Impedance probe technique to detect the absolute number density M. Wakabayashi

An Introduction to Space Instrumentation,
Edited by K. Oyama and C. Z. Cheng, 107–123.
Impedance probe technique to detect the absolute number density
of electrons on-board spacecraft
M. Wakabayashi1 , T. Suzuki2 , J. Uemoto3 , A. Kumamoto4 , and T. Ono4
1 Niihama
National College of Technology, Department of Electrical Engineering and Information Science,
7-1, Yagumo, Niihama, Ehime 792-8580, Japan
2 Meisei Electronic Co., Ltd., Isesaki, Gunma, Japan
3 National Institute of Information and Communications Technology, Koganei, Tokyo, Japan
4 Graduate School of Science, Tohoku University, Sendai, Japan
This text focuses on the impedance probe which is powerful method for observation of absolute electron
density in plasma. In several countries, impedance probe has been used during many rocket campaigns and
satellite missions by many scientists’ groups. In Japan, Oya (1966) realized remarkably accurate electron density
observation on-board sounding rocket. Since then, the impedance probe has taken part in absolute electron density
measurement to proceed ionospheric and space plasma science in Japan. This text aims to describe impedance
probe method which is developed by Oya (1966) as excellent example of in-situ observation of absolute electron
density. The following description will provide the historical background, basic theory, advantages, important
reminders and specific application ideas of sounding rocket observations during SEEK-2, DELTA and WIND
campaigns. This text will refrain from commenting a lot about impedance probe instruments on-board satellites
because the authors don’t have many experiences with carrying out satellite missions. However, description in this
issue will provide some information for future application of satellite experiment. It is expected that engineers,
scientists and graduate students who attempt to carry out such impedance probe observation will find effective
skills of the method through the text.
Key words: Electron number density, UHR frequency detection, plasma resonance.
1.
Introduction
1.1 Historical background of in-situ observations
Sounding rockets and satellites are powerful and essential
tools to study the ionospheric plasma because these in-situ
observational methods are possible to obtain the parameters
of plasma and neutral gases, electric and magnetic fields,
and plasma waves directly. In Japan, observation by using
sounding rocket has been started since 1950’s. After that,
many kinds of instrumentations were applied to obtain various parameters of the ionosphere, although only ion density
was measured on-board the sounding rocket initially. The
developments of various rocket-borne instruments made it
possible to provide electron and ion densities, temperatures,
electric and magnetic fields as direct observation data. As
excellently summarized by Hirao et al. (1966), many observations were carried out during the period of the International Years of the Quiet Sun (IQSY; from January 1964 to
December 1965).
To measure electron density on-board a sounding rocket,
Langmuir probe method has been the most commonly used
instrumentation. This method obtains Langmuir characteristic curve (I–V curve) by using conductor probe with
swept DC voltage. Electron density, electron temperature
and some plasma parameters are deduced from the characteristic curve; however, the characteristic also depends on
the effective area of probe surface and other conditions such
as magnetized, collision dominant and non-Maxwellian disc TERRAPUB, 2013.
Copyright tribution of plasma. Therefore, the Langmuir probe has difficulty in direct determination of the absolute value of electron density.
To detect the fine structure of ionospheric plasma, the
fixed-bias probe (FBP) has been used for long years until today because this method has high spatial resolution of several meters. This instrument is based on excellently simple
principle of observation. Namely, constant DC voltage was
applied to spherical conductor probe. Then, there is input of
electron current from ambient plasma though the spherical
probe. Electron density (relative value) and its fine structure
will be obtained by measuring DC and AC components of
electron current, respectively. This instrument was installed
for the rocket campaign described in Section 2 of this book.
For the identification of absolute plasma density, some
probe methods were improved through the laboratory and
rocket experiments, for example, resonance probe, mutual
impedance probe and RF impedance probe. In the 1960’s,
some applications of resonance probe were reported (e.g.,
Hirao and Miyazaki, 1965) as direct observation method of
UHR frequency. However, this resonance probe actually
measured sheath resonance frequency (SHR). Thereafter,
the resonance probe became not to be used because the SHR
frequency was too much complex function to deduce the
electron density accurately.
As for the mutual impedance probe, several scientists reported the measurement results after Storey et al. (1969).
This system consists of quadrupolar probe which measures
the frequency response of the coupling impedance between
two dipoles. Some European scientific missions such as
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GEOS 1 satellite adopted this instrumentation. Many scientific results obtained by using mutual impedance probe
have been reported (e.g., Décréau et al., 1982).
1.2 Development and improvement of impedance
probe
The RF impedance probe observation started before
1960’s as described in Jackson and Kane (1959). After that,
in 1960’s, practical application of the RF impedance probe
was realized by Oya (1966) who was inspired by the calculation of probe impedance in plasma by Balmain (1964).
Oya called this RF impedance probe “gyro-plasma probe”
initially. Oya applied Balmain’s calculation to establish the
principle which made it possible to determine Upper Hybrid Resonance (UHR) frequency of ambient plasma. Detection of the UHR frequency is possible by measuring the
frequency response of the probe impedance immersed in
plasma. When an RF voltage of swept frequency is applied
to the probe, the probe impedance becomes infinite when Fig. 1. Conceptual diagram of impedance probe circuit which is used in
the previous rocket experiments before 2002.
the RF frequency coincides with the UHR frequency. If the
UHR frequency is determined, absolute density of electrons
can be deduced by using the equation of
AKEBONO satellites) included electron density observaNe = 1.24 × 104 ( fU2 H R − f c2 ),
(1)
tions by using impedance probe (e.g., Ejiri, 1973, Ejiri et
where, Ne , fU H R and f c are absolute electron density al., 1973; Oya and Morioka, 1975). In 1998, in-situ ob(cm−3 ), UHR frequency (MHz) and electron cyclotron fre- servation of Martian ionosphere was attempted on-board
quency (MHz), respectively. The impedance probe also has Japanese Mars orbiter NOZOMI (unfortunately, the NOthe advantage that the variation of the probe potential does ZOMI failed to insert the Martian orbit).
