Bulgarian Academy of Sciences. Space Research Institute.
Aerospace Research in Bulgaria. 19, 2005, Sofia
MEASUREMENT EQUIPMENT F'OR QUASI-STATIC
AND ALTERNATING LOW.FREQUENCY ELECTRIC
F'IELDS IN EARTH.SURROUNDING PLASMA
B oytcho B oytchev, Dimitar Teodossiev
Space Research Institute, Bulgarian Academy of Sciences
Abstract
The paper describes a method, sensors, and measurement equipment
quasi-static (DC) and alternating (AC) electric /ields in the ULF/VLF
for
frequency
range in earth-surrounding plasma on-board satellites. The major requirements
with respect to the equipment and electric fields' registration m"thodt ihen using
dedicated satellite scientific complexes are formulated. Moreover, the popi,
describes the major parqmeters of satellite on-board tow-freq-umcy
electromagnetic effects measurement equipment, as well some originol oithori'
developments intended for the a.m. purposes. The peculiarities of sensors'
solutions, their interface with the measurement complex, the specffic requirements
for the latter resulting from the resolved scientific tasks, and the satellite's
characteristics are also described.
1. Tasks solved
The designed equipment is intended to study the wave processes
taking place in earth-surrounding plasma, the mechanisms of mass and
energy transfer in the magnetosphere-ionosphere-atmosphere system and the
influence of solar wind parameters; the processes of particles' acceleration
and the mechanisms of generation, propagation and interaction of various
types of electromagnetic waves and waves generated by geodynamic or
anthropogenic activity 11,2,31.
It measures electric field components from 0 to 30 kHz. This
frequency range is divided into several subranges of various maximal
frequencies. The processing block also includes a lO-channel spectrum
analyzer intended for preliminary processing and reduction of the device's
output information flow under the monitoring operation mode [4,5].
95
A main task in modem space experiments is to carry out correlated
observations, intended to separate spatial and time relationships, providing
to highlight the causal relation between the studied processes, and
guaranteeing the devices' high-resolution in phase space, thus enabling the
study of small-specifi c-scale processes [6,7].
2. Measurement method and sensors
The device operates after the Langmuir double-probe method. The
method is used to measure the potential difference between two opposite
spherical sensors. Each component can be measured either by an individual
sensor couple or using some other component's sensor couple. Each of the
device's sensors measures the potential at the measurement point. When
measuring the potential difference between two sensors, the intensity of the
electric current for the individual components of the DC and AC fields is
determined [8,9,10].
The electric sensors used in the device are made of glass-carbon-
coated spherical monoblock and fixing elements. The construction of the
spherical sonsor is shown in Fig. l. The major sensor components are: a
sensitive element - sphere with diameter of 80 mm covered by glass-carbon;
symmetry-providing electrodes; protective electrode; preamplifier PA,
assembled within the sphere; preamplifier screening box, and disconnector.
The use of spherical sensitive elements is substantiated by the requirement
for high symmetry level. The synmetry-providing electrodes are intended to
ensure identical conditions for the electrodes with respect to the Sun. The
protective electrode is intended to reduce the influence of the photoelectrons
from the satellite structure components.
The sensors' potential is determined by the balance of their surface
electric currents, which depend on the material and the characteristics of the
sensors' operating surface. For this reason, we use spherical sensors with
glass-carbon coating obtained after a unique Bulgarian method [11]. Within
the spherical sensor, a preamplifier is mounted, intended to harmonize
plasma impedance with the input impedance of the satellite measurement
block. The preamplifier is two-stage, containing high-resistance voltage
reproducer, made of operation amplifier featuring a PEC input, and an
alternating-current amplifier, ampliffing the smaller-amplitude alternating-
current voltage fluctuations. Thus, the sensor has two information outtrluts,
one for constant, and one for altemating field. The diagram also contains
high-voltage protection, switch-over and calibration circuits etc.
96
F'ig.. 1
In electric-field-measurement experiments, the probes must be
positioned sufficiently far off the satellite, so as to escape ihe
disturbed area
around it. For the purpose, it is sufficient that the iirturrr" d from
the
sensors to the closest convex part of the satellite be no less than
5 times the
satellite's most successful experiments measuring
satellite el where the distanc" b.t*".r, the sensors
is, accordi
- 36 m,ISEE_1,2 _ 74 m, GEOTAIL _
160 m, WIND -50 m, POLAR 100 and 130 m, CLUSTER _
- 1,2,3,4 _ 100
m.
