The Near-Terminator Ionosphere of Venus: How Chapman-esque?

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We present models of the ionospheres of Mars and Venus for solar zenith angles in the range 60 to 90 degrees using the Solar 2000 v2.22 solar flux model of Tobiska [2004]. We compare these models to the available radio occultation profiles, and investigate reasons for the non-Chapman behavior of the two ionosphere. Both ionosphere of have been found to be close to a Chapman layer by other investigators. The peak electron density of the Venus ionosphere has been shown to decrease in density but not altitude as the solar zenith angles increase up to about 85 degrees by Cravens et al. [1981]. This effect has been ascribed to the collapse of the atmosphere as the daytime thermosphere merges into the nighttime cryosphere. The Martian electron density profiles have been studied by several investigators, beginning with the comprehensive analysis of Zhang et al. [1990], and continuing with several studies based on data from the Mars Global Surveyor radio science experiment. We find that, on Mars, the behavior is close to that of a Chapman layer because of effects that tend to cancel each other out. One of the most important effects is the rise of the density peak due to the increase of electron temperatures with altitude. We also address the assumptions of a constant temperatures and scale heights, and the effect of different background atmospheres for increases in the solar zenith angle.


Presented at the 2005 Fall Meeting of the American Geophysical Union (AGU), San Francisco, CA.

Presentation Number SA53B-1168.

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