Ionospheric electron temperature and density by cylindrical electrostatic probes aboard Explorer 31 and Alouette 2 satellite

Brace, L. H.

Findlay, J. A.

English

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1 X -6 2 3 - 6 8 - 2 81 
PRE PR I N T  
w 
\ 
CYLINDRICAL ELECTROSTATIC PROBES 
EMPLOYED ON ALOUETTE II AND 
' EXPLORER 31 $ATEtLlTES 
, 
J. A. FINDLAY 
1. H. BRACE 
https://ntrs.nasa.gov/search.jsp?R=19680026299 2020-03-23T22:09:27+00:00Z
X-623-68-281 
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CYLINDRICAL ELECTROSTATIC PROBES EMPLOYED 
ON ALOUETTE 11 AND EXPLORER 31 SATELLITES 
J. A.  Findlay 
L. H. Brace 
July 1968 
GODDARD SPACE FLIGHT CENTER 
Greenbelt, Maryland 
CYLINDRICAL ELECTROSTATIC PROBES EMPLOYED 
ON ALOUETTE II AND EXPLORER 31 SATELLITES 
J. A. Findlay 
L. H. Brace 
ABSTRACT 
The Cylindrical Electrostatic Probe experiment, employed on both the 
Alouette 11 and the Explorer 31 satellites, pro-ided a common denominator for 
inter-satellite comparisons of directly measured parameters. The instruments 
used identical sensors, but differences in satellites resulted in some differences 
in data formats and experiment potentials. There w a s  considerable interference 
from the topside sounder seen on the probe data from Alouette 11 which made 
data processing difficult, though not impossible. 
CONTENTS 
Page 
ABSTRACT ....................................................... iii 
INTRODUCTION ................................................... 1 
THE EXPERIMENTS.. ............................................. 2 
Background ................................................... 2 
Theory ....................................................... 2 
Vehicle Potential .............................................. 3 
Electron Temperature .......................................... 3 
ElectronDensity ............................................... 4 
Range of Measurements ........................................ 4 
The Sensors ................................................... 5 
Electronics ................................................... 6 
8 SOUNDER EXPERIMENT INTER FERENC E .......................... 
ACKNOWLEDGMENTS ............................................. 9 
REFERENCES ..................................................... 10 
V 
CYLINDRICAL ELECTROSTATIC PROBES EMPLOYED 
ON ALOUETTE I1 AND EXPLORER 31 SATELLITES 
INTRODUCTION 
Of the experiments aboard the ISIS-X satellites the one common to both 
Alouette I1 and Explorer 31 was  the  Cylindrical Electrostatic Probe (CEP). 
This Langmuir-type probe was employed to make direct measurements of 
electron temperature (T,) and density (Ne) in the ionosphere at the satellites. 
There were several reasons for including the CEP in both payloads: 
1. To study the feasibility of making direct ionospheric measurements 
and topside soundings from a single satellite such as the follow-on satellites 
ISIS-A and ISIS-B. (Franklin et al., 1968.) 
2. To provide a common denominator, when the two satellites are near each 
other, for inter-satellite data studies using the topside sounder aboard the 
Alouette I1 and the various direct measurement experiments aboard the Ex- 
plorer 31 as reported in a companion paper by Donley et al. (1968). 
3. To provide continuous direct measurement of Te and Ne on the Alouette I1 
for use with the Ne profiles derived from the sounder data. 
4. To permit study of the Alouette II satellite potential behavior. 
To best accomplish the above goals the instrumentation for the two experi- 
ments was designed as nearly the same as practical. What differences there 
were  resulted from basic differences in the two spacecraft such as external 
config!r2t,tiQny in-orbit nrient2ttinn anrl telemetry fc?rm2tts 
The resultant differences in instruments included sensor location, sensor voltages, 
sampling sequences and rates. In a companion paper (1968), Brace and Findlay, 
1 
employing data from the two satellites when close together, find no significant 
differences in  Ne or  Te . 