To make potential distribution around the spherical sensors
symmetrical, several solutions are applied depending on th. satellite's
type
- whether it is a microsatellite or a large objeci. The relatively large
satellites are fed shifting voltage. This method was used on the satellites
33-
3, ISEE, GEOTAIL, INTERBALL, CLUSTER etc. In microsatellites, the
preamplifier's output is connected to the protective and symmetry-ensuring
electrode, which makes it possible to maintain one and ih. ,u-" potential
over the whole sensor.
3. Connection between sensors and equipment
The electric fields and other quantities at"
-"arur"d by sensors with
built-in amplifiers, transducers etc. Their power-supply is usually provided
by the equipment to which they are connected. rir" equip-"rrt;, por",
supply and the sensor's power supply are common and there is a gilvanic
link between them. Some of the circuits connecting the sensor with the
measurement block participate in both the contour of the output signal
and
the sensor's power supply. As a result, these circuits interact *a *r"
accuracy of the analogue information's transfer from the sensor's output
to
97
the measurement section deteriorates which boosts measurement error and
reduces the measured signals' dynamic range.
In Fig. 2.A, a typical example of an active analogue sensor with
unipolar power-supply, taken out of the measurement block, is shown. The
cable bundle, connecting device and sensor and consisting of conductors (a),
(b) and (c) is shown. The dashed line marks the route of the sensor's power-
supply current I", which causes drop of voltage U" in conductor (c). As a
result of the manner of connecting and ampliffing, the useful signal U, is
mixed with the voltage drop U, in (c), and thus gets amplified by A.
To eliminate the problem, the following solution is suggested.
Obviously, the adding up of U. u U, during the signal's transfer from the
sensor to the device should be eliminated. The suggested solution is
represented in Fig. 2.8; it is accomplished through additional link (d) and
the signal's ampliSring by differential amplifier DA, which carries away and
amplifies the signal from sensor U., eliminating its mixing up with U,.
This technical solution was applied to carry away the signals from
the electric and magnetic sensors of the ULFA/LF complex of the COMPAS
Project. It is reflected in the equipment's electric filed measurement block to
be considered further on. The application of this solution resulted in
material increase of the measurement accuracy of the sensor-fed signals.
4. Equipment parameters for studying ionospheric and
magnetospheric fields
Measurement of electric fields are important as for the decision of
questions in the ionosphere and magnetosphere plasma, and the processes
connected from anthropogenous activity. Microsatellites, and heavy
satellites are used to carry out complex measurements in the ionosphere and
magnetosphere plasma. The parameters of the equipment for electric fields
measurements on satellites of project CLUSTER and the planned project the
RESONANCE are presented in Table 1 The results can be used for
comparison of similar equipment for microsatellites (as example the
COMPAS microsatellite).
5. Equipment parameters for studying ionospheric
seismoelectromagnetic effects
Frequency rsnge?, Electromagnetic emissions within the range
from fractions of the hertz to dozens of kilohertz have been observed. The
detailed analysis of experimental data evidences that persistent
seismoelectromagnetic signals are observed at frequencies lower than 800
98
Hz,btt alongside with them, signals with frequencies of l0 kHz and l5 kHz
have also been recorded.
Table 1.
Measured Frequency range Dynamic range Satellite
quantitv
DC Electric Field 0-lDHz 700mV/m-0.1mV/m CLUSTER
(2 components) 0-200H2 700mV/m-0.1mV/m
0 - 5000 Hz 700mV/m-0.1mV/m
0 - 10000 Hz 700mV/m-0.1mV/m
AC Electric Field 10 - 5000 Hz 10mV/mxlpYlm
(2 components)
DC Electric Field 0-30H2 100 dB REZONANS
(2 components)
AC Electric Field 0.01 Hz- 30 kllz 80 dB
(2 components)
Amplitude: Judging from reference data based on the analysis of
latge anays of satellite experimental results, seismic-activity-related signals
are those for which the signal-to-noise ratio is > 3. The studies of the rig"ul
amplitude's dependence on At, the time offset between the earthquake,s
occuffence and the time of measurement, reveal thit the
seismoelectromagnetic effects are manifested most strongly at frequencies <
I kLIz in the vicinity of the earthquake's epicentre
spectral density: It varies with frequency, being higher with lower
frequencies. For emissions within the frequency range from 0.1 kHz to 0.5
kHz, the absolute value of the magnetic .ornpotr"ntit spectral density is >
0.3 - 3 pTlHzr/2.
