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The airglow
The Airglow DANIEL BARBIER I n s t i t u t d'Astrophysique et Observatoice de Paris SUMM.~kRS[ The s t a t u s of the problem of the airglow is reviewed; the observational point of view is particularly emphasized.
NEWCOMB, in 1901, measured for the first time the intensity of the light of the night sky. He attributed it to the integrated effect of unresolved stars and thought that he thus obtained a quantity of the utmost importance for studies of the structure of the Universe. However, his measurements, and those of later investigators, show in fact that the light of the night sky is brighter than that of stellar origin as indicated by star counts. V. M. SLIPHER in 1919, and later Lord RAYLEIGtt in 1922, were able to show that our atmosphere possesses its own light, due to radiations amongst which the green line had already been observed in the aurora. In 1933 SLIPItER showed further that the emission spectrum which we observe in the twilight differs from that emitted during the night. In this short survey it is impossible to consider in detail all papers published on the subject of the airglow*; excellent summaries can be found elsewhere (DuFAY, 1928; DEJARDIN, 1936; ELVEY, 1942; DUFAY, 1943; SwrNGS, 1949; CHVOSTIKOV, 1948 ; MEINEL, 1951 ; DUFAY, 1952a) and we shall restrict ourselves here to discussing the present status of the problem. We must mention, however, the pioneers who, in spite of the rather primitive tools at their disposal, paved the way for their successors for the modern phase of these studies: to the above-mentioned names we add Y N T E M A , B U R N S , A B B O T , V A N R H I J N , F A B R Y , F E S S E N K O F F , CABANNES, G A R R I G U E ,
and B E R N A R D . The following components of the light from the night sky may be distinguished: (a) Stellar light. Stars of about 12m play the most important role and their number compensates for their faintness. (b) Zodiacal light. We are concerned not only with the light pyramid visible in the ecliptic when the Sun is not far below the horizon, but also with the light which stretches over the whole sky, being brightest along the ecliptic and intensified in the point opposite to the Sun (Gegenschein). Charts of the stellar light and of the zodiacal light were obtained by ELVE¥ and ROACH (1937), and published by ROACH and PETTIT (1952). It must not be overlooked that the zodiacal light is possibly subject to variations with time. (c) Galactic light. Here we have to deal with stellar light scattered in interstellar space. First noticed by ELVEY and ROACH (1937), its existence was confirmed by HENY~r and GREENSTEIN (1940). It is limited to the neighbourhood of the galactic plane. GAUZIT,
(d) Airglow of the upper atmosphere. (e) Auroral phenomena, particularly in northern latitudes. * Following a nomenclature recently proposed by ELV~Y, the emission by the atmosphere itself, excluding the aurorae, may be called the "airglow". The night sky emission can then be called the "nightglow", the twilight emission the "twilightglow", and the as yet unobserved daytime emission the "dayglow". 929
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The airglow
The present survey is especially concerned with the study of the nightglow; in addition, a survey of the twilight glow is given. The study of the emission spectra of the terrestrial atmosphere interests geophysicists who hope to deduce from it information as to the state of the upper atmosphere. It is equally interesting to the astronomers, since the state of the upper atmosphere is conditioned by effects (ionization, dissociation, high temperature) produced by penetrating solar radiations (ultra-violet and X-rays) and also, because the excitation phenomena, as soon as these are better understood, will without doubt give valuable information about the corpuscular solar radiation, and perhaps about interplanetary matter. 1. INSTRUMENTS The intensity of the airglow is minute--it is the faintest extended light source which has been studied in astronomy; it is only very little in excess of that of a fifth magnitude star spread over 5 square degrees (DuFAY, 1928). Expressed in candle-powers per square centimetre it amounts to 4 × 10-s (HuLBURT, 1949); for comparison we may mention that the daytime blue sky has a brightness of about one candle-power per square centimetre. This means that the most powerful instruments have to be employed. Without going into the details of this subject, we mention amongst the spectrographs which have been used during the last few years the prism spectrographs and objectives calculated by COJAN (1934, 1947), the spectrographs of the Arnulf-Lyot type (ARNuLF, 1943), the grating spectrograph and the Schmidt camera of •EINEL (1948), the prism spectrograph combined with an electron image converter by KRASSOVSK¥ (1949, 1950). Quite a number of