Mars Express: MARSIS Active Ionospheric Sounding observation, 1 July 2010. The MARSIS radar on Mars Express is best known for studying subsurface ice on Mars, but it has another mode that’s used for probing the planet’s ionosphere. The NSSDC description of the MARSIS instrument explains this mode briefly:
In the active ionospheric sounding mode is done by using quasi-CW tones swept across the entire band giving an ionogram for 160 frequencies from 0.1 to 5.5 MHzSo this image has one axis for this frequency band, and the other represents time/spacecraft position. Explaining what it is we’re seeing here is way out of my league, but I ran across someone’s master’s thesis that explains the AIS dataset and some techniques used to visualize it, although the ionograms in the paper are somewhat different, graphing frequency vs. delay.