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A.J. Kliore, A. Anabtawi (Caltech/JPL), A.F. Nagy (U. of Michigan)
The Galileo Europa Mission (GEM) provided three occultations by Callisto, in addition to the single occultation during the prime mission, which occurred on orbit 9 (C9, June 25, 1997). The new occultations occurred on C20 (May 5, 1999), C22 (Aug. 14, 1999), and C23 (Sept 16, 1999).
The C9 occultation observations, which occurred very close to the equator (2° and 3° South lat.) produced no evidence of ionospheric layers with electron densities exceeding the noise background (~ 2,000 cm-3). The C20 observations, made at ~ 18° N and S lat., show possible layers of density 2,000 to 5,000 cm-3 between the altitudes of 30 and 70 km. The C22 and C23 exit measurements, at latitudes of 33-34° North and solar zenith angles (SZA) of 97.6 and 101.3°, also show possible layers of density ~ 5,000 cm-3 at altitudes from 10 to 30 km. However, The C22 and C23 observations, which occurred at ~ 34° N latitude and SZA of 78.7 and 82.5° , clearly show layers having peak electron densities of ~ 15,000 and 17,000 cm-3 at altitudes of ~ 23 and 46 km, respectively.
No significant theoretical analysis and modeling of these results was possible before the abstract deadline, but it appears that the density of neutral O2 at the surface of Callisto that would lead to the observed ionospheric layers must be of the order of 108 cm-3. More results of theoretical modeling will be presented at the meeting.
This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, and the Space Physics Research Laboratory, University of Michigan with support of the Galileo Project and NASA.