Evidence of a Second Layer in the Topside Ionosphere of Mars
A.J. Kopf, D.A. Gurnett, D.L. Kirchner, D.D. Morgan
The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard
ESA's Mars Express space probe has provided nearly two years of measurements of
the Martian ionosphere. The data are displayed as an ionogram, which plots the
time delay of the received reflections as a function of frequency, color coded
for amplitude. An ionospheric 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. As frequency increases, the trace
generally exhibits 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.
Previous analysis of MARSIS data has shown that the main ionospheric layer on
the dayside of Mars has a
peak electron density of roughly 10^5 cm^-3 at an altitude near 130 km. MARSIS
ionograms also commonly show a second cusp at a lower frequency, indicating the
presence of another distinct layer higher in the ionosphere. The peak density of
this layer is about 5 x 10^4 cm^-3 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,
are also being studied. Analysis shows this feature to be a normal part of the
Martian ionosphere and not due to 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.
Back to Conferences and Presentations
Back to Andrew's Home Page