THE EXPERIMENTS 
Background 
The cylindrical probe technique consists of measuring the current of free 
electrons and ions to the cylindrical collectors, which a re  extended into the 
plasma surrounding the spacecraft, as a known regular sawtooth voltage is 
programmed. These currents a r e  converted to signals suitable for telem- 
etry and transmitted to the ground. 
The CEP has been flown in similar configurations on several previous 
satellite and rocket programs (Spencer et al., 1962, Spencer e t  al., 1965, 
Brace et al., 1965, Brace, Reddy 1965, Reddy et al., 1967). The theoretical 
basis for the experiment was established by Mott-Smith and Langmuir (1926). 
The theory was extended to ionospheric application by others including Boggess 
et al., (1959) and Spencer e t  al., (1962). In view of the previous coverage only 
a brief review of the general aspects is given here along with those aspects 
which a re  peculiar to the ISIS mission, 
Theory 
A predicted current vs. voltage relation for the experiment is shown in 
Figure 1. The volt-ampere characteristic has three distinct regions, two of 
which a r e  predominantly electron current and one predominantly ion current. 
Although much information is contained in the ion saturation region, the ISIS-X 
experiment was not designed toward ion current studies and this paper will 
deal only with the electron current. 
2 
Vehicle Potential 
i .  
The satellite potential is just that applied voltage required to drive the 
probe to the plasma potential (V = 0 in Figure 1). This is identified as the point 
of inflection on the volt-ampere curve between the electron retardation and 
electron saturation regions. 
Electron Temperature 
The expression for electron current in the electron retardation region is 
I e  = ANe e(kTe/2rme)1’2 exp - (eV/kTe) (1) 
where 
A is the collector area 
Ne is the electron density 
e is the electron charge, 1.602 x lo-’’ coulomb 
k is Boltzmann’s constant, 1.38 x 
Te is the electron temperature 
me is the electron mass, 9.11 x 
V is the retarding potential of the collector. 
joule/”K 
Kg 
Taking the logarithm of both sides and assuming the values of Ne and T, re- 
main constant over a voltage sweep we have 
In Ie  = Inconstant - eV/kTe . 
Differentiating both sides and solving for temperature gives 
3 
This relation is used to determine Te  from the data. Note that the determina- 
tion of Te is independent of Ne.  
Electron Density 
The third region of the volt-ampere curve is the electron saturation region 
where electrons a r e  accelerated to  the collector. To a very close approxima- 
tion the electron current to the collector is given by 
Note that this expression is nearly independent of Te. It has been shown by 
Spencer et al. (1965) that for sufficiently positive voltages the current to the 
collector can be approximated by the temperature independent expression 
from which Ne is readily obtained. 
Range of Measurement 
The amplifier sensitivities of the ISIS-X instruments were chosen with 
emphasis on providing resolution in the electron retardation and electron 
saturation regions for the densities expected in the ISIS-X orbit. This includes 
densities from about 6.5 x 10' electrons/cc (saturation on the 8 x ampere 
amplifier range) to about 100 electrons/cc (10% of full scale on the 2 x lo-*  
ampere range). Temperatures can be resolved for  densities greater than 
about 1 x l o 3  electrons/cc over the range of about 400°K to about 15,000°K, a 
much wider range than that actually observed. 
4 
The Sensors 
The cylindrical probes used on the two satellites a r e  identical (see Figure 
2) and a r e  operated independently. They consisted of a collector electrode 
0.057 cm diameter, 23 cm long, and a coaxially mounted guard electrode of 23 cm 
length and of 0.170 cm diameter. The guard electrode is electrically isolated 
from the collector but is driven at the same voltage. The guard provides con- 
tinuity in the electric field and separates the collector from the spacecraft thus 
reducing wake and sheath effects. An electrically floating conductor of 0.23 cm 
diameter and 21.2 cm long is mounted concentric to the guard electrode. This 
reduces the total current to the guard and thus increases the area ratio between 
satellite and collector. The guard and collector a r e  mounted to a screw-on type 
coaxial connector through coaxial helical springs which allowed the sensors to be 
stowed in a folded configuration for launch. 
