A Study of an Upper Layer of the Martian Ionosphere Using the Mars Express
Ionospheric Sounder
A.J. Kopf, D.A. Gurnett, D.L. Kirchner, T.F. Averkamp, D.D. Morgan
Since its deployment in June of 2005, the Mars Advanced Radar for Subsurface and
Ionosphere Sounding (MARSIS) aboard ESA's Mars Express spacecraft has sent back
nearly a year and a half worth of measurements of the Martian ionosphere. These
data are most commonly analyzed in a format known as an ionogram, which plots
the time delay of the received radar echo 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 and an intensity typically at least
two orders of magnitude higher than the background noise. As frequency
increases, this trace typically displays 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 cusp corresponding to a peak electron density of roughly 10^5 cm^-3
at an altitude of about 130 km, consistent with the results from the Viking
landers in the 1970s showing that this main layer is primarily composed of O2+
and CO2+ ions. However, MARSIS ionograms have also commonly shown another cusp
at a 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 10^4 cm^-3 at an altitude of above 200 km. We believe this
layer is most likely due to a peak in the concentration of O+ ions, which was
previously detected at around 225 km by the Viking landers. This detection has
been made at many locations in the planet's ionosphere at various spacecraft
altitudes, and has shown no clear indication of a time dependence. Therefore,
the presence of this layer is believed to be a normal part of the Martian
ionosphere and not due to transient variations in the solar UV flux. Research
is underway to more precisely analyze the properties of this layer and to
confirm that this layer consists of atomic oxygen ions.
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