photoelectric photometers have been used, amongst which may be mentioned the instruments of ELVEY and ROACH (1937), of GRANDMONTAGNE ( 1941 ), of ABADIE, A. VASSY, and E. VAss¥ (1945), of MARLOW and PEMBERTO:N (1949), of ROACH and PETTIT (1951a), as well as an instrument recently installed at the Observatoire de Haute Provence (BAILLET, BARBIER, BOSSON, LALLEMAND, MAGUERY, 1953). The use of photometers has become much more popular since the invention of interference filters, which have a half-width of the order of 50 A. This width is scarcely greater than is obtained from spectrographs used with very wide slits in order to reduce the exposure times, or to facilitate the photographic-photometric measurements. Furthermore, the speed of photometers equipped in this way is many thousand times greater than that of spectrographs, and permits a detailed exploration of the sky using automatically recording instruments. 2. THE SPECTRUM OF THE ~IGHTGLOW The best descriptions of the various different spectral regions which are available at present are: ll,000 to 9000A (KRAssovsKY, 1949, 1950); 9000 to 7000A (MEINEL, 1950a, 1951) ; 7000 to 5800 ~ (CABA~NES, J. DUFAY, and M. DUFAY, 1950) ; 5000 to 4000 ~ (CABANNES, DUFAY, 1944); 4000 to 3000A (BARBIER, 1947). The green spectral region, between 5800 A and 5000 A, is the least well-known, and observations on it are not always in good agreement; these are due to H. BABCOCK (1939); VAINU BAPPU (1950); BARBIER, DUFAY, and WILLIAMS (1951); and M. DUFAY (1951). The following atomic lines sometimes called the green, red, and yellow lines,, have been identified with certainty and are the most easily observed radiations: [0 I] 5577, [O I] 6300-6364, and Na I 5890-5896.
DANIEL BARBIER
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Molecular systems also give rise to a large number of emissions. The vibration-rotation system of OH was discovered and identified by Mm~EL (1950b), and is very intense in the infra-red. It extends with appreciable intensity into the red (CABAN~ES, J. DUFAY, and M. DUFAr, 1950), with less intensity into the green (M. DUFAY, 1951), and it can possibly be observed in the blue (HuNAERTS and NICOLET, 1950). The atmospheric system of 0 n emits the (0, 1) band which appears at 8645/~ in the form of a doublet (MEINEL, 1950d), and perhaps the (0, 2) band at 9965 A (DUFAY, 1951). The (0, 0) band cannot be observed, as it is re-absorbed in the lower atmosphere. The bands in the blue region of the spectrum have been attributed for a long time, following a suggestion of CABANNESand DUFAY (1946), to the Vegard-Kaplan system of N~, but this identification appears now doubtful (BARBIER, 1953b). The ultra-violet part of the spectrum is characterized by the Herzberg bands of 0 n (DuFAr, 1947). All these band systems are forbidden, excepting that of OH, whose observed radiations have, however, only very small transition probabilities, since they are all high overtones. A certain number of medium and faint bands which have previously remained unidentified certainly form part of the new system 1Z~--aE~- of On, recently discovered by ttERZBERG (1953). The presence of the (0, 0) band of N~+ in the spectrum of the nightglow has been confirmed by M. DUFAY (1953) and MEINEL (1953). According to B~BIER (1953) the profile of the band sometimes resembles that of the N2+ band, but often differs from it. Spectra of the nightglow also show a continuum. This is partly due to the light from the stars, and it shows the presence of the Fraunhofer lines F, G, h, H, K. These, however, do not have the same relative intensities as in the Sun, because of the presence of A-type stars in the stellar population (I~¢IEINEL,1953). The discontinuity of the continuum at about 4000 A recalls spectra of stars of the solar type, with a depression at 3883 A produced by the CN band. It is now certain that the continuum is not completely explained by the light from the stars. Its altitude may be determined, as will be discussed further below, and one does not find an infinite value, but something like 10,000 km in the neighbourhood of the D-line (BARBIER, 1944), 1600 km for 4500 A, and 500 km for 3500 ~ (BARBIER, 1947). This shows that a part of this spectrum is produced in our atmosphere. An hypothesis which attempts to explain the continuum as due to the wings of very intense lines (BA~BIER, 1947) appears to be applicable in the blue and ultra-violet but cannot be supported in the green where bands are very weak (BARBIER, DUFAr, WILLIAMS, 1951). There exists therefore a true continuous spectrum of atmospheric origin in the light of the night sky. 3. HEIGHTS OF AIRGLOW EMISSIONS
The usual method, known as VAN RHIJN'S method, is based on the comparison of two intensity measurements performed at the zenith and at a rather large zenith distance ~. These are related by the equation: 1(0=I(o).
where R is the radius of the Earth.