The mounting locations of the two se ts  of probes on the two satellites and 
their orientation in space is shown in Figure 3. The Alouette II probes protrude 
at a 45" angle to the spin axis on opposite sides of the satellite. The Explorer 31 
probes a r e  90" to the spin axis, also on opposite sides and near one end of the 
satellite. The Alouette II spin axis remains generally in the orbital plane pointing 
approximately north while the Explorer 31 spin axis is maintained approximately 
normal to the orbital plane by a spin axis control system (Nelson, 1968). The re- 
sult is that the data from each probe on Explorer 31 is modulated by satellite 
wake effects relatively uniformly throughout the orbit while that from 
Nouette E is mod.dat.tzd most in the middle a d  high l2ttitEdPS and rel2tlively 
little in the lower latitudes. 
passes where the probes a r e  generally trailing the spacecraft than on 
Also, the modulation is greater on northbound 
5 
8 
southbound passes where they generally lead. These modulations can be as 
great as 90% under extreme conditions such as near perigee where the satellite 
velocity greatly exceeds the ion thermal velocity, but is usually much less. 
The wake data a r e  not used in determining electron temperature or density 
however, and in a companion paper Brace and Findlay (1968) show that the 
accuracy of the measurement is not significantly affected by either the spin 
modulation or the presence of the booms. Spatial resolution is maintained by 
using two collectors generally on opposite sides of the spacecraft so that at any 
time there will be at least one probe taking non wake data. 
Electronics 
It follows from the above discussion that to make good measurements two 
requirements must be met. First, the probe must be driven sufficiently 
positive and negative with respect to the plasma to  cover the three regions of the 
volt-ampere characteristics; and second, each voltage sweep must be taken in 
sufficiently short time that the spin modulation does not affect the shape of the 
characteristic, but not so short a time that the telemetry cannot adequately follow. 
These considerations resulted in the remaining differences between the two 
experiments. 
Regarding the first  consideration, that of the amplitude of the applied voltage, 
one of the unknown aspects of the Alouette I1 spacecraft was the potential the 
main body would assume. Two complications to predicting this potential were: 
(1) The long sounder booms across  which a time varying voltage would be induced, 
d 
E = i i x f i . l  
6 
where 
v is the satellite velocity, about 7.5 km/sec 
B is the Earth's magnetic field 
1 is the length of the sounder, 73.15 meters and 22.86 meters for the long and 
short dipoles respectively. 
A 
(This would be a time varying voltage at twice the satellite spin rate); and 
(2) a negative biasing of the spacecraft into the ion saturation region by the 
presence of the positive exposed conductors on the solar array.  These con- 
ductors being driven a fixed positive voltage with respect to the satellite would 
tend to collect electrons beyond the ability of the remaining spacecraft surface 
to collect positive ions. Thus the potential of both would then adjust with re- 
spect to  the plasma to maintain charge balance. 
Although countermeasures were taken for each of these problems, specifi- 
cally, electrical isolation of the booms with capacitive coupling for sounding (see 
companion paper by Franklin) and coating the exposed conductors on the solar 
a r ray  with an insulating material (see companion paper by Florida), it was felt 
necessary to provide a sufficiently large applied voltage sweep to cover the possi- 
bility of failure in either case. This required an applied voltage sweep of -3v 
to +10.5v on Alouette 11 as opposed to - 1 . 7 ~  to +5.lv on Explorer 31 which had 
neither of these potential problems. 
The second basic consideration, the time over which a sample is taken, is 
controlled by the amplitude of the applied voltage sweep as discussed above and 
the maximum rate of change of applied voltage for which tne amplifier response 
time and telemetry resolution are adequate. On Explorer 31 the PCM telemetry 
allocated to the experiment limited the applied voltage rate to 5.4 volts per second, 
7 
while on Alouette 11 the F M  system allowed a rate of 17.5 volts per second. The 
resultant sweep periods were  1.26 and 0.700 seconds respectively. 
