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The first in situ measurements of the ionosphere of another planet were obtained by retarding potential analyzers of the Viking landers. These results are presented with attention to: (1) the determination of the peak ion concentration in the ionosphere layer at several altitudes, (2) the measurement of ion temperatures, and (3) an equatorward horizontal ion velocity observed at various heights. Both landers entered the ionosphere layer at solar zenith angles near 44 deg, and more structure was observed in the height profiles of ionospheric quantities on Viking 2, although the profiles were similar in shape to those of Viking 1.
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Hanson et al. (1977) studied this question.
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