Evidence of a Second Layer in the Topside Ionosphere of Mars
A.J. Kopf, D.A. Gurnett, D.L. Kirchner, D.D. Morgan
INTRODUCTION
The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard
ESA's Mars Express spacecraft has now provided nearly two years of radar
measurements of the Martian ionosphere. Spacecraft radar sounders, developed in
the 1960s to study Earth's ionosphere, have proven to be a powerful tool for
studying ionospheric physics. Before MARSIS, most knowledge of the
Martian ionosphere came from radio occultation measurements, which could only be
performed near the limbs of the planet. MARSIS data nicely complement these
measurements by providing better spatial resolution and the ability to observe
regions where radio occultation can not be performed.
PRINCIPLES OF IONOSPHERIC SOUNDING
A horizontally stratified ionosphere provides a near perfect reflecting surface
for radar 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 the 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 and reflect from the surface of the planet
itself.
PREVIOUS ANALYSIS
MARSIS data are analyzed using an ionogram, which plots the time delay of the
radar 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 at least two orders of magnitude higher
than the background noise. As the 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". Cusps indicate locations of
maximums in the electron density as a function of altitude.
A rough estimate of the density profile can be obtained simply by using the
apparent range to the reflection point; however, accurate measurements require
correcting for dispersion in the ionospheric plasma. Since the time delay is
known, the altitude as a function of the plasma frequency and, by conversion,
the altitude as a function of the density can be computed for an assumed
horizontally stratified ionosphere. 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.
Previous analysis has shown that the main Martian ionospheric
layer has a cusp corresponding to a peak electron density of about 10^5 cm^-3
at an altitude of about 130 km, consistent with the results from the Viking
landers of 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 and will be published in Advances in Space
Research.
AN UPPER LAYER
Most MARSIS ionograms 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 performed in order to ascertain the properties of this feature, most
notably at what density and altitude the discontinuity occurred.
Analysis has been completed on roughly 1500 ionograms resulting from more than
50 orbits of the planet. This study indicates that the peak density of this
layer is typically around 5 x 10^4 cm^-3 at an altitude above 200 km. 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 any time dependence, implying that
the presence of this layer is likely a normal part of the Martian ionosphere and
not simply due to transient variations in the solar UV flux, though the ability
to detect this feature does show a clear dependence on solar zenith angle.
POSSIBLE EXPLANATIONS
Now that the properties of this feature are known with great precision, the
focus of our research has turned to understanding the origin of the second
layer. In discussion, numerous possible explanations have been raised, though
none have proven to be convincingly correct to this point.
This feature was originally theorized to be a distinct upper layer of the
ionosphere dominated by a peaking density of O+ ions, and this idea remains the
leading candidate among all considered explanations. These ions had been
detected by
the Viking landers to peak in the 200-250 km range and become significant in
respect to the dominant ions of the main layer at higher altitudes.
Results of our analysis support the possibility of a layered ionosphere,
and the altitude of this feature corresponds well with the Viking detection of
O+ ions. However, the density of these ions measured by the Viking probes would
seem to imply they are not populous enough to produce the observed effect.
Still, with only these two direct measurements to go on, there simply is no
conclusive argument for or against this possibility.
The altitude of this layer's peak density, however, does raise other questions.
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. As a result, solar wind interaction may also be a key factor in the
observance of this feature. The solar wind could play one of two roles. First,
the presence of energetic electrons in the wind could act to further ionize the
ionosphere at this layer, causing the sudden change in density and slope of the
reflection that is observed on the ionogram. In addition, the solar wind could
also act in transport fashion, transporting parts of the ionosphere horizontally
along the planet, and eventually away from Mars, which could contribute to the
detected atmospheric loss occurring at Mars. In any case, solar interaction
appears to play a role, as the detection of this feature has proven to be
dependent upon the solar zenith angle. This explanation may be the most
probable; however, the lack of consistent
detection raises questions on the origins and stability of this layer.
Where stability is called into question with plasmas, it is appropriate to
consider the possibility that Kelvin-Helmholtz instabilities may be playing a
role as well. These instabilities occur at the boundary of two plasmas of
different densities. The result is a mixing effect, but one that, if oriented
correctly, could cause sudden changes in the density observed by topside
ionospheric sounders like MARSIS. At this altitude where the ionosphere is no
longer in photochemical equilibrium and has more interaction with the solar
wind, the possibility that two different plasmas, in this case the ionosphere
and the solar wind, could be mixing is certainly something worth
considering.
Attempts to explain this feature have been further confounded by the occasional
detection of a similar feature even higher in the ionosphere. In ideal
observations, where interference at the lower frequencies is minimized, it has
proven
possible to identify a third cusp in the ionospheric trace, indicating another
location where a critical point occurs in the density profile. Any model
explaining the cusp near 200 km should also allow for this higher cusp as
well.
With the existence of this feature no longer in question, the focus of our
continuing research is now on explaining its origin. More intensive study will
have to be done in an attempt to ascertain the cause or causes of this sudden
change in the density profile of the Martian ionosphere.
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