The Vertical Extent of Enhanced Densities in Cusp-Like Regions of the Ionosphere of Mars

Unusually large values of peak electron density in the ionosphere of Mars are occasionally observed in cusp-like regions of crustal magnetic field. These features are observed as atypical examples of oblique echoes in ionograms. However, the vertical extent over which electron density values are enhanced has not yet been determined. Furthermore, the physical processes responsible for these striking features are not yet understood. Here we analyze ionograms from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on Mars Express to better constrain the spatial distribution of electron density in and around these rare ionospheric features. Data from orbits 2359, 2491, 4469, and 4513 are examined. When peak electron density is unusually large, electron density values from the peak to 165 km are increased by 30% +/- 10% and the electron density at the spacecraft (similar to 300-400 km) is diminished. We investigate whether the central portion of an oblique echo is equivalent to a nadir echo, which would permit the reliable determination of an electron density profile in instances of unusually large peak electron densities. We find that uncertainties in the location of the center of an oblique echo render this approach unfeasible. We also report that 86.5 mu s should be subtracted from travel time values reported in current MARSIS data products released prior to 2019.

Automated summary

Unusually large peak electron density values are occasionally observed in the ionosphere of Mars, particularly in cusp-like regions of crustal magnetic field, displaying atypical examples of oblique echoes in ionograms. However, the vertical extent and physical processes behind these features are still not well understood.