SOLAR CONTROL OF THE MARS IONOSPHERE

The Mars ionosphere is generally assumed to be in photochemical equilibrium and can thus be described by a Chapman layer. Small departures from simple Chapman-type behavior are expected since the ionizable constituent and the scale height vary with solar zenith angle and solar activity. A re-analysis of all available ionospheric data from past U.S. and U.S.S.R. Mars missions was made in order to compare the ionospheres of Mars and Venus (both of them CO2 atmospheres), for the latter of which a substantial body of data exists. Solar zenith angle dependence of peak ionospheric density and height are essentially the same for both planets. Temperatures derived from topside ionosphere scale heights show a higher variability with solar activity for Mars than for Venus. On the other hand, neutral temperatures derived from electron density profiles via the scale heights of the ionizable constituent CO2 at the peak of the Mars ionosphere, vary with solar cycle in a similar fashion to Venus. The variation of peak electron density with solar activity is also about the same for Mars and Venus. © 1990.