The other significant difference between the two experiments resulted from 
the number of telemetry channels available on the two spacecraft. On Explorer 31 
the experiment was provided two data channels so  that both probes could be 
monitored continuously. The Alouette I1 telemetry system provided only one 
data channel, thus the two probes were sampled alternately for one sweep period 
each. Current range switching was synchronized to the sounder sweep system 
providing range switching about every 27 seconds. 
of the amplifiers w a s  provided by periodically substituting a known resistance 
in place of the probes. A functional block diagram of the two experiments is 
given in Figure 4. Note that identical current sensitivities were  used in the 
two systems. Typical r aw data from the experimentsis shown in Figure 5.  
In-flight calibration 
SOUNDER EXPERIMENT INTERFERENCE 
The greatest problem in analyzing data from aboard the Alouette I1 satellite 
was  not entirely anticipated. It was expected that the 400 watt transmitter 
would generate electromagnetic interference signals, and in fact some filtering 
was provided within the sensor mounting boxes. It appeared from ground based 
tests that the remaining noise would not interfere with data reduction. In orbit 
however, the nature of interference was drastically different. Through some 
combination of plasma disturbance and instantaneous potential shifting of either 
the spacecraft o r  collectors o r  both there is observed on the probe data a spike, 
o r  pulse, each time the sounder pulses. The width and amplitude of the spike 
vary as a function of sounder frequency and plasma density from negligible to 
8 
a point where reduction of the data is impossible. The level of disruption is 
usually greatest when the sounder frequency is near the plasma frequency. 
Figure 6 shows an example of the variation of spikes through one sounder sweep 
on a typical data run. Fortunately there is a 4 second period preceeding each 
sounder sweep when the sounder power amplifiers a r e  turned off, providing com- 
pletely undisturbed data. It is this data which is used for bulk analysis. Com- 
parisons between undisturbed and moderately disturbed data show no effect on 
the amplitude of the electron saturation region; therefore, the Ne measurements 
are unaffected. We have also found that even under conditions of moderate inter- 
ference the spikes are sufficiently short that evaluation of Te , though impractical 
on a large scale basis, can be carried out by hand with no significant loss in ac- 
curacy. A more complete comparison and evaluation of data taken from the two 
spacecraft is included in a companion paper by Brace and Findlay (1968). 
To minimize the sounder interference on the follow-on ISIS-A satellite which 
uses a PCM telemetry system synchronized to the sounder pulses, the telemetry 
format w a s  modified so that direct ionospheric data samples wil l  be taken as 
long after a sounder pulse as possible. It is the hope that under most conditions 
both the electromagnetic and ionospheric plasma spikes wi l l  have sufficient time 
to damp out in the interval between pulse and direct sample. 
ACKNOWLEDGMENTS 
Design and fabrication of the instruments w a s  by Tuck Bin Lee at the Uni- 
versity of Michigan. We wish to thank Ernest Busboso for his efforts during 
integration, test and launch operations. 
9 
REFERENCES 
1.  Boggess, R .  L., L. H. Brace, and N. W. Spencer, "Langmuir Probe Measure- 
ments in the Ionosphere ,?? J. Geophys. Res., 64 -, 1627, 1959. 
2 .  Brace , L. H., and J. A. Findlay, "Comparison of Cylindrical Electrostatic 
Probe Measurements on Alouette I1 and Explorer 31 Satellites" (in this issue). 
3. Brace, L. H., and B. M. Reddy, "Early Electrostatic Probe Results from 
Explorer XXII," J. Geophys. Res., - 70,  5783-5792, 1965. 
4. Brace, L. H.,  N. W. Spencer, and A. Dalgarno, "Detailed Behavior of the 
Midlatitude Ionosphere from Explorer XVII Satellite ," Planet. Space Sci. , 
- 13,  647-666, 1965. 
5 .  Donley, J. L., L. H. Brace, J. A .  Findlay, J. H. Hoffman, and G. L. Wrenn, 
"Comparison of Results of Explorer X X X I  Direct Measurement Probes" (in 
this issue). 
