A Study of an Upper Layer in the Topside Ionosphere of Mars Using MARSIS
A.J. Kopf, D.A. Gurnett, D.D. Morgan, D.L. Kirchner
The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard
ESA's Mars Express spacecraft has now provided over two years worth of
measurements of the Martian ionosphere. MARSIS operates by transmitting a
radar pulse, using its echo to study the topside ionosphere at Mars. Initial
analysis, which focused on the main ionospheric layer, revealed a peak electron
density for the ionosphere of roughly 10^5 cm^-3 at an altitude of about 130 km,
consistent with the results from the Viking landers in the 1970s. However,
analysis of the MARSIS data also unexpectedly revealed a vertical density
profile at a higher altitude, indicating the presence of another distinct
layer higher in the ionosphere. The peak density of this layer is typically
around 10^4 cm^-3, and peaks at an altitude near 200 km. The most probable
cause for this feature is a peak in the density of O+ ions, which the Viking
landers previously found to peak near 225 km. Other possible causes, including
solar wind transport, magnetic field effects, and Kelvin-Helmholtz
instabilities, have also been considered. This feature has been detected at
many locations in the planet's ionosphere at various spacecraft altitudes, and
shows no indication of time dependence. Therefore, it is believed to be a
normal part of the Martian ionosphere and not due to transient variations in
the solar UV flux. However, this feature does exhibit some dependence on solar
interaction, as it is most prevalent at lower solar zenith angles. In addition,
efforts to explain this feature have been further confounded by the occasional
detection of a third density falloff at even lower frequencies, likely
corresponding to another layer at even higher altitudes. Any model explaining
the feature near 200 km should also allow for this higher feature as well.
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