Impedance probe technique to detect the absolute …...Impedance probe technique to detect the...

An Introduction to Space Instrumentation,Edited by K. Oyama and C. Z. Cheng, 107–123.

Impedance probe technique to detect the absolute number densityof electrons on-board spacecraft

M. Wakabayashi1, T. Suzuki2, J. Uemoto3, A. Kumamoto4, and T. Ono4

1Niihama National College of Technology, Department of Electrical Engineering and Information Science,7-1, Yagumo, Niihama, Ehime 792-8580, Japan

2Meisei Electronic Co., Ltd., Isesaki, Gunma, Japan3National Institute of Information and Communications Technology, Koganei, Tokyo, Japan

4Graduate School of Science, Tohoku University, Sendai, Japan

This text focuses on the impedance probe which is powerful method for observation of absolute electrondensity in plasma. In several countries, impedance probe has been used during many rocket campaigns andsatellite missions by many scientists’ groups. In Japan, Oya (1966) realized remarkably accurate electron densityobservation on-board sounding rocket. Since then, the impedance probe has taken part in absolute electron densitymeasurement to proceed ionospheric and space plasma science in Japan. This text aims to describe impedanceprobe method which is developed by Oya (1966) as excellent example of in-situ observation of absolute electrondensity. The following description will provide the historical background, basic theory, advantages, importantreminders and specific application ideas of sounding rocket observations during SEEK-2, DELTA and WINDcampaigns. This text will refrain from commenting a lot about impedance probe instruments on-board satellitesbecause the authors don’t have many experiences with carrying out satellite missions. However, description in thisissue 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 effectiveskills of the method through the text.Key words: Electron number density, UHR frequency detection, plasma resonance.

1. Introduction1.1 Historical background of in-situ observations

Sounding rockets and satellites are powerful and essentialtools to study the ionospheric plasma because these in-situobservational methods are possible to obtain the parametersof plasma and neutral gases, electric and magnetic fields,and plasma waves directly. In Japan, observation by usingsounding rocket has been started since 1950’s. After that,many kinds of instrumentations were applied to obtain vari-ous parameters of the ionosphere, although only ion densitywas measured on-board the sounding rocket initially. Thedevelopments of various rocket-borne instruments made itpossible to provide electron and ion densities, temperatures,electric and magnetic fields as direct observation data. Asexcellently summarized by Hirao et al. (1966), many obser-vations were carried out during the period of the Interna-tional Years of the Quiet Sun (IQSY; from January 1964 toDecember 1965).

To measure electron density on-board a sounding rocket,Langmuir probe method has been the most commonly usedinstrumentation. This method obtains Langmuir charac-teristic curve (I–V curve) by using conductor probe withswept DC voltage. Electron density, electron temperatureand some plasma parameters are deduced from the char-acteristic curve; however, the characteristic also depends onthe effective area of probe surface and other conditions suchas magnetized, collision dominant and non-Maxwellian dis-

Copyright c© TERRAPUB, 2013.

tribution of plasma. Therefore, the Langmuir probe has dif-ficulty in direct determination of the absolute value of elec-tron density.

To detect the fine structure of ionospheric plasma, thefixed-bias probe (FBP) has been used for long years until to-day because this method has high spatial resolution of sev-eral meters. This instrument is based on excellently simpleprinciple of observation. Namely, constant DC voltage wasapplied to spherical conductor probe. Then, there is input ofelectron current from ambient plasma though the sphericalprobe. Electron density (relative value) and its fine structurewill be obtained by measuring DC and AC components ofelectron current, respectively. This instrument was installedfor the rocket campaign described in Section 2 of this book.

For the identification of absolute plasma density, someprobe methods were improved through the laboratory androcket experiments, for example, resonance probe, mutualimpedance 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 ofUHR frequency. However, this resonance probe actuallymeasured sheath resonance frequency (SHR). Thereafter,the resonance probe became not to be used because the SHRfrequency was too much complex function to deduce theelectron density accurately.

As for the mutual impedance probe, several scientists re-ported the measurement results after Storey et al. (1969).This system consists of quadrupolar probe which measuresthe frequency response of the coupling impedance betweentwo dipoles. Some European scientific missions such as

107

108 M. WAKABAYASHI et al.: IMPEDANCE PROBE TECHNIQUE TO DETECT THE ABSOLUTE NUMBER DENSITY

GEOS 1 satellite adopted this instrumentation. Many sci-entific results obtained by using mutual impedance probehave been reported (e.g., Decreau et al., 1982).1.2 Development and improvement of impedance

probeThe 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 probewas realized by Oya (1966) who was inspired by the cal-culation 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 theprinciple which made it possible to determine Upper Hy-brid Resonance (UHR) frequency of ambient plasma. De-tection of the UHR frequency is possible by measuring thefrequency response of the probe impedance immersed inplasma. When an RF voltage of swept frequency is appliedto the probe, the probe impedance becomes infinite whenthe RF frequency coincides with the UHR frequency. If theUHR frequency is determined, absolute density of electronscan be deduced by using the equation of