1
(R+H),J
'
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The airglow
This formula assumes: (a) That the emission takes place in a thin layer at height H. It is easy to show that if the emission layer is thick this formula remains valid up to zenith distances of the order of 80 °, and H is then a kind of mean value for the height of the emitting atoms (BARBIER, DUFAY, WILLIAMS, 1951). (b) It also assumes that the observations have been corrected for the whole extraterrestrial radiation, for atmospheric extinction and scattering, and for the geographical non-uniformity of the layer. When using filters or spectrographs of small resolution, it is most important to eliminate the effect of extra-terrestrial radiation; this has been emphasized by the analysis of ROACH and PETTIT (1952). In order to do so one must make a detailed analysis of numerous quantities covering an observing period of about one year (BARRIER, DUFAY, WILLIAMS, 1951); or, alternatively, a comparison can be made between the radiation concerned and that from a neighbouring spectral region (RoAcH, BARBIER, 1950). The problem of atmospheric scattering is extremely complicated; a simple solution has been obtained for the case of small scattering coefficients (BARRIER, 1944). A more exact solution which is valid for the ultra-violet spectrum has also been made (BARBIER, 1952a). The non-uniformity of the layers has been made the subject of detailed studies (ROACH and BARBIER, 1950; BARBIER, DUFAY, WILLIAMS, 1951); we shall return to this question below. This effect can be eliminated by using the mean of a large number of observations. A general discussion of height measurements has been given by BARBIER (1952b). This should be supplemented by the following recently measured altitudes: Herzberg system of 0 2 : 2 0 0 kin, very uncertain, according to BARBIER (1953a). Green line: 250 km (DuFAY, BERTHIER, MORIGNAT, 1953). OH bands and the atmospheric system of O2:140-150 km (BERTHIER, 1953). The present opinion of the author of this survey is that it is most likely that all radiation comprising the nightglow is emitted from the region near 200 km. This view is strengthened by the observed correlations between the intensities of the various bands. We shall see below that the light of the night sky shows irregularities. It is hoped that it will be possible in future to measure these heights by simple triangulation. 4. TEMPERATURES OF THE EMITTING LAYERS
(a) Kinetic temperature. The kinetic temperature may be deduced from the Doppler broadening of a monochromatic line. H. D. BABCOCK (1923) obtained "sharp" interference fringes of the order of 85,000. According to the significance placed on the word "sharp", this can mean that the maximum temperature for the green emission line lies anywhere between 680°K and 1700°K (SPITZER, 1949). It is desirable for interferometrie measurements to be repeated with higher orders of interference. (b) Rotational temperatures. MEINEL (1950C) obtained for the (0,1) atmospheric band of 02 a reasonably consistent temperature of 150 ° 4- 20°K, although in one night of anomalously great intensity he measured 200 ° 4- 10°K. The OH bands give good determinations since they are resolved into lines. MEINEL (1950d) obtained a temperature of 260 ° 4- 50K.
DANIEL BARBIER
933
The derivation of rotational temperatures is only justified if collisions are frequent enough for the molecules to distribute themselves between the different rotational levels according to BOLTZMANN'S law. This point has been treated b y BRANSCOMB (1950). (C) Vibrational temperature. From the ratio of intensities of components due to the doublet structure of the ground state of OH, MEINEL (1950d) has derived a temperature of 172 ° ~- 10°K, which does not agree with the rotational temperature. However, I~¢[EINEL can explain the discrepancy if he assumes that the maximum excitation energy is 3.242 eV. 5. THE ABSOLUTE INTENSITIES OF THE NIGHTGLOW RADIATIONS Intensities are expressed b y the number of transitions per second in a vertical column of cross-section 1 cm 2. It is only possible to give the order of magnitude of this quantity, since these intensities are essentially variable. Furthermore, for certain radiations we must rely on estimates as measurements have not yet been made. The red and green lines of [O I] and the lines of Na I have intensities of the order of l0 s, according to the work of several authors. The total intensity of all the three infra-red bands of OH near l~u is 4.5 x 101° (ROACH, PETTIT, WILLIAMS,
1950). The intensity of the continuum at about 5200/k is of the order of 8 × l05 per Angstrom unit (BARBIER, DUFAY, WILLIAMS, 1951). Other emissions have also been investigated: the (4),1) band of 02 gives 2 × 109; the observable parts of the Herzberg system give l0 s. 6. POLARIZATION OF THE NIGHTGLOW LINES BRICARD and KASTLER (1950) have shown that the red and green oxygen lines are not polarized (or b y less than 1.5 per cent), and that if the yellow sodium lines are polarized, their polarization must also be weak. These workers have used a SAVARTLYOT polariscope; they have shown that this instrument can also be used as a spectral filter. 7. SEASONAL VARIATIONS OF NIGHTGLOW INTENSITY Nearly all extensive series of intensity measurements have been carried out in the Northern hemisphere between 35 ° and 50 ° latitude. These different series are in good agreement. Fig. 1 illustrates the mean of the observed variations. These are based on the work of DUFAY and TCHENa (1947), ROACH and PETTIT (1941b), and BARBIER,DUFAY,and WILLIAMS (1951). The measurements have not been corrected for the effect of the adjacent continuum, nor for the presence of weaker OH line which blend with the [0 I] and Na I lines. The figure shows the four well-established maxima for the green line found at the Observatoire de Haute Provence, but not observed at Cactus Peak in Southern California (RoAcH, HELEN PETTIT, WILLIAMS, ST. AMAND, and DOROTHY DAVIS, 1953). Furthermore, we note the similarity which exists between the green line and the continuum on the one hand, and between the red and yellow lines on the other hand. The seasonal variation of the Herzberg system is very similar to that of the green line (BARBIER, 1953a). Some still unpublished results show that the bands in the blue behave in the same way; the seasonal variation of the other bands is still unknown.
