This figure is a map of the total electron content (TEC) in the Martian ionosphere, created with data from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) onboard Mars Express. The vertical scale indicates latitude (y-axis) and the horizontal (x-axis) is for longitude.

Imagine yourself looking at Mars from space. If the whole planet were covered in darkness and your eyes could see the electrons in the ionosphere, this figure is what you would see. The brighter areas are areas of higher electron content.