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Vehicle glow observed during a rocket sounding experiment
VEHICLE
GLOW OBSERVED DURING SOUNDING EXPERIMENT
A ROCKET
B. R. CLEiMESHA, H. TAKAHASHI and Y. SAHAl Instituto de Pesquisas Espaciais-INPE, Ministerio de Ciencia e Technologia--MCT, 12201~S.J. dos Campos, S.P., Brazil
Abstract-In a recent rocket-borne photometer experiment strong contaminating signais were observed at 5577 and 7619 A. The contamination was observed in the ram direction on both upleg and downleg trajectories and no contamination was observed in the wake direction. A strongly spin-modulated signal was observed at heights up to 88 km on the upleg and at heights below 78 km, where the rocket inverted, on the downleg. The signals at both wavelengths were observed to decrease exponentially with increasing height, but with different e-folding intervals, the latter being I.23 km for the OI 5577 A emission and 2.65 km for the 0,7619 A. At a height of 81.5 km the intensities were the same and were equal to 220 Rayleighs per Angstrom (R A-‘). It is suggested that the contamination glow most probably has its origin in reactions involving atomic species produced by thermal dissociation in the shock wave in front of the photometers. 1.
INTRODUCTION
In recent years considerable interest has developed in vesicle-injured airglow. This interest stems from the discovery that surfaces of the Space Shuttle facing the ram direction are clothed in a light-emitting layer some tens of centimetres thick, an effect which has important implications for optical experiments based on the Shuttle platform (see, for example, Torr, 1983). What appears to be an entirely similar effect has already been observed by the AE-C satellite (Yee and Abreu, 1983). Rocket-borne photometer experiments designed to measure atmospheric airglow emissions have observed vehicle-induced glow with a wide range of characteristics (see, for example, the work referenced by Torr, 1983). The purpose of this report is to present the results of a recent observation of vehicle glow having characteristics appreciably different to those previously observed in rocket experiments.
2. OBSERVATIONS The vehicle glow reported in this paper was observed during a dual purpose experiment designed to measure the vertical distributions of the 01 5577 A and O2 atmospheric 7619 b; tropical airglow emissions in the lower thermosphere and the vertical distribution of electron density through an F-region bubble. The rocket, a SOFIA III vehicle, developed by the Brazilian Air Force Institute of Space Activities (TAE), carried a payload developed by the Institute of Space Research (INPE) consisting of two deployable airglow photometers and two plasma probes, and reached an apogee of 516 km. The launch was carried
out from the Barreira de Inferno Rocket Range at Natal, Northern Brazil. Both experiments were successful, and their results are being published elsewhere (Takahashi et af., 1986). The configuration of the payload can be seen from Fig. 1. The rather unusual design of the photometric part of the payload was dictated by the fact that an ejectable nose cone was not available. The two photometers, themselves of conventional design, with a rotating disc device which performed in-flight calibration and dark current checks, were deployed by a pneumatic system at a height of 76 km. The main characteristics of the photometers are given in Table 1 and the launch parameters are given in Table 2. During the deployment of the photometers the signals from both instruments increased rapidly to values many times. the expected airglow levels. Immediately after the point at which the instruments were fully deployed, the photometer signals, which were modulated at the spin rate of the rocket, started to decay. In Fig. 2 we show the time variation of the two photometer signals together with the transverse magnetometer output for part of the upleg. As can be seen from the figure, the 01 5577 A contamination signal becomes negligible only above 88 km. The signal from the 7619 8, instrument was lost at X2.0 km and not recovered until the vehicle had reached 91 S km, by which latter height the contaminating signal was no longer present, so we cannot determine the height at which it decayed to zero. It is interesting to observe that the modulation of the signals from the two photometers bears a constant phase relationship to the magnetometer signal, and that the photometer signals are 180” out of phase with each other, showing that, in
1367
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B. R. CLEMESHA et al. 02
7619 i
PHOTOMETER
_--FOV
----__
_---HALF
ANGLE ---_
-----____ MAGNETOMETER
A%ES
FIG. 1. PAYLOAD CONFIGURATION.
TABLE
Emission 015517 A 027619 A
Optical diameter
1. CHARACTEKISTICSOF PHOTOMETERS FOV half-angle
46 mm 46 mm
4” 4”
Filter peak 5579 A 7620 A
Filter half width 10.4 8, 59.0 8,
PMT 9924NB 9798NA
TABLE 2. LAUNCH PARAMETERS Vehicle : SONDA III two-stage solid fuel rocket Launch: 20:30 Brazilian Standard Time. 23:30 G.M.T., December 1985 Elevation angle on launcher : 8 1.4” Second stage burnout : 44 s at 52 km Photometer deployment : 52.7 s at 76 km Apogee : 5 16 km Downrange impact distance : 483 km
11
terms of angle, the source of the signal is not fixed in relation to the vehicle, but appears to come from a given direction in inertial space. From the phase lag between the contamination signal and the magnetometer ouput, we are able to determine that peak signal was always produced when the photometer was on the lower side of the inclined trajectory. The phase relationships can be seen most clearly in Fig. 3, where we plot a short section of the upleg data on an expanded time scale. On the downleg part of the flight the contaminating signal was again seen, but this time only at heights below 78 km. In Fig. 4 we show part of the downleg data for the 5577 A photometer and both the transverse and longitudinal magnetometers. Note that the rocket trajectory was chosen so as to be as nearly as possible orthogonal to the Earth’s magnetic field, and that the dip angle at Natal is only about 12”. No downleg data is available for the 7619 A instrument because, during the flight, an increase in the temperature of the PMT used in this instrument caused the noise level to rise to such a point as to saturate
HEIGHT
(KM)
FIG.~.UPLEGPHOTOM!ZTEZR ANDTRANSVERSEMAGNETOMETER SIGNALS.