Ne = 1.24 × 104( f 2U H R − f 2

c ), (1)

where, Ne, fU H R and fc are absolute electron density(cm−3), UHR frequency (MHz) and electron cyclotron fre-quency (MHz), respectively. The impedance probe also hasthe advantage that the variation of the probe potential doesnot affect on the detection of the UHR frequency. In addi-tion, existence of plasma sheath and electrode contamina-tion does not affect the UHR detection. Then, it is possi-ble to obtain the accurate electron density if the fc is givenby on-board magnetometer or geomagnetic model. Thismethod can provide the absolute electron density with highaccuracy of ±3% (in case of Ne > 104 cm−3). In orderto observe the accurate electron number density with thismethod, it is essential to eliminate any stray capacitance inthe electrical circuit. For this purpose, Oya adopted capac-itance bridge circuit, as shown in Fig. 1, to cancel out theeffect of the stray capacitance by adjusting the compensa-tion capacitor (and length of dummy cable, if it is needed)associated with the capacitance bridge. The RF signals aresynthesized by using an analog VCO (voltage controlledoscillator) circuit, and the signals are added to the bridgecircuit via transformer. The impedance probe has many ad-vantages of hardware aspect, namely, significantly simpleconfiguration with single antenna, remarkable EMC char-acteristics for compatibility with other instruments, and en-durance of environmental factor. It also does not need spinmotion of rockets or satellites for this method. However, theinstrument should be installed in the part which meets con-dition that the single antenna is not immersed in wake areadue to spacecraft motion. The impedance probe had beeninstalled in many sounding rockets such as K-9M-13, -14,L-3H-2, K-9M-20, and -21, and provided many scientificresults. Oya and Obayashi (1967) showed a good exam-ple of rawdata obtained by using impedance probe. Figure2 represents example of the equivalent capacitance, phaseand frequency references which are measured on-boardthe sounding rocket. Furthermore, many satellite missions(DENPA, TAIYO, JIKIKEN, HINOTORI, OHZORA and

Fig. 1. Conceptual diagram of impedance probe circuit which is used inthe previous rocket experiments before 2002.

AKEBONO satellites) included electron density observa-tions by using impedance probe (e.g., Ejiri, 1973, Ejiri etal., 1973; Oya and Morioka, 1975). In 1998, in-situ ob-servation of Martian ionosphere was attempted on-boardJapanese Mars orbiter NOZOMI (unfortunately, the NO-ZOMI failed to insert the Martian orbit).

In recent rocket experiments, applications of digital andanalog devices enabled to improve the characteristics ofimpedance probe circuit. For SEEK-2 campaign in 2002(described in Section 2), two improvements had been ap-plied to the impedance probe instrument. One is the usageof Direct Digital Synthesizer (DDS) to generate wide-bandswept frequency to add to the capacitance bridge. The ana-log VCO, which was used in previous rocket campaigns andsatellite missions, has large dependence on the device tem-perature because the variable capacitance diode used in theresonator. So, frequency reference data should be simulta-neously obtained to determine UHR frequency of ambientplasma with high accuracy. On the other hand, the DDSis referring the 32-bit data table to define the frequencyversus step number (the frequency data controlled by thePIC micro-controller). Then, frequency reference data isnot necessary any more because the linearity of wide-bandswept frequency was extremely improved by the usage ofDDS device. Another improvement is put on the capaci-tance bridge circuit by using a high-speed differential am-plifier IC which made it possible to adjust the balance ofbridge circuit with significantly simplified way, becausenew bridge circuit did not any transformer. These improve-ments made it easy to install the antenna keeping away fromits electric circuit (about 1.5 m at maximum) by connectingwith the co-axis cable. This has an advantage in settingup the antenna near the top of the rocket body to reducethe effect of rocket wake. It should be added that this wassignificantly difficult for previous configuration because ca-pacitance and inductance components of extended co-axiscable added to the stray capacitance and inductance due totransformer.

For DELTA campaign which was carried out in 2004 (de-

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, equivalentcapacitance 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 Rusually configured as 0.1 uF and 1 M�, respectively.

scribed in Section 3), logarithmic amplifier IC was adoptedin the backend of impedance probe instrument. This ampli-fier made it easy not only to detect the dip point of equiv-alent capacitance spectrum but also to broaden equivalentcapacitance coverage. After these improvements, standard

configuration of impedance probe circuit was replaced asshown in Fig. 3.1.3 Basic theory of impedance probe measurement

The impedance probe detects equivalent capacitance ofconductor antenna immersed in plasma. The equivalent ca-


pacitance is measured as frequency response characteris-tic which includes the value of UHR and SHR frequenciesand sheath capacitance. To intuitively understand the prop-erties of antenna impedance (or admittance) immersed inplasma, it is useful to substitute equivalent circuit modelfor the real antenna. In following sentences, it will be in-troduced how to derive the relation between the equivalentcircuit and real antenna although it is necessary to importsome assumptions. The calculation of this section treatsthe isotropic, cold and electron plasma with InternationalSystem of Units. Also, it should be noted that the equiva-lent circuit model cannot evaluate quantitative value of theplasma impedance because only an ideal case is discussedin this section.

