Equatorial measurements of the [OI] 5577 Å emission of the dayglow with a rocket photometer

Planet. Space Sci. 1969, Vol. 17. pp. 1609 to 1618. Pergamon Press. Printed in Northern Ireland

EQUATORIAL MEASUREMENTS OF THE [01] 5577 ii EMISSION OF THE DAYGLOW WITH A ROCKET PHOTOMETER* Northeastern

B. S. DANDEKAR University, Boston, Mass. 02115, U.S.A. and Air Force Cambridge Research Laboratories, L. G. Hanscom Field, Bedford, Mass. 01730, U.S.A. (Received 14 March 1969)

Abstract-Observations of the [Olj 5577 A emission of the dayglow have been obtained with a rocket photometer from the equatorial launch site at Natal, Brazil. Our observations show that the contribution to the [01] 5577 8, emission of the dayglow comes from three different ranges of altitudes. The lower layer is around 96 km, with half emission width of about 23 km and peak emission of about 330 photon/cc. This layer contributes about 40 per cent of the total emission, and is similar to the one which contributes most of the [OI] 5577 A emission of thenightglow. Theexcitation is of chemical origin. Thesecondlayer is in the altitude range of 110-150 km. The probable excitation mechanism is the photodissociation of 0,. The third layer is above 150 km. The excitation mechanism is the process of recombination of 09+ and ionospheric electrons, and also the excitation of Oa by photoelectrons. This contributes about 30 per cent to the dayglow emission, INTRODUCTION

In recent years various excitation mechanisms have been proposed for explaining the emissions of the dayglow. On the experimental side ground-based techniques have achieved some success in the measurement of these emissions. Ground-based observations are complicated by the predominant Rayleigh scattered sunlight. The use of instruments aboard airplanes and balloons has resulted in a reduction of the background due to Rayleigh scattered sunlight. These methods yield only the total integrated emissions of the dayglow. The theories not only predict the integrated emissions, but also the intensity distribution of the emissions with altitude. Such data are at present available only by rocket measurements. Rocket measurements have the additional advantage of an enormous reduction in Rayleigh scattered sunlight. As yet there is a lack of sufficient rocket observations to close the gap between theoretical estimates and measured values. The excitation mechanisms and the instrumentation for the dayglow emissions has been reviewed recently by Noxono). The [01] 5577 A emission which is one of the principal emissions of the dayglow has been explained by various mechanisms, ~-1~) but there is insufficient observational information to decide between these. In this paper we present our rocket observations conducted from the equatorial launch site at Brazil, for the [01] 5577 A emission of the dayglow, and compare our results with theoretical estimates for a better understanding of the excitation mechanisms responsible for this emission. INSTRUMENTATION

The schematic arrangement of the green line rocket photometer is shown in Fig. 1. The light enters the optical system through two sets of baffle systems B and B’. In front of the field lenses L and L’ are two narrow band optical interference filters F and F’, one peaked at 5577 A for the [01] green emission and the other peaked near 5530 A for the continuum in the near vicinity of the [01] 5577 A emission. At the entrance of the filters * Presentedat the Third International in February 1969.

Symposium on Equatorial Aeronomy held at Ahmedabad, 1609

India,

B. S. DANDEKAR

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B,B’ I$

BAFFLE

SYSTEMS

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IRIS DIAPHRAGMS

F, F’ INTERFERENCE L,L’ FIELD

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LENSES

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S ROTATING SHUTTER M MAGNETIC

PICK-UP

A, A’ PAIR OF APERTURES 0 DETECTOR

, REFERENCE AMPLIFIER

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0

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SYNCHRONOUS DETECTION AMPLIFIER TELEMETRY

D.C. AMPLIFIER

-

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H.T. FIG. 1. SCHEMATIC ARRANGEMENT

OF THE ROCKET

PHOTOMETER.

A, A’ APERTURES IA1

S SHUTTER S

I81 I

TIME

---

SIGNAL SIGNAL

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REPRESENTATION

SIGNAL

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FIG. 2. (A) SCXIXMATIC ARRANGEMENT OF THE APERTURES (B> %XDMATIC!

THROUGH THROUGH

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AND THE SIIUTTER. FROM THE APERTURES.