the electronics. The vehicle attitude started to change rapidly after passing through the 01 5577 A emission layer, and became horizontal at about 78 km. Below this height the photometers pointed downwards, and it appears from the magnetometer outputs that the rocket attitude involved a coning angle of the order of 45”. The spin-modulated contamination signal reappeared on the downleg shortly after the rocket inverted, and continued until telemetry was lost at about 60 km. In Figs 2,3 and 4 the photometer signals are plotted as photon counts per 4.8 ms sampling period, and no counting rate corrections have been applied. At the very high signal levels produced by the contamination at the lower heights, the apparent counting rate is
1369
Vehicle glow during rocket sounding experiment PHOTOMETER %
OUTPUT
7
O(
I
I
I
00.5
00
0i
81.5 HEIGHT
(KM)
FIG. 3. UPLEGDATASHOWINGPHASERELATIONSHIPS: (a)5577 .%SIGNAL; (b)76198, SIGNAL;(C)TRANSVERSEMAGNETOMETER
PHOTOMETER
OUTPUT
RANSVERSE MAGNETCMETER ~~vYvvlJyvYYvv~~
I
75
65
70
60
HEIGHT
(KM)
FIG.~.DOWNLEGDATASHOWING 5577A SIGNALANDTRANSVERSEAND LONGITUDINALMAGNETOMETEROUTPUTS
IEASURED
I
78
INTENSITY
I,
I
80
I
62
I,
I
84
I
pressed the signal intensity in terms of Rayleighs per Angstrom. Note that the values plotted in Fig. 5 are the peak contamination signals encountered during each rotation of the vehicle, with the plateau level airglow and PMT noise signals subtracted. In order to express the signals in Rayleighs per Angstrom we have divided the measured airglow intensities by the filter bandwidths, and in doing this we do, of course, assume a uniform continuum emission over the relevant part of the spectrum. We can see from Fig. 5 that the 015577 8, signal varies in a manner very close to exponential over a range of nearly four decades. At least over the 2 km height interval for which the 7619 8, data is available, this emission also appears to vary exponentially with height. Note that the data point at 79 km is probably in error. The very high counting rates encountered shortly after the photometers were deployed exceeded the capacity of the IO-bit telemetry word. In most cases it was easy to determine the overflow bits from the continuity of the data, but the very rapid fluctuations in the 7619 8, signal at around 79 km made it impossible to estimate the overflow unambiguously. From the logarithmic plots of Fig. 5 we can estimate the vertical scale heights with which the two emissions fall off, resulting in values of 1.23 and 2.65 km for the 5577 and 7619 8, emissions, respectively. The two intensities were the same, and equal to 220 R 8, ’ at a height of 81.5 km. From the observations presented above it is possible to draw a number of conclusions about the behaviour of the observed contamination signal. The most important conclusion is that the glow is produced only in the ram direction. We base this conclusion on the fact that whereas the contaminating signal was observed at heights up to 87 km on the upleg part of the flight, on the downleg it was not observed until the rocket turned over at 78 km. This conclusion is also consistent with the spin-modulation of the signal. We believe this spin-modulation to have been produced by periodic shadowing of the photometers by the nose cone. This would occur if the vehicle axis were not parallel to its velocity vector, as illustrated in Fig. 6. Note that we use the term shadowing loosely
I
86
HEIGHT (KM 1 FIG. 5. CONTAMINATION SIGNAL INTENSITIESAS A FUNCTION OFHEIGHTFORTHEUPLEGDATA.
much less than the true counting rate because of the limited response of the PMT preamplifier. In Fig. 5. where we plot on a logarithmic scale the peak contamination signal as a function of height for the upleg data, we have corrected for this effect and have ex-
FIG.~.SHADOWINGOFPHOTOMETERBY
NOSECONE.
t370
B. R.
MAGNETIC
P
M
FIELD-
---l--1; (Cl
Ibl
FIG. 7. EXPECTED SIGNAL
+
--.fi
(CI
tdl
PHASE RELATIONSHIP BETWEEN PHCITO,METER
AND TRAF5VERSE GLOW
CLEMESHA
Mh~~~O~fE~ER
OUTPUT
ASSIJMlNG
IS PRODUCED IN THE KAM DIRECTION.
to reprcscnt the effects of the nose cone on the air flow ; the mean free path at the height in question is much less than the dimensions of the vehicle, so that the situation is more complicated than that encountered on the Space Shuttle, where the mean free path is large. The downleg signal variation is also consistent with the idea of shadowing by the no= cone. It can be seen from Fig. 4 that there is a progressive phase shift between the peaks in the photometer signal and the magnetometer output, with the former occurring progressively earlier. Furthermore, the magnetometer signals suggest that a change of phase by one complete cycle occurs during one complete coning cycle. As illustrated in Fig. 7, this is exactly the behaviour to be expected If the contamination glow is always produced when the pho tometer faces the ram direction and is extinguished when it is shadowed by the nose cone. On the other hand, it must be admitted that we are unable to completely reconcile the details of the relative variations in the magnetometer and photometer outputs with this mechanism. The main difficulty here is, ofcourse, the fact that we cannot unambiguously reconstruct the time history of the vehicle attitude from the magnetometer data. 3.