According to the theory demonstrated by Spitzer (1962),plasma mean velocity �v is described in fluid equation as

nm

(∂ �v∂t

+ (�v · ∇) �v)

= nq �F −∇ · �ψ −nm∇�g + �P, (2)

where, n: particle number density, m: mass of particle, q:charge, �F : external force, �ψ : stress tensor, �g: gravitypotential and �P: momentum changes due to collisions. andare also represented as

�F = �E + �v × �B, (3)�P = −nmν �v, (4)

where, �E : electric field, �B: magnetic flux density and ν:collision frequency. The gradient of stress tensor ∇ · �ψ isexpressed as

∇ · �ψ = ∇nkT = ma2∇n, (5)

where, k: Boltzman’s constant, T : plasma temperatureand a = √

(kT )/m. On the other hand, the equation ofcontinuity is given as

∂ρ

∂t+ ∇ · �j = 0, (6)

where,ρ = qn, (7)

and�j = qn�v. (8)

Here, ρ and �j represent charge density and electron currentdensity, respectively. In Eq. (2), it is assumed that thegravity potential �g is negligible and all the variables wouldchange as a function of exp(iωt). The coordinate systemwas selected as Cartesian coordinate, and magnetic fieldwas homogeneous only in direction of z-axis.

Based on these equations, Eq. (2) will be changed toequation of electron current density �j as

∂ �j∂t

+ 1

nq

(�j · ∇

)�j + ��× �j + ν �j = �2 · �D −qa2∇n, (9)

where, �: angular plasma frequency and �: angular cy-clotron frequency, namely;

� =(

q2n

mε0

)1/2

, (10)

and

� = q �Bm

. (11)

Since the second term in left hand side of Eq. (9) is nonlin-ear term, it can be neglected for linearization of this equa-tion. Then, Eq. (9) is rewritten as

∂ �j∂t

+ �� × �j + ν �j = �2 · �D − qa2∇n, (12)

namely, · �j = �2 · �D − a2∇ρ, (13)

where,

=( iω + ν −� 0

� iω + ν 00 0 iω + ν

). (14)

Finally, electron current density �j is represented as

�j = �2−1 �D − a2−1∇ρ. (15)

The total electron current density ( �J ) is equal to the sumof the convection current ( �j) and displacement current((∂ �D)/(∂t)). Therefore, �J is expressed as following equa-tion of

�J = ∂ �D∂t

+ �j

= jω

(1 + �2

jω−1

)�D − a2−1∇ρ. (16)

If it is under the assumption of cold plasma, second termin the right hand side of Eq. (16) is negligible. In Eq. (16),the value of 1 + �2/( jω)−1 means well-known dielectrictensor (K) of plasma. Here, the K can be expressed as

K =(

εxx εxy 0−εyx εyy 0

0 0 εzz

), (17)

where,

εxx = εyy = 1 + �2

iω· iω + ν

(iω + ν)2 + �2, (18)

εxy = εyx = �2

iω· �

(iω + ν)2 + �2, (19)

εzz = 1 + �2

iω· 1

iω + ν. (20)

By using the dielectric tensor (K), Eq. (16) is rewritten as

�J = iωK · �D. (21)

On the other hand, the relation between specific admittance(y) and total electron current density ( �J ) is described as

�J = y �E = y

( �Dε0

). (22)

Now, we call y “specific” admittance. The reason for thename of “specific” is that we have proceeded with this topic


Fig. 4. Equivalent circuits of conductor antenna immersed in plasma underthe condition of isotropic, cold plasma, existence of magnetic field andexistence of electron-neutral collisions. This circuit diagram is deducedfrom Eqs. (32) and (33) in the text (after Ejiri, 1973).

Fig. 5. Equivalent circuits of antenna under the condition of collision-lessplasma. The magnetic field is still remained as well as Fig. 4. Thesediagrams correspond to Eqs. (34) and (35), respectively.

for the equation of vecJ ; total electron current density. Theadmittance of plasma is inevitably treated as the admittancefor unit length. So, the dimension of y is not usual unit suchas [S] but [S/m]. Then, specific admittance (y) is derived as

y = ε0

�J · �D∣∣∣ �D∣∣∣2 . (23)

So, we can obtain the value of y as a function of εxx andεzz . Now, y is described as following equation.

y = ε0

�J · �D|D|2 = iωε0

(εxx sin2 θ + εzz cos2 θ

)= y⊥ sin2 θ + y// cos2 θ (24)

where,

y⊥ = iωε0εxx = iωε0

+ 1ν

ε0�2+ iω

ε0�2+ 1

ε0�2ν

�2 + iωε0�2

�2

. (25)

Fig. 6. Equivalent circuit of antenna under the condition of collision-lessand non-magnetized plasma. There are no differences between perpen-dicular and parallel components with respect to the z-axis (along themagnetic field line). This expression is frequently cited to explain thefeature of antenna impedance in plasma as the most simplified case.