934
The airglow 200
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Fig. l. Seasonal variation of the different components of the nightglow; tile mean intensity of each radiation is taken to be 100
8. DAILY VARIATIONS--STRuCTURE OF THE LAYERS
Up to the present only the three atomic lines and the continuum have been studied from the point of view of finding daily variations. These radiations in general undergo changes in the course of the night, and these intensity variations are different for different parts of the sky, and also are very different from one hour to another. From the observations of ELVEY and FARNSWORTH (1942), ELYEY (1948), DUFAY and TCHENG MAO-LIN (1946), BARBIER, DUFAY, and WILLIAMS (1951), ROACH and PETTIT (1951a), BARBIER (1953a), and of HELEN PETTIT, WILLIAMS, ST. AMAND, DOROTHY DAVIS (1953), we can deduce that on the average: (1) the green ray passes through a maximum somewhat after 04 local time; and (2) the intensity of the red and yellow lines is almost constant throughout the night, although at the beginning of the night they are in generM more intense towards the west, and at the end of the night towards the east (this is referring to the night proper, i.e. excluding twilight phenomena). Luminous patches and regions have frequently been observed in the sky (see, for example, ELVEY, SWINGS, and LINKE, 1941). These phenomena have now been opened to investigation through the introduction of automatic photometers. It is possible to draw charts of the upper atmosphere for a given radiation and a given moment, using observations covering the whole horizon at a constant zenith distance (ROACH and BARBIER, 1950; BARBIER, DUFAY, WILLIAMS, 1951). These charts can be amplified by using observations made at different zenith distances; according to ROACH and PETTIT (1951a, 1951b, 1952) and DAVIS (1951), the modifications of these charts in the course of one night can frequently be explained by imagining the Earth to rotate under a layer fixed in space, which, for the red, yellow, and green radiations, does not vary with time. This would evidently indicate a fixed spaeial pattern for the excitation. BARBIER, DU~AY, and WILLIAMS (1951) find similar results, but frequently also motions from north to south, or south to north, and sometimes the intensity increases or decreases all over the sky without distortion of the pattern. ROACH, HELEN PETTIT, WILLIAMS, ST. AMAND, and DOROTHY DAVIS (1953) have
DANIEL BARBIER
935
recently discussed a series of intensity measurements of the green line extending over four years. MASAAKIHURUHATA,working on the OH infra-red bands, using a nonautomatic photometer, also arrived at the conclusion that there are movements in the north to south direction. 9. VARIOUS CORRELATIOI~S According to published results the intensities of the red [O I] line and the D line of sodium are strongly correlated. There is almost no correlation between the intensities of the red and green [O i] lines (DUFAYand TCHENG,1944), while the intensity of the continuum is strongly correlated with that of the green line (BARBIER,DUFAY, WILLIAMS, 1951). Observations in progress with an eight-colour photometer (BARRIER) show that the radiation of the night sky falls into two groups: (a) 5577, Herzberg bands, blue bands, continuum. (b) 6300, D-lines, OH bands. Within each group the intensities of the radiation varies proportionally during the same night, or (more often still) during the whole of a moonless observation period: the proportionality factor changes from month to month. The radiation in these two groups (a) and (b) can vary independently, but it also happens frequently that these two groups are "coupled", varying in a parallel way during several hours. Numerous authors have looked for a correlation between the intensity of the principal lines of the night sky light and solar activity, magnetic activity, and ionization of the E and F layers. These investigations, however, still present a rather confused picture (see DUFAY, 1943, 1952). 10. THEORETICAL INTERPRETATION
It is outside the scope of this article to make a complete study of the problems involved in the interpretation of the luminous emission of the night sky. On this subject the reader will wish to consult the papers by BATES (1948) and NICOLET (1951). The proposed hypotheses can be divided into three classes: (A) The solar energy is stored during the day in the atmosphere as a result of the dissociation of molecules and the ionization of the atoms and molecules; it is then re-emitted during the night by various mechanisms: (Aa) Radiative recombination, e.g. : 0 ÷ 0 = O~ t - hr.