DISCUSSION
It cm be concluded from the observations reported above that the contamination signal is produced in the ram direction and not in the wake direction. This behaviour is quite different from that reported for the contamination glow observed in other rocket photometer experiments. Many workers (Heppner and iVfcredith, 1958 ; Wallis and Anger, 1968 ; Evans ei n& 1973) observe vehicle glow only in the wake direction,
et
d.
and where it has been seen in the ram direction as welt (Stegman et al.,1982; Lopez-Moreno ef at., 1985; Greer ~1~ri., 19X3; Ogawa ef al., 1987) it has invariably been found to be strongest in the wake direction. The height variation of the contaminating signal which we observe is also different to that seen by other workers. Greer et al. (1983), Lopez-Moreno et al. (1985) and Ogawa et al. (1987) all reporl maximum contamination from heights around 100 km, leading to the suggestion that atomic oxygen might play a role in its production. The fact that we observe no contamination above a height of 88 km rules out direct participation in the process by atmosphe~c atomic oxygen, although there remains the possibility of dissociation of 0; molecules being caused by the vehicle motion. Spin modulation ofcontaminating signals has been reported by Greer et al. (1983) and by Ogawa et al. (1987). The former workers observed spin modulation in the same phase for all photometers of a given payload, although the phase relationship between the signal modulation and the magnetometer output varied from payload to payload. Ogawa et sl. (1987) state that their contamination signal occurred over an angle of only a few degrees in the rotation of the rocket. Greer et ut. suggest that the roughly sinusoidal spin modulation which they observed was caused by a glow local to the rocket, but it is difficult to see what sort ofmechanism could have produced the very short pulses of contamination observed by Ogawa et al. If, as Grccr et cd. suggest, the contamination which they observed was local to the rocket, it is also difficult to understand why the spin modulation should have been in the same phase for all of their (forward looking) photometers. Most of the mechanisms suggested for vehicle glow refer to the Space Shuttle or the .~~~~~~~~r~~Expbzr satellites, for which the ambient conditions are very much different from those encountered by sounding rockets. Slanger (1953) has suggested the Meinef system of OH, with the excited OH being produced by reactions between ramming 0, with 5 eV kinetic energy, or 02, with 10 eV, and H,O or hydrocarbons adsorbed on the vehicle surface. Slanger points to the fact that, because ofthe S eV O2 bond strength, atomic and molecular oxygen are energetically equivalent in this context. In our case this equivalence does not hold, of course, because our vehicle velocity of 2.7 km s’ corresponds to only 1.14 eV of energy for molecular oxygen. Furthermore, the height variation of our con~m~natin~ signal is completely different to that which would be expected for a mechanism invotving atmospheric atomic oxygen. A similar argument applies to the mechanism suggested by Torr
Vehicle glow during rocket sounding experiment (1983), in which the NO, continuum is produced by excited NO, resulting from reactions between atomic oxygen and nitric oxide. Wind tunnel experiments by Kunkel and Hurlbut (1957), cited by Torr (1983) in the context of shuttle glow, have shown strong nitrogen glows both in the ram region and in the wake. These experiments seem to have little relevance either to shuttle glow or to our observations, not only because of the very different velocities and pressures involved, but more importantly, because the glows were only produced when the incoming gas stream was subject to a 5 kW electrical discharge, producing dissociation and ionization. Contamination glows observed in rocket experiments have frequently been ascribed to outgassing. O’Neil et al. (1979) for example, observed a luminescent wake which they suggest was the result of reactions between atomic oxygen and gases from the rocket motor. We cannot exclude the possibility that outgassing played a role in the production of the glow which we observed, although it is practically certain that atmospheric atomic oxygen was not involved. Considerable outgassing from our fiberglass nose cone material would be expected, and although the cone itself is outside the field of view of the photometers, ejected material could be carried into the field of view by the airstream. Against this possibility is the fact that we did not observe the contamination glow on the downleg until the rocket inverted, indicating that the glow which outgassing would be expected to produce in the wake was not present. From the above discussion we can conclude that none of the previously suggested mechanisms is particularly enticing as an explanation for our observed contamination glow, which exhibited characteristics very dilferent to the glow observed in other rocket experiments. Our experiment differed from the majority of rocket photometer experiments in two respects : firstly the vehicle velocity in the region of interest was much higher than is usual, about 2.7 km s-’ as compared with less than 1 km s-‘, and secondly there was a fiberglass nose cone ahead of the photometers. It seems probable that the glow was related to the high velocity, and the spin modulation was caused by shadowing of the photometers by the nose cone. A possible clue to the nature of the vehicle glow which we observe lies in the fact that its intensity decreases exponentially with height with an e-folding distance much less than that of the main atmospheric constituents, suggesting that a minor constituent might be involved. The principal minor constituents which may decrease rapidly in the relevant height range are the hydrogen compounds such as H20,
1371
Hz02, HO* and OH and, perhaps, ozone. With respect to the last named constituent it is interesting to note that the incoming ozone molecules will have kinetic energies around 1.7 eV, well above their binding energy of slightly more than 1 eV. Against the possibility of a minor constituent being involved in the production of the contamination glow is the remarkable uniformity of the observed decrease in the 5577 8, signal with height. The variation is very close to exponential over nearly four decades of intensity and nearly 10 km of height range ; none of the possible minor constituents would be expected to vary in such a uniform manner. In our experiment the mean kinetic energy of the incoming atmospheric molecules is sufficient neither to excite emissions in the visible region nor to produce dissociation of the principal atmospheric constituents. If, on the other hand, these molecules make a sufficient number of collisions in the shock front region ahead of the photometers, then their energies will tend to a Maxwell-Boltzmann distribution in which the r.m.s. molecular energy is just over 1 eV. In such a distribution, which corresponds to a temperature of about 4000 K, about one millionth of the molecules would have energies greater than the 5 eV dissociation energy of molecular oxygen. A somewhat lower temperature of 3000 K is obtained by applying the fluid dynamics derived equations for hypersonic flow to the normal shock which would be produced in front of the photometer window perpendicular to the flow direction. It should be remembered, however, that the mean free path at the heights in question is of the order of a few millimetres, so we are in the transition region between fluid dynamics and free molecular flow, and the fluid dynamics approach is not entirely applicable. Nevertheless, it does seem possible that significant dissociation of 02, and perhaps even N,, with a dissociation energy of 9 eV, might occur. Once atomic oxygen becomes available then the emissions which we observe at 5577 A and 76 I9 8, could be produced by the normally accepted mechanisms subsequent upon the three-body recombination of the atomic oxygen. Against this possibility are the very long life times of the relevant excited states [0.74 s for 0(‘S) and 12 s for Oz(h’C,+)], which, in view of the high collisional quenching rates in the shock front, would make the emission probability very low. On the other hand it is interesting to note that according to Torr (1983) strong 0, atmospheric band emissions have been observed in shock tube experiments. The availability of atomic oxygen also makes possible the processes suggested by Slanger (1983) and Torr (1983). At slightly higher temperatures Nz could also be dissociated, so that the mechanism suggested by