6.  Florida, C. D., "The Alouette I1 Satellite" (in this issue). 
7 .  Franklin, C. A., "The Alouette Topside Sounder" (in this issue). 
8 .  Nelson, E. D., "The Explorer 31 Direct Measurement Satellite" (in this issue). 
9 .  Mott-Smith, H. M., and I. Langmuir, "The Theory of Collectors in Gaseous 
Discharges,'' Phys. Rev., 28,  727-763, 1926. 
10. Reddy, B. M., L. H. Brace, and J. A. Findlay, "The Ionosphere at 640 Kilo- 
- 
meters on Quiet and Disturbed Days ,?' J. Geophys. Rea. , - 72,  2709-2727, 1967. 
11. Spencer, N. W.,  L. H. Brace, and G. R .  Carignan, "Electron Temperature 
Evidence for Nonthermal Equilibrium in the Ionosphere," J. Geophys. Res. , 
- 67, 157-175,1962. 
12. Spencer, N.  W . ,  L. H. Brace, G. R. Carignan, D. R. Taeusch, and H. Niemann, 
I'Electron and Molecular Nitrogen Temperature and Density in the Thermo- 
sphere ,I' J. Geophys. Res. , 70,  2665-2698 , 1965. - 
10 
LIST OF FIGURES 
Figure 
1 Predicted current characteristic of small cylindrical probe. The plasma 
potential (V = 0) is identified as the inflection point. 
2 Cylindrical Probe. Visible elements include collector electrode, guard 
electrode, floating electrode, coaxial helical spring and coaxial connector. 
Mounting position and in-orbit orientation of cylindrical probes on 
Alouette I1 and Explorer 31. 
Functional block diagrams of Alouette I1 and Explorer 31 cylindrical 
probe experiments. 
Typical cylindrical probe data from Alouette I1 and Explorer 31. Dif- 
ferences in data format resulted from basic differences in spacecraft. 
Modulation seen on current amplitude represents satellite wake effect. 
Alouette I1 cylindrical probe data demonstrating typical interference 
spikes from sounder experiment. Curve showing maximum spike 
effect was  taken with sounder pulsing near plasma frequency. 
3 
4 
5 
6 
Z 
0 
E 
I- Y 
d 
W '  
Z 
0 
I- 
LT 
3 
- 
a 
L 
Z 
0 
N + 
4 + 
4 
I 
N 
I 
11 
12 
s a 
-0 e 
c 
U 
0 
a 
-
m 
C 
0 
0 
.- c 
-
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0 n e a 
!! 
m 
3 
13 
Y Y Y \ 
I _ _ -  - 42- -- - -  
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I, 
I------ L---l 
w w  
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0)  
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a, 
a, n : - 
0 
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+ c
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Y 
0 
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0 
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0 .- 
t U
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2 
LL 
LL 
14 
1.0 p Amp 
RANGE 
CALIBRATE 
1.0 p Amp 
RANGE 
DATA 
f- 
I 8.0 p Amp 8.0 p Amp I 
RANGE RANGE i 
CALIBRATE DATA 
/-/,-/d ----, u
ALOU ETTE - I I 
U 
EXPLORER 31 
CYLINDRICAL PROBE IONOSPHERIC DATA 
Figure 5. Differences in data 
format resulted from basic differences in spacecraft. Modulation seen on current amplitude repre- 
sents satellite wake effect. 
Typical cylindrical probe data from Alouette I I  and Explorer 31. 
15 
A LOU ETTE - I I 
CYLINDRICAL 
ELECTROSTATIC PROBE 
+ -  
- 6- 
~ _ _  
1L 
I 
Figure 6. Alouette II cylindrical probe data demonstrating typical interference spikes from sounder 
experiment. Curve showing maximum spike effect was taken with sounder pulsing near plasma 
frequency. 
16 


Cylindrical electrostatic probes employed on Alouette 2 and Explorer 31 satellites

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