Fig. 7. Schematic relation of impedance probe antenna and ambientplasma. Under the assumption of cold, isotropic, B = 0 [T] andcollision-less plasma, most simplified equivalent circuit is representedin this panel.

and

y// = iωε0εzz = iωε0 + 1ν

ε0�2+ iω

ε0�2

. (26)

In Eq. (24), θ represents the angle of electric field ( �E) withrespect to z-axis. These expressions mean the specific ad-mittance of plasma can be translated as equivalent circuitswhich include equivalent specific resistance (R), equiva-lent specific inductance (L), equivalent specific conduc-tance (G) and equivalent specific capacitance (C and C0).Namely,

R = ν

ε0�2, (27)

L = 1

ε0�2, (28)

G = ε0�2ν

�2, (29)

C = ε0�2

�2, (30)

andC0 = ε0. (31)

The kinds of these circuit elements are determined by di-mension analysis. However, dimension of each element is


Fig. 8. Calculation result of equivalent impedance of conductor probe in ambient plasma. There are minimum and maximum of equivalent impedanceat 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 exam-ple, ε0[F/m] �= [F] for Eq. (31),) because these circuitelements derived from “specific” admittance (y). By usingthese parameters, Eqs. (25) and (26) are represented as

y⊥ = iωC0 + 1

R + iωL + 1

G + iωC

, (32)

y// = iωC0 + 1

R + iωL. (33)

These specific admittances correspond to equivalent circuitsshown in Fig. 4. For example, around the 120 km altitudeduring the DELTA campaign ( fc = 1.40 MHz, Ne = 5.02×105 cm−3), the values of R, L , G and C are 0.71 �/m,70.7 µH/m, 1.82 µS/m and 182 pF/m, respectively. If itis under the assumption of collision-less plasma, collisionfrequency (ν) is equal to zero. Then, the above equationscan be simplified as

y⊥ = iωC0 + 1

iωL + 1

iωC

, (34)

and

y// = iωC0 + 1

iωL. (35)

Furthermore, if the magnetic field is equal to zero, thespecific admittance is described in most simplified form as

y⊥ = y// = iωC0 + 1

iωL. (36)

The equivalent circuits in case of collision-less plasma (Eqs.(34) and (35)) and B = 0 (Eq. (36)) are shown in Figs. 5and 6, respectively.

For actual condition in plasma, plasma sheath will beformed around the antenna (many cases of impedance probeinstrument adopted cylindrical probe). Then, sheath ad-mittance ys will be installed in series with plasma admit-tance. If sharp boundary model of plasma sheath is as-sumed, equivalent sheath capacitance Cs is added to the cir-cuit.

According to the calculation described above, the equiva-lent probe model circuit as well as schematic relation of theantenna, rocket body and ambient plasma can be expressedas shown in Fig. 7 which is most simplified model and fre-quently used to explain briefly the concept of impedanceprobe measurement. These calculations described above arealso introduced in detail in the issue of Ejiri (1973) includ-ing the case of warm plasma.

About the equivalent circuit shown in Fig. 7, the syntheticimpedance of the resonant circuit is obtained as

1

|Z (ω)| =

1

ωCs+ 1

ωCa − 1

ωL p

−1

, (37)

where, Z(ω), ω, Ca and L p are the impedance of the res-onant circuit, angular frequency of RF signals, equivalentcapacitance and inductance of plasma, respectively. Theequivalent capacitance has maximum and minimum valueswhen ω is

ωSH R = 1√L p (Ca + Cs)

(a series resonance : corresponds to SHR) (38)

and

ωU H R = 1√L pCa

(a parallel resonance : corresponds to UHR). (39)

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, theeffect of the inductance L p becomes negligible. Then, thesheath capacitance can be obtained at the lowest frequencyrange of 300 kHz for the ordinary impedance probe obser-vation. (The lowest frequency should satisfy two condi-tions; namely, adequately lower than plasma and cyclotron


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 ioncharacteristic frequencies.) The SHR frequency is a func-tion of electron density, temperature and the direction of themagnetic field. The sheath capacitance is a function of theelectron density, temperature and the probe potential andcan provide useful information on ambient plasma, as de-scribed by Oya and Aso (1969).