B. R. CLEMESHA et al.
1372
Green (1984) in which the emission is produced by excited N, resulting from the recombination of atomic nitrogen could also occur. If the glow which we observed was indeed produced by this sort of mechanism, then the rapid decrease in intensity with height would be explained by the increasing mean free path, leading to a decrease in the effective temperature, with a consequent rapid decrease in the fraction of molecules in the tail of the Maxwell-Boltzmann distribution with energies greater than the molecular binding energy.
4. CONCLUSIONS
During an experiment to measure the vertical profiles of the 015577 A and O2 7619 A airglow emissions, strong spin-modulated contamination signals were observed in the ram direction at both wavelengths. The height variation of the contamination signals was such that it seems unlikely that a minor atmospheric constituent, such as atomic oxygen, was involved in its production. We are unable to positively identify the mechanism responsible for the contamination, but it seems likely that dissociation of atmospheric molecules in the shock wave in front of the photometers was involved. Since the contamination was present at heights at least up to 88 km, our results suggest that the experimental configuration used would not be suitable for observing airglow emissions below 90 km. It seems likely that the contamination was provoked by the unusually high velocity of our vehicle (2.7 km s-‘) in the region in question, and so we conclude that a vehicle with an apogee as high as 500 km is not suitable for measuring mesospheric emissions with forward-facing photometers. Unfortunately the SONDA III rocket is the only vehicle available to us for payloads greater than a few kilograms, and therefore, in a future experiment to measure the OH and NaD emissions, we intend to use side-looking as well as forward-looking instruments. Since the contamination glow appears to be produced only in the ram direction, our present results indicate that the forward-looking photometers should at least give results on the downleg.
AcknowledgementsWe
of the
INPE
possible,
and
especially
IAE
are grateful to the many members teams whose efforts made this work
to Agnaldo
Eras,
who
developed
the
payload electronics, Ricardo Daniel and Col. A. H. P. Chaves who coordinated the payload integration and the launch operation and to Bernard Rossire who designed the photometer deployment system. We are also grateful to E. J. Llewellyn and G. Witt for helpful discussions. This work was partially supported by the Fundo National de Desenvolvimento Cientifico e Tecnologico under contract FINEP537lCT.
REFERENCES
Evans, W. F. J., Llewellyn, E. J. and Valiance-Jones, A. (1973) Altitude distribution of hydroxyl bands of the v = 2 sequence in the nightglow. Can. J. Phys. 51, 1288. Green, B. D. (1984) Atomic recombination into excited molecular-A possible mechanism for shuttle glow. Geophys. Res. Lett. 11, 576. Greer, R. G. H., Murtagh, D. T., Witt, G. and Stegman, J. (1983) Photometric observations of local rocket-atmosphere interactions, in 6th ESA Symp. on Rocket and Balloon Programs, ESA SP-183, pp. 341-347. European Space Agency, Neuilly, France. _ Heppner, J. P. and Meredith. L. H. (1958) Nightglow emis__ \ sion al&udes from rocket measurements. J. geophys. Res. 63, 51. Kunkel, W. B. and Hurlbut, F. C. (1957) Luminescent gas flow visualization for low density wind tunnels. J. appl. Phys. 28,827. Lopez-Moreno, J. J., Rodrigo, R. and Vidal, S. (1985) Radiative contamination in rocket-born infrared photometric measurements. J. geophys. Res. 90, 6617. Ogawa, T., Iwagami, N., Nakamura, M., Takano, M., Tanabe, H., Takechi, A., Miyashita, A. and Suzuki, K. (1987) A simultaneous observation of the height profiles of the night airglow 01 5577 A, O2 Herzberg and Atmospheric bands. J. Geomagn. Geoelect. 39, 21 I. O’Neill, R. R., Lee, E. T. P. and Huppi, E. R. (1979) Aurora1 0 (1s) production and loss processes : ground-based measurements of the artificial aurora experiment Precede. J. geophys. Res. 84,823. Slanger, T. G. (1983) Conjectures on the origin of the surface glow of space vehicles. Geophys. Res. Lett. 10, 130. Stegman, J., Witt, G., Llewellyn, E. J., Dickensen, T. H. G. and Jenkins, D. B. (1982) On the triplet states of moleculal oxygen in the upper atmosphere. Paper presented at the 10th annual meeting on the study of the atmosphere by optical methods. Grasse, France, September 1982. Takahashi. H., Clemesha, B. R., Sahai, Y., Batista, P. P., Eras, A., Chaves, A. H. P., Rossire, B. and Daniel, J. R. (1986) Rocket observations of the atomic and molecular oxygen emissions in the equatorial region. Ado. Space. Res., in press. Torr. M. R. (1983) Optical emissions induced by spacecraftatmosphere i&a&ions. Geophys. Res. Z&t.-16, 114. Wallis. D. D. and Anger. C. D. (1968) High-altitude observat&s of a lumin& wake behinh t& Black Brant II rockets. Can. J. Phys. 46, 2753. Yee, J. H. and Abreu, V. J. (1983) Visible glow produced by spacecraft-environment interaction. Geophys. Res. Lett. 10, 126. I
__
GLOW OBSERVED DURING SOUNDING EXPERIMENT
A ROCKET
B. R. CLEiMESHA, H. TAKAHASHI and Y. SAHAl Instituto de Pesquisas Espaciais-INPE, Ministerio de Ciencia e Technologia--MCT, 12201~S.J. dos Campos, S.P., Brazil
Abstract-In a recent rocket-borne photometer experiment strong contaminating signais were observed at 5577 and 7619 A. The contamination was observed in the ram direction on both upleg and downleg trajectories and no contamination was observed in the wake direction. A strongly spin-modulated signal was observed at heights up to 88 km on the upleg and at heights below 78 km, where the rocket inverted, on the downleg. The signals at both wavelengths were observed to decrease exponentially with increasing height, but with different e-folding intervals, the latter being I.23 km for the OI 5577 A emission and 2.65 km for the 0,7619 A. At a height of 81.5 km the intensities were the same and were equal to 220 Rayleighs per Angstrom (R A-‘). It is suggested that the contamination glow most probably has its origin in reactions involving atomic species produced by thermal dissociation in the shock wave in front of the photometers. 1.