This mechanism would lead to the emission of the Herzberg bands and of the atmospheric bands of 02. (Ab) Recombination by triple collisions, e.g. : 0÷0÷0=0~-?0'. The excited oxygen atom would then emit the green and red lines while returning to its normal state (CHAPMAN). (Ac) Chemical reaction, e.g. : O~ ÷ H = OH' ÷ 0 2.
936
The airglow
This reaction, proposed b y BATES and NICOLET, could explain the bands of OH. CHAPMAN has also proposed the reaction: N a 0 ÷ 0 ---- Na' ÷ 02. (Ad) Electronic, radiative, dissociative, and ionic recombinations. All mechanisms of class (A) encounter difficulties, since they would be particularly effective at altitudes of about 100 km, which is very much below those established by observation. Furthermore, they suffer from the drawback that they require a specific behaviour for each of the groups, in contradiction to the observed correlations. (B) Excitation due to the impact of interplanetary particles, particularly electrons or cosmic dust, upon the atmosphere. Proposals of this kind have been made by ELVEY, BIERMANN, and TE~ BRUGGENCATE. (C) Excitation b y electronic collisions, after atmospheric electrons have been accelerated b y electric fields. The difficulty is to maintain the existence of an electric field within a highly conducting medium. WULF (see the article by ROAC~ and PETTIT, 1952) suggests that such a field could produce itself in streams of ionized gas under the influence of the magnetic field of the Earth. One can further imagine that the separation of the interplanetary particles by the terrestrial magnetic field, on account of their different sizes and masses, or through differences in their depths of penetration into the atmosphere, may give rise to an electric field. Theories of type B or C have not yet been properly developed. It appears probable, however, that theories of these types will be required in order to explain emissions from great heights. I 1. THE TWILIGHT GLOW The emission spectrum of the twilight glow consists of the D lines of sodium, the forbidden lines of [0 ~] 6300-6364 and 5577 (this line much the weakest of the three), the forbidden line of [N I] at 5199 and the negative system of N2+. The D lines. The observations of BRICARD and KASTLER (1944) leave no doubt that these lines are emitted by optical resonance: they are narrow like those produced in absorption in a tube of sodium vapour; they are weakly polarized perpendicularly to the Sun (BRIcAI~D and KASTLER, 1950). These authors also explain how other observers could have arrived at different results (see BRICARD and KASTLER, 1952). The total number of sodium atoms in a column of unit cross-section is of the order of 5 X l0 s to 109 (BRICARDand KASTLER, 1950; BARBIER,1948), and varies from day to day. One can account in a somewhat schematic manner for the observations in terms of an exponential decrease with height of the number of sodium atoms with a scale height of 8 km, i.e. approximately the same as that for the total number of molecules. It must, however, be assumed that there are no sodium atoms below 70 km (BARBIER, 1948). Because of their very great intensity, the D lines in the twilight glow have been recorded up to 800 km (BARRIER and ROACH, 1950). At these altitudes the scale height is large (250 km). KVIFTE (1953) also adopts the fluorescence mechanism for the emission of the sodium lines, but thinks that the observations by the zenith-horizon method favour an absorbing layer at 40 km altitude.