A A’

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there are the adjustable iris diaphragms I and I’. The two light paths are folded together by the use of two pairs of right angled prisms P, P and P’, P’. The light is chopped by the rotating shutter S made of ferromagnetic material and falls on a single detector D, which is an EMI 9524 photomultiplier, through two apertures A and A’ which lie in the focal plane of the lenses. The magnetic pickup M mounted near the rotating shutter provides the reference signal for the synchronous detection amplifier. The output of the detector is fed to the d.c. and the synchronous detection amplifiers through the preamplifier. The outputs of the d.c. and synchronous detection amplifiers are fed to the telemetry for ground reception. The instrument was pressurized as a precaution to avoid breakdown of the high voltage during Bight. The two apertures in front of the detector have the shape of truncated sectors as shown in Fig. 2(A). The shutter and the apertures are machined with precision so as to illuminate a constant area of the detector for any position of the shutter as shown in the figure. The iris diaphragms in front of the interference filters are adjusted to match the amplitudes of the continuum entering through either titer. The light flux of the continuum falling on the detector through either aperture in its front is shown schematically in Fig. 2(B). The figure shows that in the case of the continuum the total flux received by the detector should be constant and would not suffer any detectable modulation due to the rotating shutter. For the line emission superposed on the continuum, only the signal due to the line emission will be modulated by the shutter, which is processed by the synchronous detection amplifier. The use of two closely spaced wavelengths has the advantage of providing a good balance between the two channels for the continuum from a black body source over a wide range of temperatures of the black body source. This arrangement was used to eliminate the strong contamination to the line emission by the continuum from the Rayleigh scattered sunlight. The total intensity channel measures the [01] 5577 A line emission and the continuum transmitted through both the interference titers. With the arrangement of shutter and apertures shown in Fig. 2(A), the continuum intensity would be constant and only the line emission would be modulated by the rotating shutter. This modulated a.c. signal is fed in conjunction with the reference signal generated by the magnetic pickup M to the phase sensitive amplifier for synchronous detection. Thus the synchronous detection channel provides the data for only the line emission, and the difference, computed externally, between the total and the synchronous detection channel yields the data for continuum in the vicinity of the line emission. Each interference filter has a half transmission bandwidth of about 10 A with peak transmission of about 55 per cent. The transmission of the 5530 A filter was less than O-1 per cent for wavelengths greater than 5570 A. The semi angle of the field of view was 2.5”. The photometer was calibrated against the source with a known spectral character. The photometer had a forward view along the longitudinal axis of the rocket. OBSERVATIONS

The instrument was flown aboard an Aerobee rocket which was fired from Natal, Brazil (geographic latitude 5”55’ South, geographic longitude 35”lO’ East) on November 15, 1967 at 1733 hr LST, in a near zenith trajectory. The nose cone was dropped at an altitude of 54 km, and the apogee of the rocket was at an altitude of 225 km. The solar zenith angle at the time of launch was about 81”. Of the total flight time of 490 set, useful data were obtained for a period of 350 sec.

B. S. DANDEKAR

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The flight data consisted of two channels; (1) synchronous detection intensity and (2) total intensity. Data were digitized at intervals of 10 msec. The data showed two modulations, (1) a weaker one with a frequency of about 2.5 c/s on both the channels due to the spinning of the rocket, and (2) a stronger modulation of the data from the total intensity channel due to the coning of the rocket, when the photometer looked towards the solar azimuth. The first effect was eliminated by averaging the digitized data over a complete cycle corresponding to the spinning rate. For eliminating the effect of the second modulation the data of the total intensity channel corresponding to the period during which the photometer looked toward the solar azimuth were not used. As the upleg and the downleg data for the same altitude did not show any systematic and significant differences, the intensities were averaged over two kilometer intervals using both the upleg and the downleg data. The averaged intensities of the [01] 5577 A emission obtained from the synchronous detection channel are presented in Fig. 3 along with the data of other observers.(14-17) The abscissa shows the integrated intensity observed by the photometer and the ordinate presents 0

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FIG. 3.

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DEPENDENCE OF INTEGRATED EMISSIONOF THE [OII 5577 A EMISSIONOF DAYGLOW WITH ALTITUDE. (x is the solar zenith angle.)

