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 tex2html_wrap_inline20 km, with widths ranging from tex2html_wrap_inline22 to tex2html_wrap_inline24 km, and peak intensities from tex2html_wrap_inline26 to tex2html_wrap_inline28 cm tex2html_wrap_inline30 . 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 tex2html_wrap_inline32 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, tex2html_wrap_inline34 cm tex2html_wrap_inline30 , is situated about 100 km above the surface; the width of the peak is tex2html_wrap_inline38 km. The electron densities on the upstream side are much smaller, tex2html_wrap_inline40 cm tex2html_wrap_inline30 . The first three occultations of Europa have indicated a tenuous ionosphere with a maximum density of tex2html_wrap_inline44 cm tex2html_wrap_inline30 near the surface and vertical plasma scale height of 200- 400 km. Simple photochemical considerations suggest a neutral atmosphere (of, \ it e.g., O tex2html_wrap_inline50 or H tex2html_wrap_inline50 O) with surface density tex2html_wrap_inline54 cm tex2html_wrap_inline30 .