Geoscience Reference
In-Depth Information
antennae, the only way is therefore to implement onboard calibrations. The
other consequence is that main characteristics of the encountered plasma,
such as the density and temperature of the thermal component of elec-
trons may be determined from the antenna impedance measurements. If a
suitable probe can be designed and if a theory exists that describes how
the antenna impedance depends on these parameters, impedance measure-
ment indeed becomes a powerful technique for space plasma diagnostic.
As a bonus, natural waves may also be investigated in a large frequency
range including characteristic plasma frequencies such as the lower- and
upper-oblique resonances, the electron gyrofrequency and its harmonics,
the plasma frequency, the upper hybrid frequency, and so on.
The capability of such a technique has been proven onboard rockets and
spacecraft since several decades. The objective of the current paper is to
briefly recall how the impedance probes are working and, in particular, to
show how the major diculty that their electrodes perturb the plasma is got
around. Then, not necessarily very recent but well representative quadripole
impedance probes that flew onboard a rocket (CISASPE), two spacecraft
(GEOS-1 and AUREOL-3/ARCAD-3), and finally CASSINI/HUYGENS
are presented.
2. How the Impedance Probe is Working
The impedance probe consists of the probe itself and the electronics that
measure the impedance. The major diculty is that electrodes actually per-
turb the plasma, this is due to the presence of ion sheaths around the elec-
trodes that insulate them partially from the unperturbed plasma. Another
diculty comes from the antenna sensitivity to quasi-static disturbances
originating in the spacecraft body and structures, that is why monopoles are
rarely used and symmetrical dipole preferred because differential measure-
ments nominally reduce these disturbances. An elegant way to get around
these diculties
4
is to use four electrodes, two for transmitting and two
for receiving, and to measure the mutual impedance
Z
=
V
/
I
.Figure1
illustrates the process. This technique has been used for many years in
geophysical prospection to measure the ground permittivity. Transmitting
electrodes are excited from a signal generator, while the receiving electrodes
are connected to a voltmeter with a very high input impedance. By exciting
the transmitting electrodes from a constant current source (source internal
impedance
π
transmitting electrodes self-impedance), a single measurement
(
V
) is actually necessary to obtain the mutual impedance
Z
=
V
/
I
.Aslong
as transmitting electrodes are placed at “large” distances from the receiving
