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
INTRODUCTION
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.
Spacecraft radar sounders were originally developed in the 1960s to study
Earth's ionosphere and have proven to be a powerful tool for studying
ionospheric physics. Before MARSIS, most of our knowledge of the Martian
ionosphere came from radio occultation measurements. The MARSIS measurements
nicely complement these measurements by providing better spatial resolution and
the ability to make observations in regions where radio occultation can not be
performed.
PRINCIPLES OF IONOSPHERIC SOUNDING
A horizontally stratified ionosphere provides a near perfect reflecting surface
for radio echo sounding. The reflection occurs because free-space
electromagnetic radiation can not propagate at frequencies below the electron
plasma frequency. Thus, at frequencies below the maximum plasma frequency of
the ionosphere, the transmitted frequency will be reflected as soon as it
reaches and altitude where its frequency equals that of the plasma frequency of
the ionosphere. At frequencies above the maximum plasma frequency, the pulse
will pass through the ionosphere unreflected and will eventually echo off the
surface of the planet itself.
THE IONOGRAM
MARSIS data are most commonly analyzed in a format known as an ionogram, which
plots the time delay of the received echo as a function of the 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.
ANALYSIS TECHNIQUES
Although a rough estimate of the density profile can be obtained simply by using
the apparent range to the reflection point, accurate measurements require
correcting for dispersion in the ionospheric plasma. Assuming a horizontally
stratified ionosphere, the round trip time delay as a function of frequency is
given by twice the integral of the infinitesimal altitude divided by the group
velocity. The integration is carried out with respect to altitude from the
reflection point to the spacecraft altitude.
Since the time delay is known, the integral can be inverted to find the altitude
as a function of the plasma frequency and, by conversion, the altitude as a
function of the density. Our approach to finding a density profile was to use a
theoretical model and then adjust the parameters to give the best overall fit to
the measurements. For a density model, we used the Chapman model from his 1931
paper. This fit yields the maximum density of the ionospheric layer, as well as
the altitude at which this density occurs.
THE MAIN LAYER
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 on the 1970s showing that this main layer is primarily composed of O2+
and CO2+ ions. A summary of the results of the analysis of the main layer are
published in a paper by
Gurnett, et al (2005), and a new paper, with updated results and new
findings, was submitted in October.
AN UPPER LAYER
Most MARSIS ionograms will display a cusp at the maximum plasma frequency of the
main layer of the ionosphere. However, MARSIS ionograms have also commonly
shown an additional cusp at a higher altitude, indicating the presence of
another distinct layer higher in the ionosphere. Analysis similar to that of
the main layer was performer in order to ascertain the properties of this
feature, most notably at what density and altitude the discontinuity
occurred.
Early analysis has been completed on roughly 1000 ionograms resulting from more
than 30 orbits of the planet. This study indicates that the peak density of
this layer typically ranges between 4 to 6 x 10^4 cm^-3 at altitudes primarily
just above 200 km, but ranging to near 300 km in some cases. While this feature
is not seen in every ionogram, detection has been made at many locations in the
planet's ionosphere at various spacecraft altitudes. In addition, it has shown
no clear indication of a time dependence, implying that the presence of this
layer is a normal part of the Martian ionosphere and not due to transient
variations in the solar UV flux.
DISCUSSION
When this feature was first identified this past summer, it was at that point
theorized to be an upper layer of the ionosphere comprised primarily of O+ ions.
These ions had been detected by the Viking landers to have rising densities in
the 200-250 km range to the point where they were significant in respect to the
dominant ions of the main layer. Results of our analysis seem to support that
theory, as both the altitude and density of this feature correspond to a layer
dominated by O+ ions.
However, while analysis of this cusp has shown that it is a common feature on
ionograms, it also has some peculiar properties. While the main layer has
proven to be visible in some form in virtually every ionogram, this upper layer
is less common, only appearing about half the time in the highest levels of
solar interaction that have been analyzed so far, and even less commonly as the
level of sunlight decreases. This dependence on solar zenith angle is not
unexpected, as the main layer also becomes lesss detectable and less clearly
resolved in low sunlight; however, the lack of consistent detection raises
questions on the origin and stability of this layer.
In discussion with colleagues, an alternate theory has been suggested in an
effort to explain this feature if it were to not be an upper layer of O+ ions.
It is rather well established that the departure from photochemical equilibrium
to transport domination takes place near 200 km on the dayside of the Mars
ionosphere. Thus, the change in slope could be simply due to a transition from
chemical equilibrium to transport control. We find this theory unlikely,
however, given the inconsistent yet sometimes drastic change in slope in the
echo. In addition, supporting our view, is the rare but occasional detection of
a third cusp, occurring at an even higher altitude in the ionosphere, which
would have no logical transition such as to transport control as suggested
above.
FUTURE WORK
More analysis must be done to attempt to answer remaining questions about this
feature of the ionosphere. In particular, it is necessary to analyze data from
orbits where the solar zenith angle ranges between 0 and 45 degrees in order to
allow for conclusion on how solar interaction affects both the detectability and
characteristic properties of this upper layer. Careful analysis will also be
performed to determine if any connection between the ability to detect this
layer and the variations in the crustal magnetic field of Mars exists as
well.
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