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Mars Express Data Reveal New Method to Measure the Magnetic Field ofMars



 
 
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Default Mars Express Data Reveal New Method to Measure the Magnetic Field ofMars

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MARSIS data reveal new method to measure the magnetic field of Mars
European Space Agency
22 Sep 2009

Unusual signals detected by the Mars Express MARSIS instrument have
been used to determine the magnetic field strength of Mars. In a
forthcoming issue of Icarus, Ferzan Akalin and colleagues demonstrate
how the MARSIS instrument can be used as a magnetometer - an
unexpected
application with important consequences for studies of local plasma
effects and the Mars Express spacecraft environment.

Our current knowledge of the martian magnetic field is based on data
gathered by magnetometers on board the many spacecraft that have
explored Mars to date, most notably those on the Mars Global Surveyor
and Phobos 2 spacecraft. Studies have shown that the magnetic field is
variable and dynamic in nature and much still remains to be learned
about it.

Mars has no strong global magnetic field like that of the Earth.
Instead
it has a patchy crustal magnetic field originating in locally
magnetized
regions of the planet's crust. In addition, on the dayside of the
planet
an induced magnetic field is present. It originates from the magnetic
field that is carried by the solar wind which gets compressed at Mars
and is draped around the planet. In the absence of a strong global
magnetic field, the solar wind interacts directly with the planet's
ionized atmosphere (ionosphere). The interaction results in the
formation of two clear boundaries: the bow shock and the magnetic
pile-up boundary (MPB), see Figure 1. At the bow shock the solar wind
is
decelerated from supersonic to subsonic speed as its flow is
obstructed
by Mars. Closer to the planet's surface is the MPB, which is the top
of
the induced magnetic field.

Both the crustal and induced magnetic fields play an important factor
in
the space environment near Mars, as well as its ionosphere. The
instrument suite of Mars Express comprises 7 instruments designed to
characterise the atmosphere, surface and sub-surface of the planet but
does not include a magnetometer.

The Mars Advanced Radar for Subsurface and Ionospheric Sounding
(MARSIS)
instrument was designed to probe the ionosphere, as well as the
surface
and sub-surface of Mars, using radar sounding.

In ionospheric sounder mode a strong, short duration (91.4 µs) radio
pulse is transmitted towards the planet by a 40-metre-long electric
dipole antenna. The time of arrival of the subsequent echoes - signals
reflected by different layers in the ionosphere or from the Martian
surface - is recorded and provides a direct measure of the distance
travelled by the radio pulse. The pulse is repeated at 160 different
frequencies between 0.1 and 5.5 MHz and an ionogram (see Figure 3) is
constructed from the measurements from which profiles of the
ionosphere
or surface can be constructed.

Unexpected echoes - key to measuring the magnetic field strength

Since 2005, when ionospheric sounding measurements with MARSIS began,
some surprising features have been noted in the ionograms. In
particular, a series of broadband, low-frequency echoes at equally
spaced delay times after the sounder transmitter pulse have been
recorded.

In an interpretation first put forward by Gurnett et al. (2005),
Akalin
and colleagues in this study consider these echoes to be the signal
from
large collections of electrons in the vicinity of the spacecraft that
have been collectively accelerated by the strong electric pulse (400
V)
of the MARSIS antenna. The electrons subsequently follow a cyclotron
orbit in the local magnetic field of Mars. As they return to the
antenna
after each cycle in their heliacal orbit their collective pulse signal
is repeatedly picked up by the antenna.

Interpreted this way, the unexpected pulsed echoes - electron
cyclotron
echoes - provide a direct measure of the local magnetic field strength
at the position of the spacecraft; as there is a one-to-one relation
between the period of an electron's cyclotron orbit in a magnetic
field
and the strength of that field. As Mars Express follows its highly
elliptical orbit around Mars the magnetic field strength can be mapped
at different altitudes and positions over the planet, building up a
global picture.

Measuring the dayside induced magnetic field in Mars's ionosphere

The ionospheric sounding measurements are routinely performed when
Mars
Express is around its closest point to Mars, and usually in the
altitude
range of about 275 - 1200 km. In one application of their study Akalin
and colleagues derive the strength of the local magnetic field from
ionospheric sounding data gathered by MARSIS between 30 July 2005 and
8
August 2007. They show that their method can be used to measure the
magnetic field on the dayside of the planet, and they determine the
strength of the induced magnetic field at altitudes up to the magnetic
pile-up boundary, at which point the magnetic field strength falls
below
the detection limit of this MARSIS method.

The results obtained with MARSIS agree well with previous measurements
made with the magnetometer aboard the NASA Mars Global Surveyor
mission
(for altitude ranges where both mission datasets overlapped). This
leads
the authors to conclude that the cyclotron interpretation of the
regular
echoes in the MARSIS data is correct thus providing an unexpected
method
and valuable tool for measuring the magnetic field strength of Mars.


*Reference publication*
Akalin, F. et al., /"Dayside induced magnetic field in the ionosphere
of
Mars",/ Icarus (2009) in press, doi:10.1016/j.icarus.2009.03.021

*Related Publication *
Gurnett, D.A. et al., /"Radar Soundings of the Ionosphere of Mars",/
Science, Volume 310, Issue 5756, pp. 1929-1933,
DOI:10.1126/science.1121868, 23 December 2005



 




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