Isolated High-Density Structures in Magnetic Anomaly Regions of the Martian Lower Ionosphere

Abstract

The occurrence of density structures in magnetic anomaly regions is one of the prominent features of the Martian ionosphere. These structures, in general, occur at altitudes above the main peak. In the present study, we analyze four cases of unusually strong oblique echoes that occur in magnetic anomaly regions and are observed by the Mars Express MARSIS instrument. An in-depth analysis suggests that these strong oblique echoes, in fact, correspond to the density structures. After correcting for dispersion effects, these structures are found to occur in the lower ionosphere at altitudes as low as 88 km. Ray propagation simulations carried out in this study demonstrate that the density structures can be modeled as Gaussian shapes distributed along planetary latitude. The density of these structures decreases with an increase in altitude. Accordingly, the density structures at higher altitudes are associated with iso-density contours of the background ionosphere. In the lower ionosphere, however, the density structures are found to immerse in a background ionosphere whose density is much smaller than those of the density structures. The possible causes and consequences of such low altitude density structures are discussed. The results of this study are combined with those of previously reported studies to produce a comprehensive picture of these magnetically controlled density structures.