DANIEL BARBIER
937
BLAMONT and KASTLER (1951), using a photoelectric polarization photometer, were able to follow the twilight emission up to 200 km, where the scale height was about 100 km. The origin of the atmospheric sodium is perhaps terrestrial, from sea spray or volcanic dust (according to the ideas of DEJARDIN and BERNARD, 1938); or cosmical, from meteorites or cosmic dust (according to CABANNES, DUFAr, and GAVZIT, 1938). The presence of free sodium atoms is prevented at low altitudes by recombination with other atoms to form molecules, for instance, NaO. Also the distribution of neutral atoms will be partly conditioned by the ease with which they are ionized. The [0 I] radiation. The emission of the red lines has been followed up to a height of 1000 km b y CABANNESand GARRIGUE (1936) ; by ELVEY and FARNSWORTH(1942) ; and by ELVEY (1948). The apparent scale height is very great; it is probably controlled at low altitudes b y collisional de-excitation. The weak enhancement of the green line in the twilight glow is difficult to establish, but according to DUFAY and GAUZIT (1947) it certainly takes place. It requires the absorption of a quantum of 2972 A, after which the excited atom can return to its ground state b y the successive emission of 5577 and of 6300-6364. The 5199 line of [N I]. Recently discovered b y CouaT~s (1950), this line has been studied b y M. DUFAY (1952). It is particularly intense in summer; its intensity is of the order of one-tenth of the intensity of the green line in the airglow. It will be emitted when an atom returns to the ground state after absorbing a quantum of 3466/k and emitting one of 10,406 A. This line has a|so been observed by NICOLET and PASTIELS (1952). Bands ofN~ +. The most recent data are due to M. DVFAr (1949). The intensity of these bands is very variable and the altitude of their emission is 100 km. These results are confirmed b y SwI~os and NICOLET (1949). They also show that the bands of N~+ behave in the same way at dawn and dusk twilights, and that there is no correlation between the intensities of Ne + and Na. BATES (1949) has shown that a simple fluorescence mechanism is more efficient for producing these bands than one by which neutral molecules are simultaneously ionized and excited. The intensity of the 3914 band can be as great as 109 transitions per second in a slant column of unit cross-section at 75 ° zenith distance; this requires about 2 × l01° ions in a vertical column of unit cross-section. It is of interest to compare the twilight radiation with that from comets, the light of which is also excited by fluorescence (BARBIER, 1953). This comparison brings out the difference between the chemical composition of the comets and the terrestrial atmosphere, and removes a difficulty concerning the explanation of the existence of the N2+ ions in the comets. 12. CONCLUSIONS
We already understand in its main outline the radiation of the twilight; many details still remain obscure but we may hope that their elucidation will throw more light on the state of the upper atmosphere. The origin of the nightglow, at least as far as the radiations in high altitudes are concerned, is still unknown. We must ask which development should be aimed at in the observing programmes, in order to give the maximum assistance to the theoreticians. It is possible to think of qualitative improvements of the observations, for example,
938
The airglow
by increasing the resolution or the light gathering power of the spectrographs, or by improving the spectral purity of the radiation recorded by photoelectric photom e t e r s . H e r e p r o g r e s s is s t i l l p o s s i b l e ; w e h a v e n o t y e t r e a c h e d t h e l i m i t o f t h e technical possibilities, but it would not be right to overlook the fact that relatively minor gains will be extremely costly. Q u a n t i t a t i v e i m p r o v e m e n t o f t h e o b s e r v a t i o n s will c e r t a i n l y g i v e g r e a t r e t u r n s for some years to come: a large number of radiations have not yet been observed regularly; the observing stations are not yet evenly distributed over the Earth, and t r i a n g u l a t i o n s b e t w e e n s t a t i o n s s o m e 500 k m a p a r t s h o u l d b e p o s s i b l e . The introduction of new techniques remains a possibility. Here we can hope for instruments to be sent up into the upper atmosphere on rockets; or for direct e x p e r i m e n t s t o b e c o n d u c t e d o n t h e a t m o s p h e r e (for e x a m p l e , w e m a y q u o t e t h e p r o p o s a l o f BATES, 1950, t o "SOW" s o d i u m a t o m s i n t o t h e u p p e r a t m o s p h e r e ) . T h e r e s u l t s will l e a d t o a n e x t e n s i o n o f o u r k n o w l e d g e o f g r e a t a l t i t u d e s , a n d o f all aspects of the intensity variation, the structure of the emission layers, and the correlation of the intensities of the radiations between themselves or with cosmic or geophysical phenomena.
kEFEREN('ES
ABAD:~, P., VASSY, A. and VASSY, E . ARNULY, A . . . . . . . . . . BASGOCK, H. D . . . . . . . . . BABCOCK, H . . . . . . . . . .
. . . . . . . . . .
1945 1943 1923 1939
Ann. Gdophys., 1, 189. Ann. Astrophys., 6, 2l. Ap. J., 57, 209. Publ. Astron. Soc. Pac~ific, 51, 47.
1953 1950 1944 1947 1948 1952a 1952b
Ann. Gdophys., 9, 309. Ap. J., 111, 201. Ann. Gdophys., 1, 144. Ann. Astrophys., 10, 47 and 141. Ann. Gdophys., 4, 193. Ann. Astrophys., 15, 247. ~tude opti9ue de la haute atmosphere.