In recent rocket experiments, applications of digital and
not affect on the detection of the UHR frequency. In addition, existence of plasma sheath and electrode contamina- analog devices enabled to improve the characteristics of
tion does not affect the UHR detection. Then, it is possi- impedance probe circuit. For SEEK-2 campaign in 2002
ble to obtain the accurate electron density if the f c is given (described in Section 2), two improvements had been apby on-board magnetometer or geomagnetic model. This plied to the impedance probe instrument. One is the usage
method can provide the absolute electron density with high of Direct Digital Synthesizer (DDS) to generate wide-band
accuracy of ±3% (in case of Ne > 104 cm−3 ). In order swept frequency to add to the capacitance bridge. The anato observe the accurate electron number density with this log VCO, which was used in previous rocket campaigns and
method, it is essential to eliminate any stray capacitance in satellite missions, has large dependence on the device temthe electrical circuit. For this purpose, Oya adopted capac- perature because the variable capacitance diode used in the
itance bridge circuit, as shown in Fig. 1, to cancel out the resonator. So, frequency reference data should be simultaeffect of the stray capacitance by adjusting the compensa- neously obtained to determine UHR frequency of ambient
tion capacitor (and length of dummy cable, if it is needed) plasma with high accuracy. On the other hand, the DDS
associated with the capacitance bridge. The RF signals are is referring the 32-bit data table to define the frequency
synthesized by using an analog VCO (voltage controlled versus step number (the frequency data controlled by the
oscillator) circuit, and the signals are added to the bridge PIC micro-controller). Then, frequency reference data is
circuit via transformer. The impedance probe has many ad- not necessary any more because the linearity of wide-band
vantages of hardware aspect, namely, significantly simple swept frequency was extremely improved by the usage of
configuration with single antenna, remarkable EMC char- DDS device. Another improvement is put on the capaciacteristics for compatibility with other instruments, and en- tance bridge circuit by using a high-speed differential amdurance of environmental factor. It also does not need spin plifier IC which made it possible to adjust the balance of
motion of rockets or satellites for this method. However, the bridge circuit with significantly simplified way, because
instrument should be installed in the part which meets con- new bridge circuit did not any transformer. These improvedition that the single antenna is not immersed in wake area ments made it easy to install the antenna keeping away from
due to spacecraft motion. The impedance probe had been its electric circuit (about 1.5 m at maximum) by connecting
installed in many sounding rockets such as K-9M-13, -14, with the co-axis cable. This has an advantage in setting
L-3H-2, K-9M-20, and -21, and provided many scientific up the antenna near the top of the rocket body to reduce
results. Oya and Obayashi (1967) showed a good exam- the effect of rocket wake. It should be added that this was
ple of rawdata obtained by using impedance probe. Figure significantly difficult for previous configuration because ca2 represents example of the equivalent capacitance, phase pacitance and inductance components of extended co-axis
and frequency references which are measured on-board cable added to the stray capacitance and inductance due to
the sounding rocket. Furthermore, many satellite missions transformer.
For DELTA campaign which was carried out in 2004 (de(DENPA, TAIYO, JIKIKEN, HINOTORI, OHZORA and
M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
109
Fig. 2. An example of impedance probe rawdata (Oya and Obayashi, 1967). The upper, middle and lower panels show phase shift, equivalent
capacitance and frequency reference, respectively.
Fig. 3. Standard configuration of impedance probe in recent years. DDSs and logarithmic amplifier are applied for SEEK-2 and DELTA campaigns,
respectively. In this configuration, capacitance of C1, C2 and VC are 100, 10 and 20 (maximum) pF, respectively. After the capacitance bridge,
capacitances C are inserted for cutting off possible DC offset signals. Resistors R determine input impedance for buffer amplifier. These C and R
usually configured as 0.1 uF and 1 M, respectively.
scribed in Section 3), logarithmic amplifier IC was adopted
in the backend of impedance probe instrument. This amplifier made it easy not only to detect the dip point of equivalent capacitance spectrum but also to broaden equivalent
capacitance coverage. After these improvements, standard
configuration of impedance probe circuit was replaced as
shown in Fig. 3.
1.3 Basic theory of impedance probe measurement
The impedance probe detects equivalent capacitance of
conductor antenna immersed in plasma. The equivalent ca-
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M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
pacitance is measured as frequency response characteristic which includes the value of UHR and SHR frequencies
and sheath capacitance. To intuitively understand the properties of antenna impedance (or admittance) immersed in
plasma, it is useful to substitute equivalent circuit model
for the real antenna. In following sentences, it will be introduced how to derive the relation between the equivalent
circuit and real antenna although it is necessary to import
some assumptions. The calculation of this section treats
the isotropic, cold and electron plasma with International
System of Units. Also, it should be noted that the equivalent circuit model cannot evaluate quantitative value of the
plasma impedance because only an ideal case is discussed
in this section.
