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COMPARATIVE ANALYSIS OF CURRENT DENSITY METERS OPERATING IN SPACE PLASMAS COMPARATIVE ANALYSIS OF CURRENT DENSITY METERS
OPERATING IN SPACE PLASMAS
V.

Korepanov, F. Dudkin
Lviv Centre of Institute of Space Researches, 5-A Naukova str., 290601, Lviv, Ukraine
ABSTRACT
For experimental investigations of wave processes in space plasma one of the main tasks is the determination of
dispersion relations between the wave vector and the frequency. The frequency analysis of fluctuations of
magnetic field and electric current density in plasma is very efficient in this case. The measurements of the
magnetic field fluctuations usually are made by a variety of magnetometers using well developed methods.
Unfortunately, up to the moment there are no reliable measurements of space current density although some
attempts to do it with an instrument called Split Langmuir Probe were made. It is mainly because of high
sensitivity of the sensor readings to photocurrent and/or to spacecraft potential. The spatial current density can be
measured not only by split Langmuir probe, but also by a contactless probe of the type of Rogovski coil and by
Faraday cup. The comparative analysis of such probes operation in near Earth plasma is very important for further
development of investigations of solar - terrestrial interactions. This study will be done both by theoretical and
practical ways. As a first stage the theoretical analysis of application areas and of threshold characteristics of the
current density probes will be executed. At a second stage the spatial experiment VARIANT carrying such probes
as a scientific payload will be fulfilled.
INTRODUCTION
For experimental investigations of wave processes in space plasma one of the main tasks is the determination of
dispersion relations between the wave vector and the frequency. It can be shown that simultaneous measurements
of magnetic field and current density fluctuations allow to determine experimentally wave vector components. The
frequency analysis of fluctuations of magnetic field and electric current density in plasma is very efficient in this
case. The measurements of the magnetic field fluctuations usually are made by a variety of magnetometers using
well developed methods. Unfortunately, up to the moment there are no reliable measurements of space current
density, although some more or less successful attempts were made (Bering et al, 1984; Vaisberg et al, 1989).
The three sensors which could be used for the measurement of this value are proposed. First of all it is well known
«Faraday Cup» instrument, installed at some spacecrafts for plasma population investigation (Safrankova et al,
1995). Then so called «Rogovsky Coil» sensor is proposed as a second meter. It was never used before for spatial
current investigations, although its parameters can suit for this (Krasnosselskikh et al, 1996). At last the sensor
«Split Langmuir Probe» is investigated as the instrument for current density study. The theory of its operation is
developed and discussed below.
An experimental array comprising all three instruments operating in parallel is proposed to install on the board of
the satellite. This instrumentation will be prepared in the frames of a joint international spatial experiment
VARIANT (Alleyne et al., 1998). The experiment is planned to perform onboard of the Ukrainian remote sensing
satellite SICH-1M, that will be launched in 2000 at the polar circular orbit with the inclination of around 83 ° and
altitude 670
±
30 km. The scientific payload includes three mentioned instruments for registration of space current
density and also the sensors for measurements of the electric and magnetic field fluctuations in the frequency
range from 0.1 Hz to 40 kHz. These measurements are complementary with the current density ones and they are
supposed to allow to make comparison between direct and implicit estimates of the distribution and fine structure
of currents. The next goal of VARIANT experiment is the simultaneous observation of the fine structure of 2
currents in space and ground-based measurement of the ionospheric convection in the E and F layers making use
of the system of radars SuperDARN.
THE IMPORTANCE OF CURRENT DENSITY INVESTIGATION
When wave activity in space plasma is investigated, the estimation of dispersion relations for electromagnetic
(EM) waves is of great importance. It can be shown that the simultaneous measurements of orthogonal
components of magnetic field and spatial current density fluctuations can give the wave vector k values for a
spectrum of plane waves, by which wave field in plasma reference frame can be represented.
Each spectral component of plane wave may be written in the next form:
(
)
(
)
A
A
j kr
t
=
0
exp
,
A
H J
=
,
,
(1)
where H, J - magnetic field and electric current density correspondingly, j = (-1)
0,5
, r- vector distance, - angular
frequency, t - time.
Let substitute expression (1) in Maxwells equation
[
]
×
=
H
J
(2)
(J may be the sum of displacement and conductivity currents) and assume that the space derivative of magnetic
field is very small: H
0i
/ x
l 0,
(3)
where i,l = 1,2,3; H
0i
- amplitude of i - magnetic field component, x
l
- component of coordinate system.
Then we can obtain from (1)-(3) the following equation
[ ]
k H
jJ
×
=
(4)
or in scalar form in Cartesian coordinates x,y,z
k H
k H
jJ
k H
k H
jJ
k H
k H
jJ
y
z
z
y
x
z
x
x
z
y
x
y
y
x
z = = =