On the other hand, Balmain (1964) proposed a theory ofthe impedance of an antenna in plasma. It is based on theanalytical solution by assuming the cold plasma approxima-tion. The derived impedance of a short dipole antenna Z pl

with the length 2L and radius r is as follows

Z pl = α

iω2πε0εxx L√

F

(ln

L

r− 1 − ln

α + √F

2F

),(40)

F = sin2 φ + α2 cos2 φ, (41)

α2 = εxx

εzz. (42)

where φ denotes the angle between the axis of the antennaand the static magnetic field. The impedance curve calcu-lated from (40) is plotted in Fig. 9 as a form of the probeequivalent capacitance since we calibrate the output signalof impedance probe to the capacitance (after Suzuki, 2010).Figure 9 also shows the calculation including the capaci-tance of ion sheath surrounding the probe. In Fig. 9, there isadditional resonance represented as MPR (modified plasmaresonance). Ejiri et al. (1968) reported the experimental ev-idence of MPR obtained by sounding rocket observation.The MPR frequency depends on the angle of φ, namely,

ω2M P R = ω2

U H R

2

(1 ±

√1 − 4�2�2

ω4U H R

sin2 φ

). (43)

This relation will be deduced from Eq. (41) under the con-dition of F = 0. In the ionospheric D-region, the collisioneffect between electrons and neutrals becomes significantbecause the electrons in plasma can no longer realize cy-clotron 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). Thefrequency shift of the UHR waves by the collision effect isdescribed as follows:

νen = 6 × 10−9 Nn, (44)

ω2U H R = ω2

ob − ν2en, (45)

where νen and Nn represent the collision frequency (s−1) be-tween electrons and neutrals, and number density (cm−3) ofneutral gases in the ionosphere, respectively, which is ob-tained by the MSIS-E-90 atmosphere model. In Eq. (45),ωU H R is the “true” UHR angular frequency of the ambientplasma, and ωob is the UHR angular frequency observedby the impedance probe instrument. In the region above100 km altitude, this effect becomes negligible where thedifference between ωU H R and ωob is smaller than 1%, be-cause νen decreases rapidly with altitude. It should be alsonoted that the impedance probe observation is effective forthe electron density over ∼103 (cm−3). This limit is due tothe Q-value of equivalent resonant circuit which becomeslower with decrease in electron density. It will be difficult todetect UHR frequency under the condition of ∼103 (cm−3)electron density in case of 1∼2 m-long antenna. If it isneeded to detect lower electron density than 103, longer an-tenna 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 probemethod was delayed for the space plasma measurements.The group of Utah State University remarkably contributedto develop the impedance probe in the United States. Theycalled their instrument plasma impedance probe (PIP). In1980’s, Baker et al. (1985) tried to detect the UHR fre-quency with the continuous observation. They applied theanalog Phase Locked Loop to track the UHR frequency au-tomatically. The time resolution was 1 ms which corre-sponds to the several m spatial resolution during a rocketflight. In recent years, they obtained impedance probe datafrom their instrument unit on-board International Space


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, Ger-man group has studied the impedance probe, and theirimpedance probe system has the time resolution of 40 msby using the DDS (Direct Digital Synthesizer), PLD (pro-grammable logic device) and A/D converter (Steigies et al.,2000).

2. System Design, Calibration and EnvironmentalRests

2.1 Application to the sounding rocket observationsThis section shows calibration method to detect equiv-

alent capacitance of probe correctly. The calibration willbe explained for the usage of impedance probe with stan-dard configuration which is represented in previous section(Fig. 3). Environmental and integration tests are also ex-plained; however, the detail of tests should be carried outbased on the requirement of on-board instrument decidedby the space agency which conducts the rocket experimentor satellite mission.2.2 Calibration

Calibration of capacitance bridge is carried out by con-necting ceramic condensers with its capacitance of 1∼1000pF (in most recent case, 0.1∼2000 pF is used). By con-necting ceramic condenser, the capacitance bridge is cali-brated for its sensitivity as well as frequency dependences.To carry out this calibration, dummy antenna case shouldbe prepared to simulate the situation of plasma observa-tion. For impedance probe observation on-board sound-ing rocket, BeCu ribbon antenna is frequently used asimpedance probe sensor with its dimensions of 1.2 m length

and 12 mm diameter. The sensor of impedance probeshould meet requirement of dimension; namely, the sen-sor have sufficiently large dimension compared with De-bye length of ambient plasma. If the sensor is too small,Ca (see, Subsection 1.3) will be too small, and the Q-valueof equivalent circuit becomes low. The typical sensor iscylindrical antenna with its length of 1∼2 m for rocket cam-paigns. The BeCu ribbon antenna mentioned above is light,easy to store in compact antenna case, and extend by itsforce of restitution. Figure 10 represents the appearanceof the BeCu ribbon antenna and its case with extensionmechanism. Conceptual diagram of the calibration by us-ing condensers is shown in Fig. 11. Because the impedanceprobe should detect the equivalent capacitance of BeCu an-tenna which is “immersed in plasma”, BeCu antenna mustbe cut at the edge of antenna case for calibration as shownin Fig. 11. The ceramic condensers are connected to an endof dummy antenna by crocodile clip. Then, output voltageof impedance probe will change as capacitance of ceramiccondensers change. Accurate calibration of the capacitancebridge will realize the input-output characteristic as shownin Fig. 12 which represents the smallest output voltage at 0pF (open-circuit condition) and monotonically increasing.