According to the theory demonstrated by Spitzer (1962),
plasma mean velocity v is described in fluid equation as
∂ v
(2)
nm
+ (v · ∇) v = nq F − ∇ · ψ − nm∇g + P,
∂t
and
q B
.
(11)
m
Since the second term in left hand side of Eq. (9) is nonlinear term, it can be neglected for linearization of this equation. Then, Eq. (9) is rewritten as
=
∂ j
− qa 2 ∇n,
× j + ν j = 2 · D
+
∂t
(12)
− a 2 ∇ρ,
· j = 2 · D
(13)
namely,
where,
=
iω + ν
0
−
iω + ν
0
0
0
iω + ν
.
(14)
Finally, electron current density j is represented as
j = 2 −1 D
− a 2 −1 ∇ρ.
(15)
where, n: particle number density, m: mass of particle, q:
charge, F: external force, ψ: stress tensor, g : gravity
momentum changes due to collisions. and The total electron current density ( J) is equal to the sum
potential and P:
of the convection current ( j) and displacement current
are also represented as
((∂ D)/(∂t)).
Therefore, J is expressed as following equa
F = E + v × B,
(3) tion of
P = −nmν v,
(4)
∂D
J =
+ j
∂t
electric field, B:
magnetic flux density and ν:
where, E:
2 −1 collision frequency. The gradient of stress tensor ∇ · ψ is
D − a 2 −1 ∇ρ.
= jω 1 +
(16)
expressed as
jω
∇ · ψ = ∇nkT = ma 2 ∇n,
(5)
ρ = qn,
(7)
If it is under the assumption of cold plasma, second term
in the right hand side of Eq. (16) is negligible. In Eq. (16),
where, k: √Boltzman’s constant, T : plasma temperature the value of 1 + 2 /( jω)
−1 means well-known dielectric
and a = (kT )/m. On the other hand, the equation of tensor (K) of plasma. Here, the K can be expressed as
continuity is given as
εx x
εx y 0
∂ρ
K = −ε yx ε yy 0 ,
(17)
+ ∇ · j = 0,
(6)
∂t
0
0 εzz
where,
and
j = qnv.
(8)
where,
2
iω + ν
,
·
iω (iω + ν)2 + 2
2
εx y = ε yx =
·
,
iω (iω + ν)2 + 2
2
1
εzz = 1 +
·
.
iω iω + ν
εx x = ε yy = 1 +
(18)
Here, ρ and j represent charge density and electron current
(19)
density, respectively. In Eq. (2), it is assumed that the
gravity potential g is negligible and all the variables would
(20)
change as a function of exp(iωt). The coordinate system
was selected as Cartesian coordinate, and magnetic field
By using the dielectric tensor (K), Eq. (16) is rewritten as
was homogeneous only in direction of z-axis.
Based on these equations, Eq. (2) will be changed to
J = iωK · D.
(21)
equation of electron current density j as
∂ j
1 − qa 2 ∇n, (9)
× j + ν j = 2 · D
+
j · ∇ j +
∂t nq
where, : angular plasma frequency and : angular cyclotron frequency, namely;
=
q 2n
mε0
1/2
,
(10)
On the other hand, the relation between specific admittance
(y) and total electron current density ( J) is described as
D
J = y E = y
.
(22)
ε0
Now, we call y “specific” admittance. The reason for the
name of “specific” is that we have proceeded with this topic
M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
111
Fig. 6. Equivalent circuit of antenna under the condition of collision-less
and non-magnetized plasma. There are no differences between perpendicular and parallel components with respect to the z-axis (along the
magnetic field line). This expression is frequently cited to explain the
feature of antenna impedance in plasma as the most simplified case.
Fig. 4. Equivalent circuits of conductor antenna immersed in plasma under
the condition of isotropic, cold plasma, existence of magnetic field and
existence of electron-neutral collisions. This circuit diagram is deduced
from Eqs. (32) and (33) in the text (after Ejiri, 1973).
Fig. 5. Equivalent circuits of antenna under the condition of collision-less
plasma. The magnetic field is still remained as well as Fig. 4. These
diagrams correspond to Eqs. (34) and (35), respectively.
Fig. 7. Schematic relation of impedance probe antenna and ambient
plasma. Under the assumption of cold, isotropic, B = 0 [T] and
collision-less plasma, most simplified equivalent circuit is represented
in this panel.
and
y// = iωε0 εzz = iωε0 +
1
.
iω
ν
+
ε0 2
ε0 2
(26)
for the equation of vec J ; total electron current density. The
admittance of plasma is inevitably treated as the admittance
for unit length. So, the dimension of y is not usual unit such In Eq. (24), θ represents the angle of electric field ( E)
with
as [S] but [S/m]. Then, specific admittance (y) is derived as respect to z-axis. These expressions mean the specific admittance of plasma can be translated as equivalent circuits
J · D
y = ε 0 2 .
(23) which include equivalent specific resistance (R), equiva lent specific inductance (L), equivalent specific conduc D
tance (G) and equivalent specific capacitance (C and C0 ).
Namely,
So, we can obtain the value of y as a function of εx x and
ν
εzz . Now, y is described as following equation.
R=
,
(27)
ε0 2
J · D
1
y = ε0
= iωε0 εx x sin2 θ + εzz cos2 θ
L=
,
(28)
2
ε0 2
|D|
ε0 2 ν
= y⊥ sin2 θ + y// cos2 θ
(24)
G=
,
(29)
2
where,
ε0 2
C=
,
(30)
2
y⊥ = iωε0 εx x = iωε0
and
1
+
. (25)
C 0 = ε0 .