INTRODUCTION
In recent years considerable interest has developed in vesicle-injured airglow. This interest stems from the discovery that surfaces of the Space Shuttle facing the ram direction are clothed in a light-emitting layer some tens of centimetres thick, an effect which has important implications for optical experiments based on the Shuttle platform (see, for example, Torr, 1983). What appears to be an entirely similar effect has already been observed by the AE-C satellite (Yee and Abreu, 1983). Rocket-borne photometer experiments designed to measure atmospheric airglow emissions have observed vehicle-induced glow with a wide range of characteristics (see, for example, the work referenced by Torr, 1983). The purpose of this report is to present the results of a recent observation of vehicle glow having characteristics appreciably different to those previously observed in rocket experiments.
2. OBSERVATIONS The vehicle glow reported in this paper was observed during a dual purpose experiment designed to measure the vertical distributions of the 01 5577 A and O2 atmospheric 7619 b; tropical airglow emissions in the lower thermosphere and the vertical distribution of electron density through an F-region bubble. The rocket, a SOFIA III vehicle, developed by the Brazilian Air Force Institute of Space Activities (TAE), carried a payload developed by the Institute of Space Research (INPE) consisting of two deployable airglow photometers and two plasma probes, and reached an apogee of 516 km. The launch was carried
out from the Barreira de Inferno Rocket Range at Natal, Northern Brazil. Both experiments were successful, and their results are being published elsewhere (Takahashi et af., 1986). The configuration of the payload can be seen from Fig. 1. The rather unusual design of the photometric part of the payload was dictated by the fact that an ejectable nose cone was not available. The two photometers, themselves of conventional design, with a rotating disc device which performed in-flight calibration and dark current checks, were deployed by a pneumatic system at a height of 76 km. The main characteristics of the photometers are given in Table 1 and the launch parameters are given in Table 2. During the deployment of the photometers the signals from both instruments increased rapidly to values many times. the expected airglow levels. Immediately after the point at which the instruments were fully deployed, the photometer signals, which were modulated at the spin rate of the rocket, started to decay. In Fig. 2 we show the time variation of the two photometer signals together with the transverse magnetometer output for part of the upleg. As can be seen from the figure, the 01 5577 A contamination signal becomes negligible only above 88 km. The signal from the 7619 8, instrument was lost at X2.0 km and not recovered until the vehicle had reached 91 S km, by which latter height the contaminating signal was no longer present, so we cannot determine the height at which it decayed to zero. It is interesting to observe that the modulation of the signals from the two photometers bears a constant phase relationship to the magnetometer signal, and that the photometer signals are 180” out of phase with each other, showing that, in
1367
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B. R. CLEMESHA et al. 02
7619 i
PHOTOMETER
_--FOV
----__
_---HALF
ANGLE ---_
-----____ MAGNETOMETER
A%ES
FIG. 1. PAYLOAD CONFIGURATION.
TABLE
Emission 015517 A 027619 A
Optical diameter
1. CHARACTEKISTICSOF PHOTOMETERS FOV half-angle
46 mm 46 mm
4” 4”
Filter peak 5579 A 7620 A
Filter half width 10.4 8, 59.0 8,
PMT 9924NB 9798NA
TABLE 2. LAUNCH PARAMETERS Vehicle : SONDA III two-stage solid fuel rocket Launch: 20:30 Brazilian Standard Time. 23:30 G.M.T., December 1985 Elevation angle on launcher : 8 1.4” Second stage burnout : 44 s at 52 km Photometer deployment : 52.7 s at 76 km Apogee : 5 16 km Downrange impact distance : 483 km
11
terms of angle, the source of the signal is not fixed in relation to the vehicle, but appears to come from a given direction in inertial space. From the phase lag between the contamination signal and the magnetometer ouput, we are able to determine that peak signal was always produced when the photometer was on the lower side of the inclined trajectory. The phase relationships can be seen most clearly in Fig. 3, where we plot a short section of the upleg data on an expanded time scale. On the downleg part of the flight the contaminating signal was again seen, but this time only at heights below 78 km. In Fig. 4 we show part of the downleg data for the 5577 A photometer and both the transverse and longitudinal magnetometers. Note that the rocket trajectory was chosen so as to be as nearly as possible orthogonal to the Earth’s magnetic field, and that the dip angle at Natal is only about 12”. No downleg data is available for the 7619 A instrument because, during the flight, an increase in the temperature of the PMT used in this instrument caused the noise level to rise to such a point as to saturate
HEIGHT
(KM)
FIG.~.UPLEGPHOTOM!ZTEZR ANDTRANSVERSEMAGNETOMETER SIGNALS.