The accuracy of the UHR frequency detection should beconfirmed by applying LC resonant circuit to the dummyantenna. This calibration is also significant because it canevaluate the influences of any stray “inductance” in the elec-tric circuit. Resonant frequency of LC circuit can be deter-mined by measurement of spectrum analyzer in advance.The capacitance bridge should be slightly adjusted to detect


Fig. 11. Conceptual diagram of impedance probe calibration by using ceramic condensers. For this calibration, dummy antenna should be prepared tosimulate plasma observation.

Fig. 12. Example of translation characteristic of impedance probe developed for DELTA campaign. The BeCu antenna is equivalent to 11 pFcapacitance.

the resonant frequency of LC circuit as correct as possible.To avoid any environmental noise as well as electrical cou-pling from the outside, the LC resonant circuit is shieldedby aluminum box. The resonant frequencies of LC circuits

will be designed to cover the wide range of swept frequencyas much as possible. Figure 13 and Table 1 represent blockdiagram and summary of calibration by using LC circuits(for DELTA campaign), respectively.


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 bythermoplastic materials or silicon bonds. The temperature sensor IC was represented in the center of this panel. This IC is only for environmentaltests (non-flight item).

2.3 Environmental tests and integration testTo evaluate environmental durability, several tests are

carried out by using vibration and impact testing ma-chine, thermostatic bath and space science chamber. InJapan, ISAS/JAXA requires payload instruments of sound-ing rockets and satellites to pass the environmental criteriato avoid any failures during its flight. Some private-sectorfacilities can operate the vibration and impact testing ma-chine 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 sub-strate temperature should be monitored during the test, ad-ditional temperature sensor (for example, LM35DZ) is tem-porary attached on the printed board. Figure 13 shows theexternal appearance of printed board which is prepared forenvironmental test. Also, ISAS/JAXA provides space sci-ence chamber as sharing system for the operation test invacuum or ionized gas.

Integration and spin timer test (that are conducted by


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 preparationfor DELTA campaign).

LC circuit No. UHR detection Error

(Resonant freq.)

(1) (5.787 MHz) 5.785 MHz (− 2 kHz)

(5) (2.198 MHz) 2.203 MHz (+ 5 kHz)

(6) (1.069 MHz) 1.079 MHz (+10 kHz)

ISAS/JAXA) are also important to confirm compatibilitywith other instruments and simulate flight condition. Inthese tests, it is possible to make sure of antenna extensionwith appropriate length, in applicable direction, at exacttime sequence. It is necessary to have preparatory exerciseof removing non-flight item before flight operation.

3. Application of Impedance Probe to RocketCampaigns

3.1 Observations of mid-latitude sporadic-ESEEK-2 (Sporadic-E Experiment over Kyushu) cam-

paign was planned to investigate generation mechanism ofQP echoes (e.g., Yamamoto et al., 1991) associated withmid-latitude sporadic-E (Es) layer. Two rockets were pre-pared to observe electron density, electric field, electrontemperature, neutral wind velocity and other parameters.The outline of SEEK-2 campaign is excellently describedby Yamamoto et al. (2005). The impedance probes werealso installed in two rockets of S310-31 and -32 which be-long in typical sounding rocket series of S310 with singlestage. During the SEEK-2 campaign, the impedance probeinstrument called as NEI (Number density of Electrons byImpedance probe) as well as in recent Japanese rocket ex-periments. Dimensional data and external appearance ofimpedance probe for SEEK-2 campaign were representedin Table 2 and Fig. 15, respectively. Because the maximumelectron density of mid-latitude Es layer sometimes reaches5 × 106 cm−3, measurable frequency range should be up to20 MHz. This is the first application of DDS and high-speed differential amplifier IC for rocket experiment as de-

Table 2. Dimensional data of impedance probe on-board S310-31.

Size/Weight (circuit) 140 × 140× 30 mm/0.65 kg

Size/Weight (sensor) 70 × 30× 25 mm/0.23 kg

Electrical requirements ±18 V, 250 mA

Frequency sweep range 300 k–21.3 MHz

Frequency step 10 kHz @ 300 k–3.8 MHz

20 kHz @ 3.8 M–21.3 MHz

Electron density coverage 103 − 5.6 × 106 cm−3

Sweep time 500 ms

Micro controller PIC16F877

Direct digital synthesizer AD9850BRS

Buffer amp. and

differential amp. IC CLC420AJE

Telemeter bitrate 204.8/102.4 kbps

(for NEI: 1600 samples/sec)

scribed in Section 1. Associated with these improvements,impedance probe sensor was separated from the electric cir-cuit, and it was near the top of the payload section as shownin Fig. 16. This improvement is expected to avoid the wakeeffects during the rocket flights. The rocket experimentwas carried out at Uchinoura Space Center (USC; 31.15◦N,13.04◦E) on 3 August 2002 with successive launches of theS310-31 and -32 rockets at 23:24 and 23:39 JST, respec-tively. During the rocket flights, two impedance probes ob-served the electron density profiles with distinct sporadic-Esignature. Figure 17 represents the electron density profilesobtained on-board S310-31. Rocket wake effect was notseen in the profile obtained during the SEEK-2 campaign.