(31)
ν
iω
1
+
+
2
The kinds of these circuit elements are determined by diε0 2
ε0 2
ε0 2 ν + iωε0 mension analysis. However, dimension of each element is
2
2
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M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
Fig. 8. Calculation result of equivalent impedance of conductor probe in ambient plasma. There are minimum and maximum of equivalent impedance
at the SHR (series resonance) and UHR (parallel resonance) frequencies in this panel, respectively. In this calculation, values of Ca = 10 pF, Cs =
50 pF and L p = 10 mH were used.
not corresponding to our “ordinary” dimension (for examAccording to the calculation described above, the equivaple, ε0 [F/m] = [F] for Eq. (31),) because these circuit lent probe model circuit as well as schematic relation of the
elements derived from “specific” admittance (y). By using antenna, rocket body and ambient plasma can be expressed
these parameters, Eqs. (25) and (26) are represented as
as shown in Fig. 7 which is most simplified model and frequently used to explain briefly the concept of impedance
1
y⊥ = iωC0 +
,
(32) probe measurement. These calculations described above are
1
R + iωL +
also introduced in detail in the issue of Ejiri (1973) includG + iωC
ing the case of warm plasma.
1
About the equivalent circuit shown in Fig. 7, the synthetic
y// = iωC0 +
.
(33)
R + iωL
impedance of the resonant circuit is obtained as
These specific admittances correspond to equivalent circuits
−1

shown in Fig. 4. For example, around the 120 km altitude

 1
1
1
during the DELTA campaign ( f c = 1.40 MHz, Ne = 5.02 ×
 ,
=
+
(37)
5
−3
10 cm ), the values of R, L, G and C are 0.71 /m,
1 
|Z (ω)|  ωCs
ωC
−
a
70.7 µH/m, 1.82 µS/m and 182 pF/m, respectively. If it
ωL p
is under the assumption of collision-less plasma, collision
frequency (ν) is equal to zero. Then, the above equations where, Z (ω), ω, Ca and L p are the impedance of the resonant circuit, angular frequency of RF signals, equivalent
can be simplified as
capacitance and inductance of plasma, respectively. The
1
y⊥ = iωC0 +
,
(34) equivalent capacitance has maximum and minimum values
1
when ω is
iωL +
iωC
1
and
ωS H R = 1
L
(C
p
a + Cs )
y// = iωC0 +
.
(35)
iωL
(a series resonance : corresponds to SHR) (38)
Furthermore, if the magnetic field is equal to zero, the
specific admittance is described in most simplified form as and
1
.
(36)
iωL
The equivalent circuits in case of collision-less plasma (Eqs.
(34) and (35)) and B = 0 (Eq. (36)) are shown in Figs. 5
and 6, respectively.
For actual condition in plasma, plasma sheath will be
formed around the antenna (many cases of impedance probe
instrument adopted cylindrical probe). Then, sheath admittance ys will be installed in series with plasma admittance. If sharp boundary model of plasma sheath is assumed, equivalent sheath capacitance Cs is added to the circuit.
y⊥ = y// = iωC0 +
1
L p Ca
(a parallel resonance : corresponds to UHR). (39)
ωU H R = Figure 8 shows a calculation result revealing the minimum
(SHR) and maximum (UHR) of the equivalent impedance.
When the RF signal is applied at a very low frequency, the
effect of the inductance L p becomes negligible. Then, the
sheath capacitance can be obtained at the lowest frequency
range of 300 kHz for the ordinary impedance probe observation. (The lowest frequency should satisfy two conditions; namely, adequately lower than plasma and cyclotron
M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
113
Fig. 9. Equivalent probe capacitance calculated after the Balmain (1964) formula. This panel represents two cases of “Z pl and Cs (with sheath)” and
“Z pl only (without sheath)”. The calculation performed by using parameters as follows, L = 0.50 m, r = 0.0060 m, φ = 90 deg and Cs = 30 pF.
frequencies of ambient plasma, and enough higher than ion
characteristic frequencies.) The SHR frequency is a function of electron density, temperature and the direction of the
magnetic field. The sheath capacitance is a function of the
electron density, temperature and the probe potential and
can provide useful information on ambient plasma, as described by Oya and Aso (1969).
On the other hand, Balmain (1964) proposed a theory of
the impedance of an antenna in plasma. It is based on the
analytical solution by assuming the cold plasma approximation. The derived impedance of a short dipole antenna Z pl
with the length 2L and radius r is as follows
√ α
α+ F
L
Z pl =
,(40)
ln − 1 − ln
√
r
2F
iω2π ε0 εx x L F
F = sin2 φ + α 2 cos2 φ,
εx x
α2 =
.
εzz
(41)
(42)
where φ denotes the angle between the axis of the antenna
and the static magnetic field. The impedance curve calculated from (40) is plotted in Fig. 9 as a form of the probe
equivalent capacitance since we calibrate the output signal
of impedance probe to the capacitance (after Suzuki, 2010).
Figure 9 also shows the calculation including the capacitance of ion sheath surrounding the probe. In Fig. 9, there is
additional resonance represented as MPR (modified plasma
resonance). Ejiri et al. (1968) reported the experimental evidence of MPR obtained by sounding rocket observation.
The MPR frequency depends on the angle of φ, namely,
ωU2 H R
42 2
2
2
ωM P R =
1± 1− 4
sin φ .
