Radar Exploration of Mars: Recent Results and Progresses - Planetary Exploration and Science: Recent Results and Advances - page 91

the ionization of the atmosphere is higher and the recombination is lower; as a
consequence, high values of the electron density are present even on the night side
(Safaeinili et al. 2007 ).
Many aspects of Martian ionosphere have been investigated in the past (Gurnett
et al. 2008 ; Leblanc et al. 2008 ;Nemec et al. 2010 ). In particular, it was proved
that the total electron content (TEC) is higher where the local magnetic field is
nearly vertical (Safaeinili et al. 2007 ), and it was observed that during solar particle
events, the TEC increases by a factor of 2 or more (Lillis et al. 2010 ). Recently,
it has been proved that MARSIS data in subsurface mode, usually used to detect
subsurface features, can be processed in order to obtain reliable estimates of the
TEC of the Martian ionosphere (Cartacci et al. 2012 ). This task was pursued using
the Contrast Method (CM). CM is a tool developed to compensate distortion effects
on radar signals due to the Martian ionosphere in order to analyze Martian surface
and subsurface more efficiently.
Radar signals have a penetration depth in the subsurface approximately pro-
portional to their wavelength. But, in order to propagate through the Martian
ionosphere, MARSIS needs to operate with frequencies above the maximum plasma

frequency, f p ,givenbyf p . z / D 8:98 p N e . z /,where N e , the electron density in m 3 ,
is a function of the altitude z (Gurnett and Bhattacharjee 2005 ). For this reason,
Martian areas to be observed by the radar are selected mainly on the night side
where the values of the sun elevation angle (SEA), which is the angle between
the direction of the geometric center of the Sun apparent disk and the horizon, are
negative and the electron density is known to be lower than on the day side. In fact,
all frequencies lower than f p are reflected. Frequencies higher than f p are attenuated,
delayed, and are subject to phase distortion proportionally to the electron density of
the ionosphere.
In order to properly analyze MARSIS data, it is, therefore, necessary to correct
the phase distortion of the signal or, at least, to reduce it. Many methods have been
suggested (Safaeinili et al. 2003 ; Ilyushin and Kunitsyn 2004 ; Mouginot et al. 2008 ;
Zhang et al. 2009 ) through the years; compared to them, CM is a simpler algorithm
which proved to be a very reliable method.
The correctness of the application of CM to Mars TEC estimation is also proved
by matching, with remarkable approximation, the theoretical model proposed by
Chapman in 1931 (Fig. 5.15 ).
From the analysis of MARSIS data in subsurface mode, it has been possible to
estimate the TEC of the Martian ionosphere and to produce the related maps, to
evaluate its dependence by solar activities, and to confirm the result of Safaeinili
et al. ( 2007 ) about a connection between the orientation of the crustal magnetic
field and the measured TEC, which appears to be higher where the magnetic field
direction is quasi-vertical and lower where the magnetic field direction is quasi-
horizontal (Fig. 5.16 ).