Extremely High Plasma Densities in the Mars Ionosphere Associated With Cusp-Like Magnetic Fields

The Martian crustal magnetic field modifies the structure of the ionosphere, the loss of atmosphere to space, and the interactions of the upper atmosphere with the solar wind. The Mars Advanced Radar for Subsurface and Ionosphere Sounding topside sounder on Mars Express has regularly observed oblique echoes from magnetically controlled plasma structures in regions of strong and vertical crustal magnetic fields. Here we present 11 observations of oblique echoes with unusually high plasma densities and compare them to more common oblique echoes with typical ionospheric densities. In two cases, the density is at least 3.6 x 10(5) cm(-3), perhaps the highest recorded in the Mars ionosphere. We find that high-density features occur only where the magnetic inclination angle is nearly vertical and the magnetic field is sufficiently strong (>200 nT at 150 km). Additionally, the more frequently the magnetic field is open at a given location, the greater the probability that a high-density echo is found there. Oblique echoes with typical densities, in contrast, are found at a wider range of magnetic geometries and strengths and are coherent and stable across as much as 45. in longitude (-53 degrees latitude and 165-210 degrees E longitude) and 5.5 years of observation. Based upon these findings, we suggest that these high-density features occur where the magnetic field is open and interactions with the solar wind, such as plasma instabilities arising from heating by the solar wind electric field, have increased ionospheric plasma densities. The more common oblique echoes with typical densities are more likely to be created by transport along closed field lines in response to pressure gradients and winds.