Radar Exploration of Mars: Recent Results and Progresses - Planetary Exploration and Science: Recent Results and Advances - page 86

Fig. 5.8 MARSIS ionogram

Before radar observations on Mars, most of the Martian ionosphere knowledge
derived from radio occultation measurements. MARSIS provided better spatial
resolution measurements and extended the study to regions where radio occultation
measurements cannot be made. Martian ionosphere is a perfectly reflecting surface
for radio echo sounding at frequencies below the electron plasma frequency f p . z / D

8:98 p N e . z / Hz, which is a function of the altitude, where N e is the electron density
in m -3 (Gurnett and Bhattacharjee 2005 ).
MARSIS data are presented as an ionogram, which is a plot of the echo strength
versus frequency and time delay. In a typical ionogram (Fig. 5.8 ), it is possible
to identify three types of echoes: a very strong spike at the local electron plasma
frequency f p-local , caused by excitation of electrostatic oscillations at the electron
plasma frequency; a second echo from f p-local to the maximum plasma frequency in
the ionosphere f p-max , due to a reflection by the ionosphere; and a third echo from
f p-max to the maximum sounding frequency, due to a surface reflection. In regions
characterized by strong crustal magnetic fields, a fourth oblique echo occurs.
MARSIS data are providing an essential contribution in the characterization
of the response of Martian ionosphere to various inputs such as solar EUV flux,
energetic particles, crustal magnetic fields, and solar wind. They are also analyzed
to study the effects due to seasons, latitude, and local time. An example of MARSIS
data contribution to the study of a peculiar aspect of Martian ionosphere will be
presented in Sect. 5.5.2 .