Geoscience Reference
In-Depth Information
SHORT ELECTRIC-FIELD ANTENNAE AS
DIAGNOSTIC TOOLS FOR SPACE PLASMAS
AND GROUND PERMITTIVITY
JEAN-GABRIEL TROTIGNON
Laboratoire de Physique et Chimie de l'Environnement, Centre National de la
Recherche Scientifique, Orleans University
3 A, Avenue de la Recherche Scientifique, F-45071 Orleans cedex 02, France
Jean-Gabriel.Trotignon@cnrs-orleans.fr
Certain techniques used in geophysical prospection to measure ground permit-
tivity have been transposed to space plasmas. The principle is to measure the
self impedance of a single electric antenna or the mutual impedance between
two sets of Hertz dipoles. Since probe impedances depend on dielectric prop-
erties of the immersed medium, space plasma parameters such as the density
and temperature of thermal electrons may be reliably and accurately deduced.
Natural waves may also be investigated in a large frequency range includ-
ing characteristic plasma frequencies. As any electrode immersed in a plasma
acquires a charge, it perturbs the plasma in its immediate neighborhood. The
way to get around this di
culty is to use four electrodes, two for transmit-
ting and two for receiving. Transmitting electrodes are excited from a signal
generator, while receiving electrodes are connected to a high input impedance
voltmeter. The transmitted current
I
and the received voltage
V
being known,
plasma properties may be deduced from both the imaginary and real parts of
the mutual impedance
Z
=
V/I
. Such quadripole probes have been used on
sounding rockets and spacecraft.
1. Introduction
The impedance of an electric antenna depends on the dielectric properties
of the medium in which the antenna is immersed. Electric sensors have
therefore to be calibrated in flight in order to get usable E-field measure-
ments. It was, for example, carefully done onboard the EIDI 1 ionospheric
rocket,
1
and onboard the EXOS-D (Akebono)
2
and GEOTAIL
3
magneto-
spheric spacecraft by measuring the antenna impedances all along the space-
craft trajectories. This is not the purpose of this paper to recall how these
calibrations were achieved, but we can mention than the main objective is
to determine what deformations actually affect the amplitude and phase
of the detected electric fields when the antenna is immersed in a plasma.
3
As no plasma chamber is large enough to contain such long electric-field
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