(43)
2
ωU H R
This relation will be deduced from Eq. (41) under the condition of F = 0. In the ionospheric D-region, the collision
effect between electrons and neutrals becomes significant
because the electrons in plasma can no longer realize cyclotron motion. The collision effect on the observed UHR
frequency should be taken into account for the data analysis,
as it has been carried out by Yamamoto et al. (1998). The
frequency shift of the UHR waves by the collision effect is
described as follows:
νen = 6 × 10−9 Nn ,
2
2
ωU2 H R = ωob
− νen
,
(44)
(45)
where νen and Nn represent the collision frequency (s−1 ) between electrons and neutrals, and number density (cm−3 ) of
neutral gases in the ionosphere, respectively, which is obtained by the MSIS-E-90 atmosphere model. In Eq. (45),
ωU H R is the “true” UHR angular frequency of the ambient
plasma, and ωob is the UHR angular frequency observed
by the impedance probe instrument. In the region above
100 km altitude, this effect becomes negligible where the
difference between ωU H R and ωob is smaller than 1%, because νen decreases rapidly with altitude. It should be also
noted that the impedance probe observation is effective for
the electron density over ∼103 (cm−3 ). This limit is due to
the Q-value of equivalent resonant circuit which becomes
lower with decrease in electron density. It will be difficult to
detect UHR frequency under the condition of ∼103 (cm−3 )
electron density in case of 1∼2 m-long antenna. If it is
needed to detect lower electron density than 103 , longer antenna should be prepared as EXOS-B satellite (Ejiri et al.,
1981).
1.4 Applications of impedance probe in other groups
In other countries, application of impedance probe
method was delayed for the space plasma measurements.
The group of Utah State University remarkably contributed
to develop the impedance probe in the United States. They
called their instrument plasma impedance probe (PIP). In
1980’s, Baker et al. (1985) tried to detect the UHR frequency with the continuous observation. They applied the
analog Phase Locked Loop to track the UHR frequency automatically. The time resolution was 1 ms which corresponds to the several m spatial resolution during a rocket
flight. In recent years, they obtained impedance probe data
from their instrument unit on-board International Space
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M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
Fig. 10. Photograph of 1.2 m BeCu ribbon antenna (extension phase) and antenna case. This type of antenna case was used in SEEK-2 campaign
(described in Subsection 3.1).
Station (Barjatya et al., 2009). On the other hand, German group has studied the impedance probe, and their
impedance probe system has the time resolution of 40 ms
by using the DDS (Direct Digital Synthesizer), PLD (programmable logic device) and A/D converter (Steigies et al.,
2000).
2.
System Design, Calibration and Environmental
Rests
2.1 Application to the sounding rocket observations
This section shows calibration method to detect equivalent capacitance of probe correctly. The calibration will
be explained for the usage of impedance probe with standard configuration which is represented in previous section
(Fig. 3). Environmental and integration tests are also explained; however, the detail of tests should be carried out
based on the requirement of on-board instrument decided
by the space agency which conducts the rocket experiment
or satellite mission.
2.2 Calibration
Calibration of capacitance bridge is carried out by connecting ceramic condensers with its capacitance of 1∼1000
pF (in most recent case, 0.1∼2000 pF is used). By connecting ceramic condenser, the capacitance bridge is calibrated for its sensitivity as well as frequency dependences.
To carry out this calibration, dummy antenna case should
be prepared to simulate the situation of plasma observation. For impedance probe observation on-board sounding rocket, BeCu ribbon antenna is frequently used as
impedance probe sensor with its dimensions of 1.2 m length
and 12 mm diameter. The sensor of impedance probe
should meet requirement of dimension; namely, the sensor have sufficiently large dimension compared with Debye length of ambient plasma. If the sensor is too small,
Ca (see, Subsection 1.3) will be too small, and the Q-value
of equivalent circuit becomes low. The typical sensor is
cylindrical antenna with its length of 1∼2 m for rocket campaigns. The BeCu ribbon antenna mentioned above is light,
easy to store in compact antenna case, and extend by its
force of restitution. Figure 10 represents the appearance
of the BeCu ribbon antenna and its case with extension
mechanism. Conceptual diagram of the calibration by using condensers is shown in Fig. 11. Because the impedance
probe should detect the equivalent capacitance of BeCu antenna which is “immersed in plasma”, BeCu antenna must
be cut at the edge of antenna case for calibration as shown
in Fig. 11. The ceramic condensers are connected to an end
of dummy antenna by crocodile clip. Then, output voltage
of impedance probe will change as capacitance of ceramic
condensers change. Accurate calibration of the capacitance
bridge will realize the input-output characteristic as shown
in Fig. 12 which represents the smallest output voltage at 0
pF (open-circuit condition) and monotonically increasing.
The accuracy of the UHR frequency detection should be
confirmed by applying LC resonant circuit to the dummy
antenna. This calibration is also significant because it can
evaluate the influences of any stray “inductance” in the electric circuit. Resonant frequency of LC circuit can be determined by measurement of spectrum analyzer in advance.
The capacitance bridge should be slightly adjusted to detect
M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
115
Fig. 11. Conceptual diagram of impedance probe calibration by using ceramic condensers. For this calibration, dummy antenna should be prepared to
simulate plasma observation.
Fig. 12. Example of translation characteristic of impedance probe developed for DELTA campaign. The BeCu antenna is equivalent to 11 pF
capacitance.
the resonant frequency of LC circuit as correct as possible.
