Fig. 4

Altitude of the top of the ionosphere, as defined in the text, as a function of solar zenith angle. Black and red symbols indicate Mariner 9 and Viking Orbiter occultations, respectively. Crosses indicate actual detections of the top of the ionosphere, and diamonds indicate lower limits from non-detections. Orbit 4’s result is a non-detection of 210 km at a solar zenith angle of 55.8°