,

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the corresponding altitude of the rocket above the ground. The solid triangles show the intensity averaged over 2 km intervals, and the smooth curve joins the data points. The figure shows that the intensity of the [01] 5577 A emission was constant at about 1.9 kR up to an altitude of about 78 km, and reached a minimum of about 100 Rayleighs (R) at 225 km. The data of other observers presented in Fig. 3 will be considered during discussion. From all the data consisting of the 2 km average intensities presented in Fig. 3, the differential intensity was computed for a 10 km sliding window. These differential intensities now over 10 km interval provide the volume emission rate. These are presented in Fig. 4. The abscissa shows the emission rate in photon/cc of the [01] 5577 A emission and the ordinate shows the altitude in km. At this stage we will consider our observations only. Our data reveal the presence of two distinct layers from which the green line emission of the dayglow originates. The lower layer is around 96 km, having a peak emission rate of about 330 photon/cc. The half emission width of this layer is about 23 km. The multiplication of peak volume emission rate and the half width of the layer provides the contribution by this layer to the green emission. This layer thus contributes about 780 R of [01] 5577 A of the dayglow. The second layer peaks at an altitude greater than 200 km. The presence of a weak emission layer around 170 km should be noted. These and the results of other workers presented in Fig. 4 will be considered later. The use of 10 km interval for determining the volume emission rate will have the effect of suppressing the fine structure of the layer. For any layer the general effect of this smoothing procedure will be to widen the layer and suppress the peak emission rate. However, for the layer around 96 km, because of the very prominent peak, this procedure will neither affect the position of the peak nor will it appreciably widen the half emission width of this layer. The difference in the intensities of the total and the synchronous detection channels provides the information about the continuum. This will be postponed for a latter communication. DISCUSSION

The [01] 5577 A emission of the dayglow has also been measured by Wallace and McElroy,04) Silverman et u1.,06) Wallace and Nidey,(le) and Lloyd ef a/.07) All these results have already been presented in Fig. 3 along with our observations. Wallace and McElroyo4) had continuous observations similar to our observations. The other observations(15-1’) were made with scanning spectrometers and are therefore discontinuous. Smooth curves representing the integrated emission profiles have been drawn through the observed points. The details pertaining to these observations are also included in the figures. All these observations are from different latitudes, at different solar zenith angles and also under different magnetic conditions. The flights of Silverman et aZ.(16)and Lloyd et u1.o’) were made during periods of moderate magnetic activity. These observations did reveal the presence of aurora1 activity below an altitude of 120 km. As we are here concerned only about the dayglow phenomenon we will not use these observations below 120 km, which are dominated by the aurora1 activity. The detailed study of the calibrations of 1963, 64, 66 flights (15,17)by Dandekar and Ahmed us) showed that the intensities for these flights were underestimated by the factors 2.5, 2.5 and 1.2 respectively. Therefore the corrected intensities have been presented in the figure. At any altitude over the range covered by these observations the intensity variation is within an order of magnitude. Thus all these results including ours are in good agreement with each other. 4

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The photon emission rates of the [01] 5577 A emission of the dayglow for all these observations are in Fig. 4. The emission rates by Wallace and McElroyo4) are plotted from their published results. Results of Wallace and McElroyu4) for the 1964 flight show the presence of an emission layer around 95 km, with a peak emission rate of about 330 photon/cc, and a half emission width of the layer of about 25 km. Thus this layer contributes about 770 R to the [01] 5577 A emission of the dayglow. These results are in good agreement with our results. The remaining results presented in Fig. 4, basically refer to an altitude range above 120 km, and are unable to provide any information about the lower layer. Another common and important feature to most of the flights of Fig. 4 is the presence of the layer in the altitude range of 170 km. The reference to the solar zenith angle of these flights indicates a strong dependence of the peak emission rates of this layer on the solar zenith angle. The August 1966 ffight(l’) with the solar zenith angle of 47” had a peak emission rate of about 600 photo&, whereas the other flights with the solar zenith angle in the range of 72”-76” have a range of 90-400 photon/cc. The emission rate for the flight from Natal in November 1967 with a solar zenith angle of 81” barely shows the presence of a layer at that altitude. However the 1964 aurora1 flighP) with a solar zenith angle of 68” does not show the presence of any layer around 170 km. It is quite possible that the strong aurora1 emission may have blanketed the presence of the relatively weaker emission, from the upper layer. Thus the results in Fig. 4 show that at least three layers, two around altitudes 95, 170 km and one above 200 km respectively, contribute to the [01] 5577 A emission of the dayglow. We should now compare our experimental results with theoretical estimates. For this purpose our observed emission profile in Fig. 4 can be divided into four broad ranges of altitudes; (1) below 110 km, (2) from 110 to 150 km, (3) 150-200 km and (4) above 200 km with the emissions of 800, 500, 300 and 400 R respectively. These altitude ranges are associated with different excitation mechanisms which contribute to the [01] 5577 A emission of the dayglow. As we shall see these are the Chapman mechanism, photodissociation of 02, the dissociative recombination of Oa+, and excitation by photoelectrons. The peak observed around 96 km with the emission of about 800 R is similar to that of the [01] 5577 A nocturnal emission and therefore would be of photochemical origin. Chapman(2) suggested the three body collision process for explaining this emission, but the laboratory measurements of the rate coefficient for this mechanism by Barth and HildebrandP and by Young and Clarkog) were an order of magnitude lower than that needed to account for the nocturnal [01] 5577 A emission, therefore Barth(‘*lO)suggested the two step mechanism as the alternative. Later Young and Black(20) corrected their results to take into account the inherent quenching of O?S by OsP in the laboratory measurements and concluded that the Chapman mechanism would be adequate to account for the [01] 5577 A emission of the nightglow. Still there is not enough evidence to exclude one of these mechanisms in favor of the other. In either case the emission would be proportional to the third power of the concentration of atomic oxygen. It should be noted that the rate of Young and Black c20)for the Chapman reaction need a high concentration of more than 1012oxygen atom/cc around 95 km altitude. Thus there could be an additional source for this emission layer, operating through the metastable oxygen molecules. Above 130 km the concentration of atomic oxygen is too low to make any significant contribution through the Chapman or Barth mechanism. Hence some other source must be sought to account for the emission from altitudes above 130 km. Enough information is not available about the diurnal variation of the neutral

