ISSN 0010-9525, Cosmic Research, 2018, Vol. 56, No. 3, pp. 180–189. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © V.M. Smirnov, O.V. Yushkova, V.N. Marchuk, 2018, published in Kosmicheskie Issledovaniya, 2018, Vol. 56, No. 3, pp. 199–208.
Using the Subsurface Soil Sounding Radar for Investigating
the Structure and Total Electron Content of the Martian Ionosphere
V. M. Smirnov
, O. V. Yushkova
, and V. N. Marchuk
Kotelnikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, 111250 Russia
Received October 19, 2016
Abstract—The possibilities of using the Martian soil subsurface sounding radar for investigating the structure
of the plasma shell surrounding the planet have been considered. Based on the numerical modeling results
and actual soil sounding data, it has been shown that the soil sounding mode of the radio-locating MARSIS
radar can be used to assess the structure of the Martian ionosphere. As the emitted signals pass to the planet’s
surface, it is possible to use the reflected signals to estimate the total electron content of the Martian iono-
sphere along the flight track of the spacecraft.
In December 2003, the Mars Express spacecraft of
the European Space Agency entered the Martian orbit
carrying the Mars Advanced Radar for Subsurface and
Ionospheric Sounding (MARSIS) instrument on
board . The MARSIS radar has three operating
modes, i.e., subsurface sounding (SS), which is
intended for investigating the upper layer of soil during
a flyby of the spacecraft above the planet’s surface
during nighttime; active ionospheric sounding (AIS)
for studying the ionosphere during daytime; and pas-
sive sounding for detecting the electromagnetic cos-
mic noise within the frequency range of the receiver.
Although the first attempts at radar studies of the soil
using the orbital spacecraft were made in 1972 , the
Mars Express became the first orbiter with a special-
ized subsurface radar on board. An analysis of the
MARSIS measurement results in the SS mode has
shown that, in addition to its primary task, i.e. study-
ing the dielectric properties and structure of the soil,
the subsurface radar allows the planet’s ionosphere to
be studied during night and twilight hours.
Until recently, information on the Martian iono-
sphere was based on the results of the radio occultation
experiments, which assessed the average electron den-
sity of the ionosphere along the line of sight between
the Earth and a spacecraft in orbit around Mars [3, 4].
After 2005, these data were complemented by the
MARSIS radar measurements . The available
information provides a good picture of the daytime
ionosphere. However, the data on the night, morning,
and evening (twilight) ionosphere are obtained based
on an analysis of single experiments, which do not
allow for a full understanding of the daily variations in
the altitudinal profile of the electron density. In this
paper, the possibility of using the subsurface sounding
radar to study the planetary ionosphere during twilight
hours has been discussed. Based on a comparison of
the numerical modeling of the radio signal propaga-
tion along the ionosphere–soil–ionosphere trace and
an analysis of the MARSIS experimental data in the
SS mode, a method of determining the critical fre-
quency of the ionosphere and detecting the nonuni-
form formations is demonstrated. A procedure for
estimating the total electron content (TEC) of the ion-
osphere has been proposed. The conclusions have
been confirmed by the results of processing the MAR-
SIS measurements for track 1855 (June 26, 2005).
1. OPERATING SEQUENCE
OF THE MARSIS RADAR
The Mars Express orbiter is in an elliptical orbit
with a periapsis of 300 km and apoapsis of 1300 km.
Depending on the operating mode, the MARSIS
radar follows either the soil sounding (SS) or iono-
sphere sounding (AIS) sequence. A diagram of the
process is shown in Fig. 1. Depending on the fre-
quency of the emitted signal, either the ionosphere or
soil of the planet are sounded.
In the AIS mode, MARSIS functions as an iono-
sonde and is used for studying the upper ionosphere of
Mars. For the ionospheric sounding in the range
between 0.1 and 5.5 MHz, 160 frequencies are emitted
in a step sequence with a quasi-logarithmic spacing of
their nominal value (Δf/f ≈ 2%). The minimum fre-
quency step is 10.937 kHz.