1953
La Physique des Com~tes, Colloque de Liege; 263. Ann. Astrophys., 16, 96. C.R. Acad.Sci. (Paris), 287, 599. Contrib.
BAILLET, i . , BARBIER, D., BOSSON, F., LALLEMAND,
A. and MAOI3ErCY,J . . . . . . . . . BAPPU, V . . . . . . . . . . . . . BARBIER, D . . . . . . . . . . . .
Colloque de Liege; 43. 1953a 1953b BARBIER, D., DUFAY, J. and WILLIAMS, D. BARBIER, D. and ROACH, F. E . . . . . . . BATES, D. R . . . . . . . . . . . .
BERTHIER, P . . . . . . . . . . . . BLAMONT, J. S. a n d KASTLER. A . . . . . . BRANSCOMB, L. M . . . . . . . . . . BRICARD, J. a n d KASTLER, A . . . . . . .
1951 1950 1948 1949 1950 1953 1951 1950 1944
1950 1952 CABA~ES, J. and DUFAY, J . . . . . . .
1944 1946 1947
CABA~ES, J., DUFXY, J. and DUFAY, M . . CABANNES,J., DUFAY, J. and GAUZIT,J. C-~B~-~.S, J. and G)~tRIOUE, H . . . . . . C~VOSTIKOV, I. A . . . . . . . . . .
1950 1938 1936 1948
Inst. Astrophys. A, No. 151. Ann. Astrophys., 14, 399. Trans. Amer. Geophys. Un., 31, 13. The Emission Spectra of the Night S k y and Aurorae, London; 21. Proc. Roy. Soc., 196, 562. J. gcophys. Res., 55, 347. C.R. Acad. Sci. (Paris), 287, 928. Ann. Gdophys., 7, 73. Phys. Rev., 79, 619. Ann. Gdophys., 1, 53. Ann. Gdophys., 6, 286. L'dtudc optique de l'atmosph~re terrestre,
Liege, 87.
Ann. Gdophys., 1, 1. Ann. Gdophys., 2, 290. Relations entre lee phdnom~nes solaires et gdophysiques, Lyon, 217. C.R. Acad. Sci. (Paris), 280, 1233. C.R. Acad. Sci. (Paris), 206, 1525. C.R. Acad. Sci. (Paris), 208, 484. Lumi~re du ciel nocturne. Acad. Sci.
U.S.S.R., Moscow.
DANIEL BARBIER
939
1934
I~tude lumi~re ciel nocturne, E d i t . R e v u e
1947 1950 1951 1936 1938 1928 1943 1947 1951 1952
Ann. Astrophys., 10, 33. C.R. Acad. Sci. (Paris}, ~,~1, 62. J. geophys. Res., 56, 567. Rev. rood. Phys., 8, 1. C.R. Acad. Sci. (Paris}, 207, 81. Bull. Obs. Lyon, 10, (9); Thesis. Ann,. Univ. de Lyon, lr Ann,. Gdophys., 8, 1. Ann,. Gdophys., 7, 1. lntern. Astron. Union Draft Report, R o m e ,
DUFAY, J., BERTHIER, P. a n d MORIGNAT, B. DUFAY, J. a n d GAUZIT, J . . . . . . . .
1953 1947
DUFAY, J. a n d TCHENG MAo-LIN
.
1946 1947 1949 1951 1952
C.R. Acad. Sci. (Paris}, 237, 828. Relations entre les phdnom~nes solaires et ge~ophysiques, L y o n ; 245. Ann. G~ophys., 2, 189. Ann. Gdophys., 8, 153. Ann. Gdophys., 5, 2. C.R. Acad. Sci. (Paris}, 232, 2344. L'dtude optique de l'atmosph~re terrestre,
.
1953 1942 1948
CoJA~, J .
.
.
.
.
.
.
.
.
.
.
.
.
O p t i q u e , P a r i s ; 43. COURT~S, G . DAvIs, D. N . DEJA~tDIN, G . DEJARDIN, G. D UFAY, J . .
.
.
. . . . . . . . . . . . . . . a n d BERNARD, R . . . . . . . . .
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. . . . . . . . . . .
. . . . . .
138.
DUFAY, M .
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ELVE¥, C. T .
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. . . . .
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.
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Liege, 141. Thesis, L y o n .