the electronics. The vehicle attitude started to change rapidly after passing through the 01 5577 A emission layer, and became horizontal at about 78 km. Below this height the photometers pointed downwards, and it appears from the magnetometer outputs that the rocket attitude involved a coning angle of the order of 45”. The spin-modulated contamination signal reappeared on the downleg shortly after the rocket inverted, and continued until telemetry was lost at about 60 km. In Figs 2,3 and 4 the photometer signals are plotted as photon counts per 4.8 ms sampling period, and no counting rate corrections have been applied. At the very high signal levels produced by the contamination at the lower heights, the apparent counting rate is
1369
Vehicle glow during rocket sounding experiment PHOTOMETER %
OUTPUT
7
O(
I
I
I
00.5
00
0i
81.5 HEIGHT
(KM)
FIG. 3. UPLEGDATASHOWINGPHASERELATIONSHIPS: (a)5577 .%SIGNAL; (b)76198, SIGNAL;(C)TRANSVERSEMAGNETOMETER
PHOTOMETER
OUTPUT
RANSVERSE MAGNETCMETER ~~vYvvlJyvYYvv~~
I
75
65
70
60
HEIGHT
(KM)
FIG.~.DOWNLEGDATASHOWING 5577A SIGNALANDTRANSVERSEAND LONGITUDINALMAGNETOMETEROUTPUTS
IEASURED
I
78
INTENSITY
I,
I
80
I
62
I,
I
84
I
pressed the signal intensity in terms of Rayleighs per Angstrom. Note that the values plotted in Fig. 5 are the peak contamination signals encountered during each rotation of the vehicle, with the plateau level airglow and PMT noise signals subtracted. In order to express the signals in Rayleighs per Angstrom we have divided the measured airglow intensities by the filter bandwidths, and in doing this we do, of course, assume a uniform continuum emission over the relevant part of the spectrum. We can see from Fig. 5 that the 015577 8, signal varies in a manner very close to exponential over a range of nearly four decades. At least over the 2 km height interval for which the 7619 8, data is available, this emission also appears to vary exponentially with height. Note that the data point at 79 km is probably in error. The very high counting rates encountered shortly after the photometers were deployed exceeded the capacity of the IO-bit telemetry word. In most cases it was easy to determine the overflow bits from the continuity of the data, but the very rapid fluctuations in the 7619 8, signal at around 79 km made it impossible to estimate the overflow unambiguously. From the logarithmic plots of Fig. 5 we can estimate the vertical scale heights with which the two emissions fall off, resulting in values of 1.23 and 2.65 km for the 5577 and 7619 8, emissions, respectively. The two intensities were the same, and equal to 220 R 8, ’ at a height of 81.5 km. From the observations presented above it is possible to draw a number of conclusions about the behaviour of the observed contamination signal. The most important conclusion is that the glow is produced only in the ram direction. We base this conclusion on the fact that whereas the contaminating signal was observed at heights up to 87 km on the upleg part of the flight, on the downleg it was not observed until the rocket turned over at 78 km. This conclusion is also consistent with the spin-modulation of the signal. We believe this spin-modulation to have been produced by periodic shadowing of the photometers by the nose cone. This would occur if the vehicle axis were not parallel to its velocity vector, as illustrated in Fig. 6. Note that we use the term shadowing loosely
I
86
HEIGHT (KM 1 FIG. 5. CONTAMINATION SIGNAL INTENSITIESAS A FUNCTION OFHEIGHTFORTHEUPLEGDATA.
much less than the true counting rate because of the limited response of the PMT preamplifier. In Fig. 5. where we plot on a logarithmic scale the peak contamination signal as a function of height for the upleg data, we have corrected for this effect and have ex-
FIG.~.SHADOWINGOFPHOTOMETERBY
NOSECONE.
t370
B. R.
MAGNETIC
P
M
FIELD-
---l--1; (Cl
Ibl
FIG. 7. EXPECTED SIGNAL
+
--.fi
(CI
tdl
PHASE RELATIONSHIP BETWEEN PHCITO,METER
AND TRAF5VERSE GLOW
CLEMESHA
Mh~~~O~fE~ER
OUTPUT
ASSIJMlNG
IS PRODUCED IN THE KAM DIRECTION.
to reprcscnt the effects of the nose cone on the air flow ; the mean free path at the height in question is much less than the dimensions of the vehicle, so that the situation is more complicated than that encountered on the Space Shuttle, where the mean free path is large. The downleg signal variation is also consistent with the idea of shadowing by the no= cone. It can be seen from Fig. 4 that there is a progressive phase shift between the peaks in the photometer signal and the magnetometer output, with the former occurring progressively earlier. Furthermore, the magnetometer signals suggest that a change of phase by one complete cycle occurs during one complete coning cycle. As illustrated in Fig. 7, this is exactly the behaviour to be expected If the contamination glow is always produced when the pho tometer faces the ram direction and is extinguished when it is shadowed by the nose cone. On the other hand, it must be admitted that we are unable to completely reconcile the details of the relative variations in the magnetometer and photometer outputs with this mechanism. The main difficulty here is, ofcourse, the fact that we cannot unambiguously reconstruct the time history of the vehicle attitude from the magnetometer data. 3.
