Detection of an Upper Layer in the Topside Ionosphere of Mars Using the Mars
Express Ionospheric Sounder
A.J. Kopf, D.A. Gurnett, D.L. Kirchner, D.D. Morgan, and T.F. Averkamp
The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard
ESA's Mars Express space probe has now provided nearly two years of radar
measurements of the Martian ionosphere. This data is most commonly analyzed as
an ionogram, which plots the time delay of the received reflections as a
function of frequency, color coded for amplitude. An ionospheric radar echo
appears as a trace exhibiting a smooth increase in time delay with frequency
that has an intensity typically at least two orders of magnitude higher than
the background noise. As frequency increases, this trace typically exhibits an
abrupt increase in time delay at some frequency, forming a discontinuity in the
trace that we call a "cusp". These cusps indicate locations of maximums in the
electron density as a function of altitude.
Previous analysis of MARSIS data has shown that the main Martian ionospheric
layer has a peak electron density of roughly 10^5 cm^-3 at an altitude of near
130 km, consistent with the results from the Viking landers in the 1970s.
However, MARSIS ionograms have also commonly shown a cusp at higher altitude,
indicating the presence of another distinct layer higher in the ionosphere.
Early analysis indicates that the peak density of this layer is typically around
5 x 10^4 cm^-3 at an altitude of above 200 km. We believe the most probable
cause for this feature is a peak in the concentration of O+ ions, which were
previously detected to peak at around 225 km by the Viking landers. In addition,
we consider the possibility that this second cusp could be due to magnetization
in the Martian ionosphere. 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, regardless of its source, this feature is
believed to be a normal part of the Martian ionosphere and not due to variations
in the solar UV flux. Research is underway to precisely analyze the properties
of this feature of the upper Martian ionosphere and to determine its cause.
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