Magnetically controlled density structures in the topside layer of the Martian ionosphere

Magnetically controlled density structures in the main layer of the Martian ionosphere are regularly detected by the Mars Advance Radar for Subsurface and Ionospheric Sounder (MARSIS) on board the Mars Express spacecraft when it passes over the regions of strong crustal magnetic fields. These structures appear as oblique echoes in an ionogram and as hyperbola-shaped feature in a radargram. For the first time, we observed such oblique echoes from the topside layer of the Martian ionosphere. Comparison with a crustal magnetic field model suggests that these echoes mostly occur in regions close to magnetic field anomalies. The MARSIS ionograms that show these oblique echoes are limited in number. One reason for such limited occurrence is the fact that these echoes occur at the low-frequency end of an ionogram where local plasma frequency lines and electron-cyclotron lines make it difficult to discern these echoes. In one case, however, we observed the topside layer with oblique echoes on several continuous ionograms and a hyperbola-shaped feature on a radargram. The apex of the hyperbola falls in the region of vertical magnetic fields. Unlike the main layer, the recurrent occurrence of the topside layer at the same location is not common. More importantly, the oblique echoes from the topside layer are always associated with those from the main layer of the ionosphere, but not vice versa. The simultaneous observation of oblique echoes from the main and topside layers is explained by considering the formation of ionization bulges at the respective layer altitudes.