Observed electron density was used to estimate the spa-tial structure of Es layers which is described in the paperof Wakabayashi et al. (2005). In addition, accurate and si-multaneous observation by impedance probe without wakeeffect provided a clue to point out the difference betweenupper thin layer and lower thick layer as described in Wak-abayashi and Ono (2005).


Fig. 16. Antenna extension phase of S310-31 rocket. The sensor was installed near the top of the payload section. This improvement was expected toavoid 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 altitudeof 100 km (after Wakabayashi et al., 2006).

3.2 Observations of electron density during diffuse au-roral event

The DELTA (Dynamics and Energetics of the LowerThermosphere in Aurora) campaign aimed to obtain accu-rate heating rates and to compare quantitatively with the at-mosphere temperature and wind. The DELTA campaign in-volved the S310-35 sounding rocket experiment to measurethe neutral temperature, electron density, electron tempera-

ture, auroral particle fluxes and emissions. To realize the ac-curate measurement of the neutral temperature, the electronbeam fluorescence method has been developed on-board asounding rocket (Kurihara, 2003). The impedance probewas also installed to obtain absolute electron density on therocket trajectory. Table 3 shows the summary of payloadinstruments. This experiment is characterized by its func-tion of electron beam injection by NTV (N2 temperature of


Fig. 18. External appearance of impedance probe on-board S310-35 forDELTA campaign. The co-axis cable was shorter than that of SEEK-2campaign because the antenna was installed near its electrical circuitunlike for the SEEK-2 campaign.

Vibration) instrument to measure the N2 gas temperature.To avoid any potential changes due to electron beam injec-tion with its energy of 1 keV, S310-35 was equipped with arocket separation mechanism. The impedance probe was in-stalled in motor side of the rocket (in this campaign, the mo-tor side was called as “mother rocket”). Dimensional dataand external appearance of impedance probe for DELTAcampaign were shown in Table 4 and Fig. 18, respectively.The target of this campaign was diffuse auroral event whichwas not expected with intense and local ionization. There-fore, swept frequency range was up to 10.3 MHz for thisexperiment.

Figure 19 shows the capacitance bridge circuit ofimpedance probe on-board S310-35. To protect the capaci-tance bridge circuit from the possible high-energy particlesinput due to the electron beam injection as well as intenseauroral precipitation, antenna was connected to the elec-tric ground level of the rocket body with a 10 M� resistor,while previous experiments used the floating probe condi-tion. This improvement is only for the protection of bridgecircuit which detects the equivalent capacitance of antenna.Since the inserted resisters do not have frequency response,they do not prevent from accurate adjustment of capacitancebridge circuit. In addition, capacitance bridge consisted ofthree mica condensers with its high break down voltageendurance of 500 V and a 20 pF trimmer condenser (it isnoted that ordinary laminated ceramic capacitor is enoughfor bridge circuit if there is no possibility of high-voltage in-put). Since it was also necessary to protect the buffer ampli-fier ICs against high voltage input, protection diodes wereadded to the input terminal of amplifier. As described inSection 1, this is the first case of impedance probe with log-arithmic amplifier application. Therefore, the capacitancebridge should be covered with the electrostatic shield to re-duce noises and to make it easy to adjust the capacitancebridge. The close-up of the capacitance bridge is given inFig. 20.

For the DELTA campaign, sensor of impedance probeshould be designed to extend through the hole of rocketbody because electric circuit and sensor of impedance probe

Table 3. Summary of payload instruments on-board the S310-35 soundingrocket. Impedance probe was installed in sub-PI section.

PI section (Daughter rocket)

N2 temperature of vibration (NTV)

Auroral particle detector (APD)

Horizon sensor (HOS)

Constant biased Langmuir probe (CLP)

CI section

S-band PCM telemetry system (S-PCM-TM)

Programmable timer (EPT)

S-band antenna (ANT-SMT)

Geomagnetic attitude sensor (GA2S)

CI battery (CI-BAT)

CI power supply controller (CI-PSC)

Motor pressure sensor (Pc)

Sub-PI section (Mother rocket)

Fast Langmuir probe (FLP)

Auroral green line photometer (AGL)

Surface finder (SFF)

Number density of electrons

by impedance probe (NEI)

Small telemetry system (SMT)

Sub-PI power supply (SUBPI-BAT)

SMT geomagnetic attitude sensor (SMT-GA)

Table 4. Dimensional data of impedance probe on-board S310-35. This isthe first time to application of logarithmic amplifier.