,
,
.
(5)
This system of equations allows to determine the unknown values k
m
, m = x,y,z by the measured values H
m
, J
m
. The
amount of components necessary to measure can be reduced to any two if to use the next Maxwells equation:
× = B
0,
(6)
then from expressions (1)-(3) we obtain

k H
× =
0
(7)
or in scalar form
k H
n
n
n
=
= 0
1
3
(8)
This explanation gives strong evidence of the theoretical possibility of k vector direct estimation. It is possible to
say that there can not be objections as to the possibility to measure magnetic field fluctuations with very high
resolution (Korepanov and Berkman, 1997). Now it is necessary to analyze whether the metrological parameters of
the known instruments allow to measure J value with enough high precision and whether the erroneous factors do
not hinder the results. This is the aim of the investigation. By this it is postulated that the errors introduced by
following processing in electronic circuitry are negligible. Only the physical limitations of all examined methods
will be studied. 3
FARADAY CUP
A Faraday cup (FC) is the device for the direct electron or ion current measurements in space plasma. Such
measurements were realized, in particular, on board of Prognoz satellites and in experiment Interball
(Safrankova et al, 1995).
The FC allows to measure the space current density J by correlation:
J = IS
-1
,
where I is the current that goes through FC entrance window, S is the area of this window. The FC schematic
drawing is presented on fig.1.
The charged particles go through the entrance window (its area is about 10 sq. cm) to collector. The current
formed by these particles is transformed into voltage by current amplifier with high input resistance R (about 10
9
Ohms). Thus the input voltage is connected with current density by relation:
u = JSR.
(9)
The sign of collected particles depends on the sign of voltage on FC grids G2,G3. The entrance grid G1 protects
the space near FC from influence of another grids potentials. The grid G4 protects in the same way the space near
collector from influence of potentials of the grids G2,G3. The potentials of grids G1 and G4 have to be maintained
to be equal to the potential of satellite. The suppressor grid G5 (the nearest to the collector) reduces the
photoelectron and secondary electron currents from the collector. Additionally for reduction of the photoelectrons
current the collector surface is corrugated and covered with a special black nickel coat. All these measures provide
a photocurrent reduction in 200-300 times.
The space diagram pattern of FC is a circular cone with basement that oriented to FC. The angle at the top of the
cone diametrical cross section or angular aperture is 134
°
as shown on fig.1.
In order to provide the possibility of the measurement of charged particles fluxes along one axis it is necessary to
have two of such FCs, pointing opposite sides. Then such arrangement will react to the charged particles fluxes or
what is the same, spatial current flowing along given direction. The experimental model was constructed and the
tests revealed enough good sensitivity of the device: about 10
-11
A/cm
2
to current density fluctuations. As it is seen
from this very short instrument description, from the first sight the main principal limitations of the method could
be the influence of direct sunlight and high-energy charged particles and spacecraft floating potential changes. No
other complications encountered with the relation (9) are expected.
ROGOWSKY COIL
The principle of Rogowsky Coil (RC) operation is based on the following concept. The instrument itself consists
of the ring core from high permeability material with the toroidal winding on it (Fig. 2) (Krasnosselskich et al,
1996).
Fig.1.Faraday Cup Sensor diagram 4
Fig.2. Rogowsky Coil sensor diagram Fig.3. Rogowsky Coil noise level
The current J flowing through the core inner hole in the direction parallel to the axis of this toroid causes magnetic
field intercepting the turns of the RC winding. When the current J is oscillating it provokes time varying magnetic
fie