Durationz The earthquake electromagnetic precursors can be
recorded within a couple of days prior to the earthquake's occurrence. The
duration of observation of seismoelectromagnetic emissions on-board
satellites depends on the satellite's orbital tharacteristics (height and
inclination), i.e., the height at which it crosses a force tube relateO wittr ttre
earthquake's epicenter.
Experiments have shown that it takes from a couple of seconds to a
couple of minutes.
The separation of ionospheric seismoelectromagnetic effects from
the background values of such emissions at satellite orbital height is a key
task, since the field's generation mechanisms and the propagatioi
99
conditions for various types of waves are strongly influenced by solar
activity, geomagnetic circumstances, season, local weather etc. Moreover, in
earth-surrounding plasma, various physical processes take place, which
generate signals similar to seismoelectromagnetic effects.
Iz (a) LZ
nlA
(b)
Az Tz
Uirrl in:k (UE
l-I
A
lz (aJ Iz suppry
jl'r tl
I lue
(b)
(d)
Ui
DA
in=kUs
I (c)
B
Fig.2
6. Peculiarities of on-board equipment in recording
geomagnetic-activity-related electromagnetic fields in earth-
surrounding plasma
To solve the scientific tasks related with studying electromagnelic
and plasma earthquake precursors, three basic on-board-satellite
measurement modes should be used ll2,13l:
Monitoring mode or minimal telemetry mode. Under this mode,
key physical parameters are measured continuously on a 24-hour basis
within selected channels and with a small inquiry frequency (of about one
inquiry per second). This mode presumes continuous monitoring. In
accordance with the selected mode the equipment measures the quasi-static
field's components, while the alternating field' measurements include only
measurement of local frequencies by a spectral analyzer to reduce data flow.
Local monitoring mode. Under this mode, the full set of the
physical parameters provided by the scientific equipment complex is
rnonitored, during all satellite orbits passing over some of the Earth's
seismoactive regions. The EMC operates in its most informative mode,
measuring the actual signal "wave" forms within a wide frequency range,
but only for the time while passing over the region.
100
Physical experirnent mode. This mode is used when targeted
experiments are carried out, implying the use of other space or ground-based
instrumentation. Under this mode, the satellite's full scientific complex
operates with maximal telemetry and jointly with other on-board
geophysical and radio-physical provision instrumentation. The operating
mode resembles the description provided in item Physical experiment mode.
Operating conditions restrictions on-hoard small satellites. For the
purpose of studying various environmentalparumeters and processes taking
place in Earth-surrounding space and on Earth, recently, small satellites
have been widely used.
The small size and mass of such space apparata call for reduction of
some of their office systems, operation time, power resource, as well as for
material telemetry restrictions etc. However, this results in significant
reduction of the price per on-board load (equipment). The equipment to be
mounted on such objects should comply with their specific features.
7. Structural diagram of the measurement complex
The structural diagram (Fig. 3) of the measurement complex
provides for changing the number of the measured field components and
their frequency range, making preliminary processing of collected data, such
as spectral analysis of some components, switching over of components,
selection and changing of operation regimes by a command from the Earth
[14]. By varying the operation regime, the complex's options as well as the
amount of output datavary {eaily, too. Below, the major components and
the operation of the ULFA/LF-complex are described. On the presented
structural diagtam, the analogue signals to be processed and converted in the
digital section are shown. These signals are outtrlut at control coupling T.
The signals M1-M4 controlling the complex's operation regimes as
well as the signals Ft, R, A, Ct, Fms, D0-D7 providing for the protocol and
data exchange with on-board telemetry are also shown. They are duplicated
and output at control couplings IOI and IO2.
The ULFA/LF-complex is intended to measure one of two optional
components of the AC electric field and one component of the alternating
magnetic freld in the following frequency ranges:
- from l}lz to 30Hz - denominated on the flow-chart as MVF I and
EVFI;
- from 30Hz to 1000H2 - denominated on the flow-chart as MVF2
and EVF2;
- from 30Hz to 8000H2 - denominated on the flow-chart as MVF3
and EVF3:
101
- parallel spectral analysis of one electric and one magnetic field
component with ten local frequencies (7 .5, 14, 30, 7 0, 140, 5 60, I 3 00, 4500,
8500, and l5000Hz) - signals from EDI to EDl0 and from MDl to MD10.