DISCUSSION
It cm be concluded from the observations reported above that the contamination signal is produced in the ram direction and not in the wake direction. This behaviour is quite different from that reported for the contamination glow observed in other rocket photometer experiments. Many workers (Heppner and iVfcredith, 1958 ; Wallis and Anger, 1968 ; Evans ei n& 1973) observe vehicle glow only in the wake direction,
et
d.
and where it has been seen in the ram direction as welt (Stegman et al.,1982; Lopez-Moreno ef at., 1985; Greer ~1~ri., 19X3; Ogawa ef al., 1987) it has invariably been found to be strongest in the wake direction. The height variation of the contaminating signal which we observe is also different to that seen by other workers. Greer et al. (1983), Lopez-Moreno et al. (1985) and Ogawa et al. (1987) all reporl maximum contamination from heights around 100 km, leading to the suggestion that atomic oxygen might play a role in its production. The fact that we observe no contamination above a height of 88 km rules out direct participation in the process by atmosphe~c atomic oxygen, although there remains the possibility of dissociation of 0; molecules being caused by the vehicle motion. Spin modulation ofcontaminating signals has been reported by Greer et al. (1983) and by Ogawa et al. (1987). The former workers observed spin modulation in the same phase for all photometers of a given payload, although the phase relationship between the signal modulation and the magnetometer output varied from payload to payload. Ogawa et sl. (1987) state that their contamination signal occurred over an angle of only a few degrees in the rotation of the rocket. Greer et ut. suggest that the roughly sinusoidal spin modulation which they observed was caused by a glow local to the rocket, but it is difficult to see what sort ofmechanism could have produced the very short pulses of contamination observed by Ogawa et al. If, as Grccr et cd. suggest, the contamination which they observed was local to the rocket, it is also difficult to understand why the spin modulation should have been in the same phase for all of their (forward looking) photometers. Most of the mechanisms suggested for vehicle glow refer to the Space Shuttle or the .~~~~~~~~r~~Expbzr satellites, for which the ambient conditions are very much different from those encountered by sounding rockets. Slanger (1953) has suggested the Meinef system of OH, with the excited OH being produced by reactions between ramming 0, with 5 eV kinetic energy, or 02, with 10 eV, and H,O or hydrocarbons adsorbed on the vehicle surface. Slanger points to the fact that, because ofthe S eV O2 bond strength, atomic and molecular oxygen are energetically equivalent in this context. In our case this equivalence does not hold, of course, because our vehicle velocity of 2.7 km s’ corresponds to only 1.14 eV of energy for molecular oxygen. Furthermore, the height variation of our con~m~natin~ signal is completely different to that which would be expected for a mechanism invotving atmospheric atomic oxygen. A similar argument applies to the mechanism suggested by Torr
Vehicle glow during rocket sounding experiment (1983), in which the NO, continuum is produced by excited NO, resulting from reactions between atomic oxygen and nitric oxide. Wind tunnel experiments by Kunkel and Hurlbut (1957), cited by Torr (1983) in the context of shuttle glow, have shown strong nitrogen glows both in the ram region and in the wake. These experiments seem to have little relevance either to shuttle glow or to our observations, not only because of the very different velocities and pressures involved, but more importantly, because the glows were only produced when the incoming gas stream was subject to a 5 kW electrical discharge, producing dissociation and ionization. Contamination glows observed in rocket experiments have frequently been ascribed to outgassing. O’Neil et al. (1979) for example, observed a luminescent wake which they suggest was the result of reactions between atomic oxygen and gases from the rocket motor. We cannot exclude the possibility that outgassing played a role in the production of the glow which we observed, although it is practically certain that atmospheric atomic oxygen was not involved. Considerable outgassing from our fiberglass nose cone material would be expected, and although the cone itself is outside the field of view of the photometers, ejected material could be carried into the field of view by the airstream. Against this possibility is the fact that we did not observe the contamination glow on the downleg until the rocket inverted, indicating that the glow which outgassing would be expected to produce in the wake was not present. From the above discussion we can conclude that none of the previously suggested mechanisms is particularly enticing as an explanation for our observed contamination glow, which exhibited characteristics very dilferent to the glow observed in other rocket experiments. Our experiment differed from the majority of rocket photometer experiments in two respects : firstly the vehicle velocity in the region of interest was much higher than is usual, about 2.7 km s-’ as compared with less than 1 km s-‘, and secondly there was a fiberglass nose cone ahead of the photometers. It seems probable that the glow was related to the high velocity, and the spin modulation was caused by shadowing of the photometers by the nose cone. A possible clue to the nature of the vehicle glow which we observe lies in the fact that its intensity decreases exponentially with height with an e-folding distance much less than that of the main atmospheric constituents, suggesting that a minor constituent might be involved. The principal minor constituents which may decrease rapidly in the relevant height range are the hydrogen compounds such as H20,
1371
Hz02, HO* and OH and, perhaps, ozone. With respect to the last named constituent it is interesting to note that the incoming ozone molecules will have kinetic energies around 1.7 eV, well above their binding energy of slightly more than 1 eV. Against the possibility of a minor constituent being involved in the production of the contamination glow is the remarkable uniformity of the observed decrease in the 5577 8, signal with height. The variation is very close to exponential over nearly four decades of intensity and nearly 10 km of height range ; none of the possible minor constituents would be expected to vary in such a uniform manner. In our experiment the mean kinetic energy of the incoming atmospheric molecules is sufficient neither to excite emissions in the visible region nor to produce dissociation of the principal atmospheric constituents. If, on the other hand, these molecules make a sufficient number of collisions in the shock front region ahead of the photometers, then their energies will tend to a Maxwell-Boltzmann distribution in which the r.m.s. molecular energy is just over 1 eV. In such a distribution, which corresponds to a temperature of about 4000 K, about one millionth of the molecules would have energies greater than the 5 eV dissociation energy of molecular oxygen. A somewhat lower temperature of 3000 K is obtained by applying the fluid dynamics derived equations for hypersonic flow to the normal shock which would be produced in front of the photometer window perpendicular to the flow direction. It should be remembered, however, that the mean free path at the heights in question is of the order of a few millimetres, so we are in the transition region between fluid dynamics and free molecular flow, and the fluid dynamics approach is not entirely applicable. Nevertheless, it does seem possible that significant dissociation of 02, and perhaps even N,, with a dissociation energy of 9 eV, might occur. Once atomic oxygen becomes available then the emissions which we observe at 5577 A and 76 I9 8, could be produced by the normally accepted mechanisms subsequent upon the three-body recombination of the atomic oxygen. Against this possibility are the very long life times of the relevant excited states [0.74 s for 0(‘S) and 12 s for Oz(h’C,+)], which, in view of the high collisional quenching rates in the shock front, would make the emission probability very low. On the other hand it is interesting to note that according to Torr (1983) strong 0, atmospheric band emissions have been observed in shock tube experiments. The availability of atomic oxygen also makes possible the processes suggested by Slanger (1983) and Torr (1983). At slightly higher temperatures Nz could also be dissociated, so that the mechanism suggested by