Electrical requirements +18 V, 250 mA

Frequency sweep range 300 k–10.3 MHz

Frequency step 10 kHz @ 300 k–4.3 MHz

20 kHz @ 4.3 M–10.3 MHz


Sweep time 515 ms

Micro controller PIC18F452


Buffer amp. IC AD8065AR

Differential amp. IC AD8130AR

Logarithmic amp. IC AD8307AR

Telemeter bitrate 204.8 kbps


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 extensionphase of S310-35 is represented in Fig. 22 during spin timertest at ISAS/JAXA (this test was conducted except rocketseparation 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 issuesof Abe et al. (2006). Altitude profiles obtained by usingimpedance probe are shown in Fig. 23. There are data gapsin ascending phase of S310-35 due to the effect of elec-tron beam injection. However, detail analysis of impedanceprobe made it possible to complement the gaps as shownin Wakabayashi and Ono (2006). The electron density pro-file contributed to evaluate the effect of Jeule heating during


Fig. 19. Capacitance bridge circuit designed for DELTA campaign. In this configuration, C1 and C2 are mica condensers with their capacitances of 100and 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 electrostaticshield). 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 pro-cess between the ionospheric plasma and the thermosphericneutral wind through a direct observation using the S520-23 sounding rocket. In the WIND campaign, a lithium re-lease experiment was conducted in the descending phase toestimate the thermospheric neutral wind by observing themotion of the released lithium cloud from the ground. This

campaign also included optical observation of cumulonim-bus and sea by using on-board camera to verify new multi-band observation technique.

This experiment also involved electron density obser-vation by using impedance probe technique on-board therocket. This case is characterized by combination withplasma wave receiver. Also, the instrument was called asPWM (Plasma Wave Monitor) which is developed as inte-gration of impedance probe and receiver. Figure 24 shows


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 straightahead.

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 asynchronizing signal to slave one of plasma wave receiver.This configuration was proposed to avoid interference dueto local signals of both instruments. This is good exam-ple of impedance probe to enhance the compatibility withcoadjacent passive receiver. Table 5 shows the dimensionaldata of impedance probe for WIND campaign. In this case,impedance probe was relatively larger and heavier becausetwo kinds of instrument were integrated as one. The altitudeprofile obtained during the WIND campaign is represented

and discussed in the issue of Uemoto et al. (2010).

4. Future Works4.1 Automatic detection of UHR frequency

In the missions in 1970’s, automatic detection system ofUHR frequency was needed because the limit of the capac-ity of telemetry speed as 64 bits/s (Ejiri et al., 1973; Oyaand Morioka, 1975). Ejiri (1973) reported the establish-ment of automatic UHR detection system to reduce the dataamount by using the derivation of equivalent capacitanceof conductor probe. However, there were miss detections


Fig. 23. Altitude profiles of electron density obtained by using impedance probe during the DELTA campaign. In ascending phase, there were data gapsdue to electron beam injection. Detail data analysis of impedance probe can estimate the electron density even in data gaps (described in Wakabayashiand 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 noisesor effects of electrostatic waves also give a condition to bedetected as UHR frequency. After the application on-boardthe TAIYO satellite, there has been no trial of the automaticUHR detection on-board the satellite.

In recent years, automatic and high-speed UHR detec-

tion is desired again not only to reduce data amount butalso to observe the fine structure of ionospheric plasma byusing sounding rocket. The key point of this improvementis application of phase detection circuit; namely, PLL cir-cuit. This application will make it possible to improve spa-tial resolution of impedance probe observation enough to


Table 5. Dimensional data of impedance probe for S520-23 is summarizedin this table. This impedance probe was integrated with plasma wavereceiver.



Electrical requirements +18 V, 500 mA

Frequency sweep range 300 k–12 MHz

Frequency step 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


Sweep time 502.3 ms

Micro controller PIC18F452QFP


Buffer amp. IC AD8065AR

Differential amp. IC AD8130AR

Logarithmic amp. IC AD8307

Telemeter bitrate 204.8 kbps


detect the fine structure of ionospheric plasma. In addi-tion, impedance probe with this improvement will obtainUHR frequency successively even if telemetry speed is rel-atively slow. The phase detection type is expected furtherprogress of impedance probe installation on-board satel-lites and planetary exploration spacecrafts. To increase thechance of installation, it is necessary to make the circuitlighter and smaller. As reported by Suzuki (2010), applica-tion of FPGA device will provide considerable progress indownsizing of impedance probe to be standard instrumentof spacecrafts in the future.

Acknowledgments. Impedance probe instruments described inthis issue were produced by System Keisoku Inc. The envi-ronmental experiments for carrying out the rocket campaignswere conducted and supported by the Space Plasma Labora-tory, ISAS/JAXA. The rocket experiments were conducted byISAS/JAXA rocket team. The authors would like to show theirspecial thanks to all the persons concerned and professor Koh-Ichiro Oyama.

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Uemoto, J., T. Ono, T. Yamada, T. Suzuki, M.-Y. Yamamoto, S. Watanabe,A. Kumamoto, M. Iizima, Impact of lithium releases on ionosphericelectron density observed by impedance probe during WIND campaign,Earth Planets Space, 62, 589–597, 2010.

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