Understanding the structure and dynamics of the Martian thermosphere (100-220 km) is extremely important as this region, in particular the upper thermosphere, acts as a buffer zone between the reservoir of atmospheric species down below and the exosphere above from where the gaseous escape happens (e.g., Bougher, Cravens, et al., 2015). From this view point, the exobase acts as a lid on top of the thermosphere and the gaseous escape is regulated by the amount of energy, particles, and fields (both electric and magnetic) that reach the upper thermosphere. Thermospheric neutral densities, in general, decrease exponentially with an increase in altitude. Superimposed on this, there are perturbations of various scales that are due to forcings from above and below. Forcings from below include the planetary waves, thermal tides, and gravity

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The Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express spacecraft observes vertical echoes from the normal ionosphere and oblique echoes from ionization bulges. The peak frequencies of these two kinds of echoes are, in general, nearly equal. In the present study, we report the detection of oblique echoes whose peak frequencies are much larger than those of the vertical echoes. These echoes were observed on 15 June, 21 June, 28 June, and 10 July 2007, respectively. All these oblique echoes were observed from a region of strong crustal magnetic fields. After correcting for dispersion effects, the maximum electron density in the first case is found to be 3.75 × 10 5 cm −3 and is observed at an altitude of~88 km. In subsequent cases, a gradual decrease in peak electron density and an increase in altitude are observed. The causative mechanism of these oblique echoes is examined by considering various forces from top and bottom. During the times of the observations, there were no solar flares or meteor in fall. The presence of solar energetic particles also could not be firmly established. We found that these strong oblique echoes were observed at the initial phase and during the progression of a planet-encircling dust storm in Martian Year 28. A similar, but weaker features were observed in other dust storm seasons as well. Mechanisms causing such oblique echoes in regions of strong magnetic fields are discussed.

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