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B. f-3.DANDEKAR

atmospheric species, at higher altitudes. The [01] 5577 A emission around 95 km can provide this valuable information about the change in the concentration of atomic oxygen from night to day if the emissions at these corresponding periods are known, and one assumes that the lower layer is entirely due either to the Chapman(2) or Barth’7Jo) mechanism at all times. On November 7 and 9, 1967 the intensity of the [OI] 5577 A emission in the nightglow measured from the ground was 300 R. As we are concerned about the emission around 95 km, the contribution if any, from layers above 130 km should be subtracted to obtain the contribution from the lower layer. The existence of a secondary layer in the altitude range above 130 km in the equatorial region has been established indirectly by correlation studies of [01] 5577 A and [OI] 6300 A nocturnal emissions from ground based observations (21--a6) and directly by rocket(2B-2s) and satellite(29) observations. The detailed analysis by Wei11(22)shows that for the equatorial region the contribution to the [01] 5577 A emission from the layer above 130 km is one fourth that of the [01] 6300 A emission. As there were no observations for the [01] 6300 A nocturnal emission the corresponding correction could not be applied to the [OI] 5577 A emission, and we assumed all the emission to be from the lower layer. Thus only an approximate change in the concentration of atomic oxygen can be determined. With the assumption that the emission is proportional to the third power of atomic oxygen concentration, the concentration was 40 per cent more than that at night. Wallace and McElroy(14) estimate an increase of 60 per cent from their observations without applying any correction to the [01] 5577 A emission from the altitude above 130 km. Thus both results are in good agreement with each other and are consistent with the study of photochemistry of oxygen by Blamont and Donahue(30). Some workers(13*14)have tried to set an upper limit to the contribution to the nocturnal emission from the dissociation of O2 by the Lyman-a emission. Our dayglow data can be used for checking their results. The upper limit to the contribution by Lyman-a through the dissociation of O2 can be obtained by using the Lyman-a emission measured during day and night. The corresponding values are 6.0(31)and 0*01(32)erg cm-2 se+. With the assumption that all the emission around 95 km in the dayglow is due to the dissociation of 0, by Lyman-g, 800 R measured at a solar elevation of 9” would yield less than 10 R at night. These results are consistent with 15 R determined by Walkero3) and 5 R by Wallace and McElroy,‘14) showing that the contribution by Lyman-a to [01] 5577 A emission of the nightglow is negligible. For explaining the [OI] 5577 A emission of the dayglow in the altitude range above 130 km Bates and Dalgarno,(4) Walker,u3) and Wallace and McElroyu4) have considered the processes of resonant and fluorescent excitation of atomic oxygen, dissociative recombination of molecular oxygen ions, photodissociation of molecular oxygen and the excitation of molecular oxygen by thermal and photoelectrons. Using the recent values for the ‘g’ factor Wallace and McElroy (14)estimate an upper limit of 25 R as the contribution by the fluorescence mechanism. Bates and Dalgarno,c4) Dalgarno and Walker(12) also arrive at a similar conclusion that the contribution by resonance and fluorescence excitation to the [01] 5577 8, emission of the dayglow is negligible. The excitation by thermal electrons was considered by Dalgarno and Walker,02) Dalgarno,(ll) and Noxon.(33) From their observations Wallace and McElroy,d4) Zipf and Fastie’=) concluded that this process normally would be a minor source for [01] 6300 A dayglow emission, though occasionally it can be a major source as evidenced by Noxon. (as) Using the rate coefficients of Seaton, Walker(la) estimated the contribution to the [01] 5577 A emission of the dayglow by thermal