B. R. CLEMESHA et al.
1372
Green (1984) in which the emission is produced by excited N, resulting from the recombination of atomic nitrogen could also occur. If the glow which we observed was indeed produced by this sort of mechanism, then the rapid decrease in intensity with height would be explained by the increasing mean free path, leading to a decrease in the effective temperature, with a consequent rapid decrease in the fraction of molecules in the tail of the Maxwell-Boltzmann distribution with energies greater than the molecular binding energy.
4. CONCLUSIONS
During an experiment to measure the vertical profiles of the 015577 A and O2 7619 A airglow emissions, strong spin-modulated contamination signals were observed in the ram direction at both wavelengths. The height variation of the contamination signals was such that it seems unlikely that a minor atmospheric constituent, such as atomic oxygen, was involved in its production. We are unable to positively identify the mechanism responsible for the contamination, but it seems likely that dissociation of atmospheric molecules in the shock wave in front of the photometers was involved. Since the contamination was present at heights at least up to 88 km, our results suggest that the experimental configuration used would not be suitable for observing airglow emissions below 90 km. It seems likely that the contamination was provoked by the unusually high velocity of our vehicle (2.7 km s-‘) in the region in question, and so we conclude that a vehicle with an apogee as high as 500 km is not suitable for measuring mesospheric emissions with forward-facing photometers. Unfortunately the SONDA III rocket is the only vehicle available to us for payloads greater than a few kilograms, and therefore, in a future experiment to measure the OH and NaD emissions, we intend to use side-looking as well as forward-looking instruments. Since the contamination glow appears to be produced only in the ram direction, our present results indicate that the forward-looking photometers should at least give results on the downleg.
AcknowledgementsWe
of the
INPE
possible,
and
especially
IAE
are grateful to the many members teams whose efforts made this work
to Agnaldo
Eras,
who
developed
the
payload electronics, Ricardo Daniel and Col. A. H. P. Chaves who coordinated the payload integration and the launch operation and to Bernard Rossire who designed the photometer deployment system. We are also grateful to E. J. Llewellyn and G. Witt for helpful discussions. This work was partially supported by the Fundo National de Desenvolvimento Cientifico e Tecnologico under contract FINEP537lCT.
REFERENCES
Evans, W. F. J., Llewellyn, E. J. and Valiance-Jones, A. (1973) Altitude distribution of hydroxyl bands of the v = 2 sequence in the nightglow. Can. J. Phys. 51, 1288. Green, B. D. (1984) Atomic recombination into excited molecular-A possible mechanism for shuttle glow. Geophys. Res. Lett. 11, 576. Greer, R. G. H., Murtagh, D. T., Witt, G. and Stegman, J. (1983) Photometric observations of local rocket-atmosphere interactions, in 6th ESA Symp. on Rocket and Balloon Programs, ESA SP-183, pp. 341-347. European Space Agency, Neuilly, France. _ Heppner, J. P. and Meredith. L. H. (1958) Nightglow emis__ \ sion al&udes from rocket measurements. J. geophys. Res. 63, 51. Kunkel, W. B. and Hurlbut, F. C. (1957) Luminescent gas flow visualization for low density wind tunnels. J. appl. Phys. 28,827. Lopez-Moreno, J. J., Rodrigo, R. and Vidal, S. (1985) Radiative contamination in rocket-born infrared photometric measurements. J. geophys. Res. 90, 6617. Ogawa, T., Iwagami, N., Nakamura, M., Takano, M., Tanabe, H., Takechi, A., Miyashita, A. and Suzuki, K. (1987) A simultaneous observation of the height profiles of the night airglow 01 5577 A, O2 Herzberg and Atmospheric bands. J. Geomagn. Geoelect. 39, 21 I. O’Neill, R. R., Lee, E. T. P. and Huppi, E. R. (1979) Aurora1 0 (1s) production and loss processes : ground-based measurements of the artificial aurora experiment Precede. J. geophys. Res. 84,823. Slanger, T. G. (1983) Conjectures on the origin of the surface glow of space vehicles. Geophys. Res. Lett. 10, 130. Stegman, J., Witt, G., Llewellyn, E. J., Dickensen, T. H. G. and Jenkins, D. B. (1982) On the triplet states of moleculal oxygen in the upper atmosphere. Paper presented at the 10th annual meeting on the study of the atmosphere by optical methods. Grasse, France, September 1982. Takahashi. H., Clemesha, B. R., Sahai, Y., Batista, P. P., Eras, A., Chaves, A. H. P., Rossire, B. and Daniel, J. R. (1986) Rocket observations of the atomic and molecular oxygen emissions in the equatorial region. Ado. Space. Res., in press. Torr. M. R. (1983) Optical emissions induced by spacecraftatmosphere i&a&ions. Geophys. Res. Z&t.-16, 114. Wallis. D. D. and Anger. C. D. (1968) High-altitude observat&s of a lumin& wake behinh t& Black Brant II rockets. Can. J. Phys. 46, 2753. Yee, J. H. and Abreu, V. J. (1983) Visible glow produced by spacecraft-environment interaction. Geophys. Res. Lett. 10, 126. I
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