Apart from these, the ULF/VLF-complex measures: - two
components of the quasi-DC electric field within the range 0-2Hz - signals
PPI-? andPP2-3; - the temperature in the electronic block and the electric
and magnetic sensors coupled with it - signals BTl-BT5.
The complex consists of: - electric sensors EDl, EDz, and ED3
with preamplifiers for measurement of one of two optional electric
components; - a block of magnetic sensors, BMD, consisting of two
magnetic sensors in different frequency regions with preamplifiers for
measurement of one magnetic component;- An electronic block housed
within a single case, and consisting of a block for study of electric fields,
EPS, a block for study of magnetic fields, MPS, a data acquisition block,
BSD, consisting of a microprocessor system for collection of data from 36
analogous channels, generation of calibration signals, complex control,
processing of obtained data, and connection with the object's telemetry, a
block for galvanic discorurectio and duplication of signals for data exchange
with telemetry - BGRD, and a power-supply block for galvanic
disconnection from the on-board power- supply, YIZ.
The analogue data obtained at the outputs of the above-named
blocks is processed by a quick l2+l data acquisition bit system of the
LI[dl2458 type, and a microprocessor system based on the processor
80CI88XL. The system also includes a block for generation of calibration
signals providing for autocalibration during flight.
8. Approbation of equipment and sensors under the conditions
of real experiments
Electric sensors measuring electric fields on-board satellites, that
have been developed at the SRI - BAS with our participation have operated
successfully on eight satellites: IC Bulgaria 1300 (1981); IC-24 Activen
(1989); Magion-2 (1989); IC-25 - APEX (1991); Magion -3 (1991);
Magion -a Q99 5); INTERBALL-2 (1996) and Magion -5 (l 996).
Measurement equipment and sensors intended for the COMPAS
microsatellite, featuring the a.m. parameters, have been developed and have
passed technological tests. The equipment is intended to study ionospheric
electromagnetic effects caused by geodpamic activity.
r02
EHg9*
Fig.3
103
9. Measurements in the lligh-Latitude near-Earth Magnetotail
Region
ULF measurements aboard Magioq-4 were performed during UT
13:00 - 13:45 while the spacecraft crossed the high-latitude near-Earth
magnetotail at distance 11 RB in the pre- sector [15]. Data are presented
in Fig. 4. Initially till 13:28 UT we observe electric fluctuations which
are purely electrostatic. Magion-4 and plasma data (not shown
here) suggest that during this period take place on field lines
connected with the lobes, though at latitudes than the Dav
20.04.1997 case. A sharp increase of wave ity begins at UT 13.28, the
spectrum of the waves changes to electrQmagnetic. Plasma instruments
aboard Magion-4 register density enhancement. We interpret the changes in
medium characteristics as spatial, due to s/c entering a magnetosphere
mantle id supposed to be well developed
after UT 13:28 take place in the plasma
eld and plasma measurements on board
the Interball-l satellite, which could facil tate region identification, are
absent for this period. While the electrostatic spectrum is below I Hz, the
electromagnetic one is up to 5+6 Hz and its intensity decreases for higher
frequency. Note that the absence of elecfic field component at higher
frequencies (above ro.1) could be connected lyith the worse sensitivity of our
ULF electric field measurements compared to the magnetic field one. The
intensity of the .E waves is structured, ahd Ihe E field component is
orientated in the meridian plane. The electrompgnetic spectrum observed in the
mantle belongs probably to electromagnetic ion cyclotron modes.
24 frtil,l99?
E
H
Bx D.
zr.l
Ey p
e
0 lrl
Bz I fi
2 Ir
0
UT 13:03:20 l3:l l:40 13120:00 13:28:20 l3:32i30 13:,10:50 13:,19:10 l3:5?:30
XE"rn 35A7 3J57 s927 4fr95 4221 4388 45s4 4.718
Ygsrn -l.'tll -1382 -1355 -133t -lsl.t -t294 -t277 -1.263
?gsn 10,050 l0.l5l 10.249 10344 l0.,lt.t t0505 10593 t06?9
Fig. 4 ectric and magnetic ield components
on 24 da{a. At - UT 13:28 the spectrum
change
r04
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105
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106