To avoid any environmental noise as well as electrical coupling from the outside, the LC resonant circuit is shielded
by aluminum box. The resonant frequencies of LC circuits
will be designed to cover the wide range of swept frequency
as much as possible. Figure 13 and Table 1 represent block
diagram and summary of calibration by using LC circuits
(for DELTA campaign), respectively.
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M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
Fig. 13. Block diagram of calibration by using LC resonant circuit. Dummy antenna is same as shown in Fig. 11.
Fig. 14. Printed board of Impedance probe circuit prepared for environmental tests before DELTA campaign in 2004. Some electric parts are fixed by
thermoplastic materials or silicon bonds. The temperature sensor IC was represented in the center of this panel. This IC is only for environmental
tests (non-flight item).
2.3 Environmental tests and integration test
To evaluate environmental durability, several tests are
carried out by using vibration and impact testing machine, thermostatic bath and space science chamber. In
Japan, ISAS/JAXA requires payload instruments of sounding rockets and satellites to pass the environmental criteria
to avoid any failures during its flight. Some private-sector
facilities can operate the vibration and impact testing machine and thermostatic bath for such environmental tests.
On the printed circuit of impedance probe, tall or large parts
(variable condensers, variable resistors, DC-DC converter,
etc.) must be fixed by thermoplastic materials. If the substrate temperature should be monitored during the test, additional temperature sensor (for example, LM35DZ) is temporary attached on the printed board. Figure 13 shows the
external appearance of printed board which is prepared for
environmental test. Also, ISAS/JAXA provides space science chamber as sharing system for the operation test in
vacuum or ionized gas.
Integration and spin timer test (that are conducted by
M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
117
Fig. 15. Photograph of impedance probe instrument on-board S310-31. Electric part and sensor part was connected by using co-axis cable.
Table 1. Summary of LC circuit calibration results (in case of preparation
for DELTA campaign).
LC circuit No.
(Resonant freq.)
(1) (5.787 MHz)
(5) (2.198 MHz)
(6) (1.069 MHz)
UHR detection
Error
5.785 MHz
2.203 MHz
1.079 MHz
(− 2 kHz)
(+ 5 kHz)
(+10 kHz)
ISAS/JAXA) are also important to confirm compatibility
with other instruments and simulate flight condition. In
these tests, it is possible to make sure of antenna extension
with appropriate length, in applicable direction, at exact
time sequence. It is necessary to have preparatory exercise
of removing non-flight item before flight operation.
3.
Table 2. Dimensional data of impedance probe on-board S310-31.
Size/Weight (circuit)
Size/Weight (sensor)
Electrical requirements
Frequency sweep range
Frequency step
Electron density coverage
Sweep time
Micro controller
Direct digital synthesizer
Buffer amp. and
differential amp. IC
Telemeter bitrate
140 × 140× 30 mm/0.65 kg
70 × 30× 25 mm/0.23 kg
±18 V, 250 mA
300 k–21.3 MHz
10 kHz @ 300 k–3.8 MHz
20 kHz @ 3.8 M–21.3 MHz
103 − 5.6 × 106 cm−3
500 ms
PIC16F877
AD9850BRS
CLC420AJE
204.8/102.4 kbps
(for NEI: 1600 samples/sec)
Application of Impedance Probe to Rocket
Campaigns
3.1 Observations of mid-latitude sporadic-E
SEEK-2 (Sporadic-E Experiment over Kyushu) campaign was planned to investigate generation mechanism of
QP echoes (e.g., Yamamoto et al., 1991) associated with
mid-latitude sporadic-E (E s ) layer. Two rockets were prepared to observe electron density, electric field, electron
temperature, neutral wind velocity and other parameters.
The outline of SEEK-2 campaign is excellently described
by Yamamoto et al. (2005). The impedance probes were
also installed in two rockets of S310-31 and -32 which belong in typical sounding rocket series of S310 with single
stage. During the SEEK-2 campaign, the impedance probe
instrument called as NEI (Number density of Electrons by
Impedance probe) as well as in recent Japanese rocket experiments. Dimensional data and external appearance of
impedance probe for SEEK-2 campaign were represented
in Table 2 and Fig. 15, respectively. Because the maximum
electron density of mid-latitude E s layer sometimes reaches
5 × 106 cm−3 , measurable frequency range should be up to
20 MHz. This is the first application of DDS and highspeed differential amplifier IC for rocket experiment as de-
scribed in Section 1. Associated with these improvements,
impedance probe sensor was separated from the electric circuit, and it was near the top of the payload section as shown
in Fig. 16. This improvement is expected to avoid the wake
effects during the rocket flights. The rocket experiment
was carried out at Uchinoura Space Center (USC; 31.15◦ N,
13.04◦ E) on 3 August 2002 with successive launches of the
S310-31 and -32 rockets at 23:24 and 23:39 JST, respectively. During the rocket flights, two impedance probes observed the electron density profiles with distinct sporadic-E
signature. Figure 17 represents the electron density profiles
obtained on-board S310-31. Rocket wake effect was not
seen in the profile obtained during the SEEK-2 campaign.
Observed electron density was used to estimate the spatial structure of E s layers which is described in the paper
of Wakabayashi et al. (2005). In addition, accurate and simultaneous observation by impedance probe without wake
effect provided a clue to point out the difference between
upper thin layer and lower thick layer as described in Wakabayashi and Ono (2005).
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M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
Fig. 16. Antenna extension phase of S310-31 rocket. The sensor was installed near the top of the payload section. This improvement was expected to
avoid the wake effects during the rocket flight.