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electrons to be about 70 R, provided the electron temperature was of the order of 4000°K. For the temperature of the order of 2000°K measured for the ionospheric electrons,(a638) he concludes that the contribution would be insignificant. Thus the processes which could conceivably contribute to the [01] 5577 A emission of the dayglow are, the dissociative recombination of molecular oxygen ions, photodissociation of Oe, and the dissociation by photoelectrons. Wallace and Nidey(16) considered the direct photoionization of 0, followed by the ionic recombination for explaining the [01] 5577 A emission of the dayglow. Walker(“) suggested the additional potential source of 0, f through the ion-atom interchange between 0s and 0+, and also the charge exchange between Ne+ and 0s. The computations involve parameters such as the concentrations of N,, 0,, and 0, the rate coefficients for the reactions responsible for production and quenching of 0, + , the electron densities, and the spectral distribution of the solar flux responsible for ionization. On the basis of his computations, the contribution from this source in our measurements would range from 500 to 3000 R depending upon the model atmosphere used. His estimates predict two peaks in the emission, one above and the other below 200 km. Our observations do indeed reveal these peaks above 150 km, with the combined intensity of about 700 R. The same process would be responsible for the nightglow emission of [OI] 5577 A originating at altitudes above 130 km. Recent rocket(26-28) and satellite (2g) observations have provided evidence beyond doubt about the contribution from this altitude region to the [01] 5577 A nocturnal emission. The process of photodissociation was studied by Bates and Dalgarno,(4) and Wa1ker.o”) There is some uncertainty in the photodissociation computations yielding 0% due to two factors: (1) there is no information about the detailed process of absorption of 0, for wavelengths less than 1344 A and (2) the per cent yield of atomic oxygen in O*S state from the dissociation of 0, is not known. Bates and Dalgarno, t4)and Walkerol) conclude that the process could yield about 10 kR to the dayglow emission for the overhead Sun. The peak due to this process would be below 150 km (Wallace and McEhoy(14)). Our measurement of the total intensity of 1.9 kR, as well as ground observations by Noxon(3g) set an upper limit of 10 kR for the [01] 5577 A emission of the dayglow for the overhead Sun. Walker(ls) established an upper limit of 5 kR for the contribution from dissociative recombination of O$ by taking into account his theoretical estimates and the experimental result of the production rate of 0.24 O’S atoms per combination of 0, + derived from the rocket measurements of the nocturnal [01] green and red emissions, and the upper limit of [01] green dayglow emission based on the observations by Noxon.(3g) This sets an upper limit of about 5 kR for the contribution by the photodissociation mechanism. Based on the estimates by Walker (13)the corresponding contribution in our observations would be about 800 R. In the altitude range of 110-150 km we observe an intensity of 500 R, which agrees with the theoretical estimates within a factor of 2. In the absence of adequate information about the dissociative recombination of O,+ ions, Wallace and McElroy (14)had considered in detail the process of excitation by photoelectrons. The contribution by this process could be large enough to account for the total intensity observed above 110 km. At this stage it is not possible to exclude one of the processes; the excitation by photoelectrons or the dissociative recombination of O$, in favor of the other. The two could be possibly distinguished if concurrent measurements were conducted for the dayglow and the positive ion concentrations. At present both the processes seem to be able to explain the intensity profiles in the altitude range above 130 km, and it is quite conceivable that both the processes could be operating simultaneously.

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B. S. DANDEKAR

CONCLUSION

The observations show the presence of a strong emission of [01] 5577 A peaked around 96 km having an emission of about 800 R. Its shape and position lead to the conclusion that the mechanism for this emission is the same as that for the nocturnal emission. The emission in the altitude range between 110 and 150 km with 300 R intensity is probably due to the photodissociation of 0,. The double peak above 150 km could be explained by either mechanism; dissociative recombination of OS+, or the excitation by photoelectrons. The experimental results are in good agreement with those by other observers. This qualitative picture calls for further investigation to obtain a better agreement between experimental results and improved estimates from theoretical considerations. Acknow@gemenrs-The author is especially thankful tion of the electronic package. He is thankful to the their generous co-operation and to the authorities of facilities at the launch site. The author is indebted to for their valuable suggestions.

to his colleague, Mr. J. W. F. Lloyd, for the construcinstrumentation group of Northeastern University for the Brazilian Air Force for providing all the necessary

Drs. A. I. Stewart, S. M. Silverman and C. G. Stergis A part of this work was carried out under AFCRL contract. REFERENCES

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