Short caption: MARSIS Ionogram with Oblique Ionospheric Echo
Full caption: MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) ionospheric sounding data are shown in this plot known as an ionogram. The horizontal axis is the frequency of the pulse and the vertical axis is the time delay after transmitting the pulse, plotted positive downward and converted to apparent range to the reflection point. The intensity of the received signal at any given frequency and apparent range is indicated by the color, with dark blue being the least intense and green being the most intense. The green echo at an apparent range of about 800 km from 2.5 to 5.5 MHz is the reflected signal from the surface of Mars. The curved bright green feature with an apparent range varying from about 600 to 750 km at frequencies from about 0.7 to 1.8 MHz is the echo from the top side of the ionosphere. A second echo the ionosphere, at an apparent range of about 100 km is labeled “Oblique ionospheric echo.” Such echoes are believe to come from distorted structures in the ionosphere caused by the magnetic fields in the crust of Mars.

Credit: ASI/NASA/ESA/Univ. of Rome/Univ. of Iowa/JPL