Fig. 17. Altitude profiles obtained by using impedance probe on-board S310-31. In this panel, there are obvious sporadic-E signatures about the altitude
of 100 km (after Wakabayashi et al., 2006).
3.2
Observations of electron density during diffuse auroral event
The DELTA (Dynamics and Energetics of the Lower
Thermosphere in Aurora) campaign aimed to obtain accurate heating rates and to compare quantitatively with the atmosphere temperature and wind. The DELTA campaign involved the S310-35 sounding rocket experiment to measure
the neutral temperature, electron density, electron tempera-
ture, auroral particle fluxes and emissions. To realize the accurate measurement of the neutral temperature, the electron
beam fluorescence method has been developed on-board a
sounding rocket (Kurihara, 2003). The impedance probe
was also installed to obtain absolute electron density on the
rocket trajectory. Table 3 shows the summary of payload
instruments. This experiment is characterized by its function of electron beam injection by NTV (N2 temperature of
M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
119
Table 3. Summary of payload instruments on-board the S310-35 sounding
rocket. Impedance probe was installed in sub-PI section.
Fig. 18. External appearance of impedance probe on-board S310-35 for
DELTA campaign. The co-axis cable was shorter than that of SEEK-2
campaign because the antenna was installed near its electrical circuit
unlike for the SEEK-2 campaign.
Vibration) instrument to measure the N2 gas temperature.
To avoid any potential changes due to electron beam injection with its energy of 1 keV, S310-35 was equipped with a
rocket separation mechanism. The impedance probe was installed in motor side of the rocket (in this campaign, the motor side was called as “mother rocket”). Dimensional data
and external appearance of impedance probe for DELTA
campaign were shown in Table 4 and Fig. 18, respectively.
The target of this campaign was diffuse auroral event which
was not expected with intense and local ionization. Therefore, swept frequency range was up to 10.3 MHz for this
experiment.
Figure 19 shows the capacitance bridge circuit of
impedance probe on-board S310-35. To protect the capacitance bridge circuit from the possible high-energy particles
input due to the electron beam injection as well as intense
auroral precipitation, antenna was connected to the electric ground level of the rocket body with a 10 M resistor,
while previous experiments used the floating probe condition. This improvement is only for the protection of bridge
circuit which detects the equivalent capacitance of antenna.
Since the inserted resisters do not have frequency response,
they do not prevent from accurate adjustment of capacitance
bridge circuit. In addition, capacitance bridge consisted of
three mica condensers with its high break down voltage
endurance of 500 V and a 20 pF trimmer condenser (it is
noted that ordinary laminated ceramic capacitor is enough
for bridge circuit if there is no possibility of high-voltage input). Since it was also necessary to protect the buffer amplifier ICs against high voltage input, protection diodes were
added to the input terminal of amplifier. As described in
Section 1, this is the first case of impedance probe with logarithmic amplifier application. Therefore, the capacitance
bridge should be covered with the electrostatic shield to reduce noises and to make it easy to adjust the capacitance
bridge. The close-up of the capacitance bridge is given in
Fig. 20.
For the DELTA campaign, sensor of impedance probe
should be designed to extend through the hole of rocket
body because electric circuit and sensor of impedance probe
PI section (Daughter rocket)
N2 temperature of vibration
Auroral particle detector
Horizon sensor
Constant biased Langmuir probe
CI section
S-band PCM telemetry system
Programmable timer
S-band antenna
Geomagnetic attitude sensor
CI battery
CI power supply controller
Motor pressure sensor
Sub-PI section (Mother rocket)
Fast Langmuir probe
Auroral green line photometer
Surface finder
Number density of electrons
by impedance probe
Small telemetry system
Sub-PI power supply
SMT geomagnetic attitude sensor
(NTV)
(APD)
(HOS)
(CLP)
(S-PCM-TM)
(EPT)
(ANT-SMT)
(GA2S)
(CI-BAT)
(CI-PSC)
(Pc)
(FLP)
(AGL)
(SFF)
(NEI)
(SMT)
(SUBPI-BAT)
(SMT-GA)
Table 4. Dimensional data of impedance probe on-board S310-35. This is
the first time to application of logarithmic amplifier.
Size/Weight (circuit)
Size/Weight (sensor)
Electrical requirements
Frequency sweep range
Frequency step
Electron density coverage
Sweep time
Micro controller
Direct digital synthesizer
Buffer amp. IC
Differential amp. IC
Logarithmic amp. IC
Telemeter bitrate
140 × 140× 30 mm/0.65 kg
65 × 30× 25 mm/0.21 kg
+18 V, 250 mA
300 k–10.3 MHz
10 kHz @ 300 k–4.3 MHz
20 kHz @ 4.3 M–10.3 MHz
103 − 1.2 × 106 cm−3
515 ms
PIC18F452
AD9851BRS
AD8065AR
AD8130AR
AD8307AR
204.8 kbps
(for NEI: 800 samples/sec)
must be installed in motor side of S310-35. Therefore,
mechanism of antenna case was changed as shown in Fig.
21 to ensure the antenna extension. The antenna extension
phase of S310-35 is represented in Fig. 22 during spin timer
test at ISAS/JAXA (this test was conducted except rocket
separation sequence).
