterogeneous. The topmost, main
electron density peaks are situated at altitudes (above the 1-bar level)
that range from 900 km to 
km, with widths ranging from
to 
km, and peak intensities from 
to 
cm 
. At several locations, thin layers lie beneath the main peak, and
cause multipath propagation of the received radio signal. Of particular
interest are the well-defined thin layers sometimes seen at altitudes 600-750
km
with vertical spacing 
km, which may be forced by
upwardly propagating gravity waves.
The first occultation of Io indicates an ionosphere that is qualitatively
identical to that observed by Pioneer 10. On the side of Io downstream from
the
corotating jovian magnetosphere, the maximum electron density, 
cm , is situated about 100 km above the surface; the width of the peak
is 
km. The electron densities on the upstream side are much
smaller, 
cm . The first three occultations of Europa have
indicated a tenuous ionosphere with a maximum density of
cm near the surface and vertical plasma scale height of 200-
400
km. Simple photochemical considerations suggest a neutral atmosphere (of, \
it
e.g., O 
or H O) with surface density 
cm .