Magnetically Controlled Density Structures in the Martian Ionosphere: Are they Stably Recurring?

Abstract

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° to 40° longitude and −10° to 10° 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.

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

“…In Figure , the simulated echograms (black lines) are overplotted with the corresponding echograms of orbit 4447. The simulated echograms at frequencies of 3 and 3.5 are well fitted with the respective echograms observed on orbit 4447, in accordance with the previous simulations (Duru et al, ; Mohanamanasa et al, ). The simulated echogram at 4.5 MHz also correlates well with the observed one, suggesting that the assumed density structures are appropriate ones.…”

“…The above analysis suggests that the enhanced densities are in fact the MCDS of the type reported earlier (Andrews et al, 2014;Diéval et al, 2015;Duru et al, 2006;Gurnett et al, 2005;Mohanamanasa et al, 2018) but occur at very low altitudes. By considering these characteristics, the shapes of the density structures that are responsible for the observed electron densities are illustrated through a schematic diagram in Figure 4.…”

“…It is evident from this figure that the observed high-density profiles are in regions of strong magnetic anomaly regions but may not fall exactly in regions of vertical magnetic fields. Such behavior was also observed for density structures from the main ionospheric layer (Duru et al, 2006;Gurnett et al, 2005;Mohanamanasa et al, 2018).…”

“…In the following, we use ray propagation simulations to depict the density structures that are responsible for the observed hyperbola shapes in MARSIS echograms. The ray propagation simulations can tell not only about the shapes of the density structures (Duru et al, ) but also their tilt with respect to the mean position (Mohanamanasa et al, ) and the relative positions of the density structures at different altitudes as will be shown below.…”

Oblique Reflections of Mars Express MARSIS Radar Signals From Ionospheric Density Structures: Raytracing Analysis

“…It is therefore possible that the density structure squeezed between two consecutive ionograms. It is also interesting to note that—although apparently quite stable features (Andrews et al, ; Mohanamanasa et al, )—such density structures have not been so far observed in the local electron density data obtained by the Mars Atmosphere and Volatile EvolutioN Mission spacecraft. This seems to be consistent with very small dimensions of these structures suggested by our analysis.…”