This campaign was carried out at Andøya Rocket Range
(ARR; 69.29◦ N, 16.01◦ E) in Norway, on 13 December,
2004. Outline of this campaign is described in the issues
of Abe et al. (2006). Altitude profiles obtained by using
impedance probe are shown in Fig. 23. There are data gaps
in ascending phase of S310-35 due to the effect of electron beam injection. However, detail analysis of impedance
probe made it possible to complement the gaps as shown
in Wakabayashi and Ono (2006). The electron density profile contributed to evaluate the effect of Jeule heating during
120
M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
Fig. 19. Capacitance bridge circuit designed for DELTA campaign. In this configuration, C1 and C2 are mica condensers with their capacitances of 100
and 10 pF, respectively. The VC is trimmer condenser as well as for the SEEK-2 campaign.
Fig. 20. Close-up picture of capacitance bridge circuit designed for DELTA campaign. This circuit is the bridge part of Fig. 13 (inside of electrostatic
shield). Each electric parts are fixed by thermoplastic materials.
diffuse auroral event.
3.3 Simultaneous observation with plasma waves
WIND campaign was aimed to clarify the interaction process between the ionospheric plasma and the thermospheric
neutral wind through a direct observation using the S52023 sounding rocket. In the WIND campaign, a lithium release experiment was conducted in the descending phase to
estimate the thermospheric neutral wind by observing the
motion of the released lithium cloud from the ground. This
campaign also included optical observation of cumulonimbus and sea by using on-board camera to verify new multiband observation technique.
This experiment also involved electron density observation by using impedance probe technique on-board the
rocket. This case is characterized by combination with
plasma wave receiver. Also, the instrument was called as
PWM (Plasma Wave Monitor) which is developed as integration of impedance probe and receiver. Figure 24 shows
M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
121
Fig. 21. (a) Close-up view of hole for impedance probe antenna extension. (b) Structure of antenna case and lid which is designed to be released straight
ahead.
Fig. 22. Antenna extension phase of impedance probe on-board S310-35.
the block diagram of impedance probe on-board S520-23.
In this panel, master controller of impedance probe sends a
synchronizing signal to slave one of plasma wave receiver.
This configuration was proposed to avoid interference due
to local signals of both instruments. This is good example of impedance probe to enhance the compatibility with
coadjacent passive receiver. Table 5 shows the dimensional
data of impedance probe for WIND campaign. In this case,
impedance probe was relatively larger and heavier because
two kinds of instrument were integrated as one. The altitude
profile obtained during the WIND campaign is represented
and discussed in the issue of Uemoto et al. (2010).
4.
Future Works
4.1 Automatic detection of UHR frequency
In the missions in 1970’s, automatic detection system of
UHR frequency was needed because the limit of the capacity of telemetry speed as 64 bits/s (Ejiri et al., 1973; Oya
and Morioka, 1975). Ejiri (1973) reported the establishment of automatic UHR detection system to reduce the data
amount by using the derivation of equivalent capacitance
of conductor probe. However, there were miss detections
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M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
Fig. 23. Altitude profiles of electron density obtained by using impedance probe during the DELTA campaign. In ascending phase, there were data gaps
due to electron beam injection. Detail data analysis of impedance probe can estimate the electron density even in data gaps (described in Wakabayashi
and Ono (2004)).
Fig. 24. Block diagram of impedance probe on-board S520-23. Local signal is shared to avoid any possible interference with each other.
remained for the UHR frequency detection because noises
or effects of electrostatic waves also give a condition to be
detected as UHR frequency. After the application on-board
the TAIYO satellite, there has been no trial of the automatic
UHR detection on-board the satellite.
In recent years, automatic and high-speed UHR detec-
tion is desired again not only to reduce data amount but
also to observe the fine structure of ionospheric plasma by
using sounding rocket. The key point of this improvement
is application of phase detection circuit; namely, PLL circuit. This application will make it possible to improve spatial resolution of impedance probe observation enough to
M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY
Table 5. Dimensional data of impedance probe for S520-23 is summarized
in this table. This impedance probe was integrated with plasma wave
receiver.
Size/Weight (circuit)
Size/Weight (sensor)
Electrical requirements
Frequency sweep range
Frequency step
Electron density coverage
Sweep time
Micro controller
Direct digital synthesizer
Buffer amp. IC
Differential amp. IC
Logarithmic amp. IC
Telemeter bitrate
140 × 140× 50 mm/2.00 kg
92 × 50× 28 mm/0.20 kg
+18 V, 500 mA
300 k–12 MHz
9.4 kHz @ 300 k–2.0 MHz
20.0 kHz @ 2.0–4.0 MHz
50.0 kHz @ 4.0 M–8.0 MHz
100 kHz @ 8.0 M–12.0 MHz
103 − 2.0 × 106 cm−3
502.3 ms
PIC18F452QFP
AD9851BRS
AD8065AR
AD8130AR
AD8307
204.8 kbps
(for NEI: 800 samples/sec)
detect the fine structure of ionospheric plasma. In addition, impedance probe with this improvement will obtain
UHR frequency successively even if telemetry speed is relatively slow. The phase detection type is expected further
progress of impedance probe installation on-board satellites and planetary exploration spacecrafts. To increase the
chance of installation, it is necessary to make the circuit
lighter and smaller. As reported by Suzuki (2010), application of FPGA device will provide considerable progress in
downsizing of impedance probe to be standard instrument
of spacecrafts in the future.
Acknowledgments. Impedance probe instruments described in
this issue were produced by System Keisoku Inc. The environmental experiments for carrying out the rocket campaigns
were conducted and supported by the Space Plasma Laboratory, ISAS/JAXA. The rocket experiments were conducted by
ISAS/JAXA rocket team. The authors would like to show their
special thanks to all the persons concerned and professor KohIchiro Oyama.
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