Magnetically Controlled Density Structures in the Martian Ionosphere: Are they stably recurring?

MARSIS instrument aboard the Mars Express spacecraft regularly detects the magnetically controlled density structures (MCDSs) in the Martian ionosphere. In this paper, we studied the recurrence characteristics of these MCDSs using long-term (more than a decade) MARSIS observations over a selected region of 35 degrees to 40 degrees longitude and -10 degrees to 10 degrees latitude, where a strong magnetic anomaly is present. Instead of a conventional echogram analysis, we used the peak density echogram analysis as the latter is able to consider all the vertical and oblique echoes even if their peak densities are varying. The results of the present study show that MCDSs are in fact formed persistently on all dayside periapsis passes. More importantly, there are discontinuities in the hyperbola shape in a peak density echogram display that appear at different locations on different orbital passes. In addition, the apices of the hyperbolas associated with an MCDS are observed at different latitudes on different orbital passes. From these results, we infer that the locations and shapes of MCDSs are disturbed by some time-varying external factors so that some parts of the MCDS cannot give specular reflections. These hypotheses were demonstrated with some simple ray propagation simulations. Nearly similar features are observed with the nightside MCDS, but they occur only 60% of the passes. We surmise that wave dynamics associated with tides and gravity waves and so forth might be responsible for altering the shape and location of MCDS, both on the dayside and nightside.