This image shows the electron concentration measured by the MGS electron reflectometer as it flew through the ionosphere of Mars during an early aerobraking pass. The Martian ionosphere is a layer of ionized gas that extends from about 75 miles to several hundred miles above the surface, as shown schematically by the shaded region circling the planet (not to scale). As the spacecraft enters the ionosphere, the electron concentration increases dramatically, peaks near the bottom of the ionosphere, and then abruptly drops. The altitude where this abrupt drop occurs depends on the state of the atmosphere, that is, whether it is heating and expanding or cooling and contracting. This information, together with other spacecraft and ground-based measurements, helps mission controllers to guide the spacecraft during aerobraking. These measurements also help us to study Mars' ionosphere and compare it with those of Earth and Venus.
This image is a blow-up of the previous one showing the part of the orbit closest to the planet. This illustrates one way in which the magnetometer and electron reflectometer work together to study the planet. Unlike the Earth's ionosphere, the Martian ionosphere is not shielded from the solar wind by a strong planetary magnetic field. The solar wind rams into Mars' ionosphere and generates complicated magnetic fields (wavy lines) that can obscure any magnetic anomalies in the crust. Orbits that dip beneath the ionosphere are highly valuable because they take us closer to the crustal magnetic fields that we wish to measure and farther from ionospheric magnetic fields. The electron reflectometer helps the magnetometer by signaling when the spacecraft is beneath the ionosphere.
The goal of aerobraking is to reach a circular orbit 235 miles above the surface. This is shown by the dashed line. On the day side of the planet, the spacecraft would fly within the ionosphere, where it can be a difficult task to measure crustal magnetic anomalies. Our best chance of detecting crustal magnetic anomalies in such an orbit will be on the planet's night side, where the ionosphere is much weaker or absent. During this phase of the mission, the electron reflectometer will look for electrons reflected from magnetic anomalies on the surface. These reflected electrons tell us how much stronger the surface field is compared with the field measured at the spacecraft by the magnetometer. This technique allows us to continue studying crustal anomalies at higher altitudes.