Rev. Mod. Phys., 14, 140. The Emission Spectra of the Night Sky and Aurorae, L o n d o n ; 16. Ap. J., 96, 451. Ap. J., 85, 213. Ap. J . , 93, 337. Ann. Phys. (Paris}, S~r. 11, 16, 253. Ann. Astrophys., 3, 117 (also Ap. J., 1941,
ELVEY, C. T. a n d FARNSWORTH,A. H . . . . . ELVEY, C. T. a n d ROACH, F. E . . . . . . . ELVEY, C. T., SWINGS, P. a n d LINKE, % V . . GRANDMONTAGNE, R . . . . . . . . . . HENYEY, L. G. a n d GREENSTEIN, J . L .
1942 1937 1941 1941 1940
HERZBERG, G . . . . . . HULBURT, E. O . . . . . HUNAERTS, J. a n d NICOL~T, KRASSOVSKY, V. J . . . .
NICOLET, M . . . . . . . . . . . .
1953 1949 1950 1949 1950 1953 1949 1948 1950a 1950b 1950c 1950d 1951 1953 1951
NICOLE% M. a n d PASTIELS, R . . . . . . .
1952
Canad. J. Phys., 31, 657. J. Opt. Soc. Amer., 89, 211. Ciel et Terre, 66, 213. Dokl. Acad. Sci. U.S.S.R., 66, No. 1. Dokl. Acad. Sci. U.S.S.R., 70, No. 6. Univ. Fys. Inst. Oslo, Meddel. No. 205. Rev. sci. Instrum., 20, 724. Publ. Astron. Soc. Pacific, 60, 317. Trans. Amer. Geophys. Union, 31, 21. Ap. J., 111, 206. Ap. J., 112, 120. Ap. J., 112, 464. Reports Progr. Physics, 14, 121. Ap. J., 118, 200. Relations entre les phdnom~nes solaires et terrestres ; rapport 7, Paris. l~tude optique de la haute atmosphere,
ROACH, F. E. a n d BARBIER, D . . . . . . . ROACH, F. E. a n d PETTIT, H . B . . . . . .
1950 1951a 1951b 1952
Trans. Amer. Geophys. Union, 31, 7. J. geophys. Res., 56, 325. Ann. Astrophys., 14, 399. ~tude optique de la haute atmosphere,
ROACH, F. E., PETTIT, H . a n d WILLIAMS,D . R . . ROACH, F. E., PETTIT, H . B., WILLIAMS, D. R.,
1950
J. geophys. Res., 55, 183.
SPITZER, L., J r . . . . . . . . . . .
1953 1949
Ann. Astrophys., 16, 185. Atmospheres of the Earth and Planets.
SWINGS, P .
1949
Atmospheres of the Earth and Planets.
1949
Ap. J., 109, 327.
. . . . M. . . .
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KVIFTE, G . . • . . . . . . . . . . MARLOW, D. a n d PEMBERTON, J. C . . . . . MEINEL, A. B . . . . . . . . . . . .
98, 70).
Colloque de Liege, p. 147.
Colloque de Liege. ST. AMAND, P., DAVIS, D. N . . . . . . .
( E d i t e d b y G. P. KUIPER), Chicago. .
.
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( E d i t e d b y G. P. KUIPER), Chicago. SWINGS, P. a n d NICOLET, M .
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NEWCOMB, in 1901, measured for the first time the intensity of the light of the night sky. He attributed it to the integrated effect of unresolved stars and thought that he thus obtained a quantity of the utmost importance for studies of the structure of the Universe. However, his measurements, and those of later investigators, show in fact that the light of the night sky is brighter than that of stellar origin as indicated by star counts. V. M. SLIPHER in 1919, and later Lord RAYLEIGtt in 1922, were able to show that our atmosphere possesses its own light, due to radiations amongst which the green line had already been observed in the aurora. In 1933 SLIPItER showed further that the emission spectrum which we observe in the twilight differs from that emitted during the night. In this short survey it is impossible to consider in detail all papers published on the subject of the airglow*; excellent summaries can be found elsewhere (DuFAY, 1928; DEJARDIN, 1936; ELVEY, 1942; DUFAY, 1943; SwrNGS, 1949; CHVOSTIKOV, 1948 ; MEINEL, 1951 ; DUFAY, 1952a) and we shall restrict ourselves here to discussing the present status of the problem. We must mention, however, the pioneers who, in spite of the rather primitive tools at their disposal, paved the way for their successors for the modern phase of these studies: to the above-mentioned names we add Y N T E M A , B U R N S , A B B O T , V A N R H I J N , F A B R Y , F E S S E N K O F F , CABANNES, G A R R I G U E ,
and B E R N A R D . The following components of the light from the night sky may be distinguished: (a) Stellar light. Stars of about 12m play the most important role and their number compensates for their faintness. (b) Zodiacal light. We are concerned not only with the light pyramid visible in the ecliptic when the Sun is not far below the horizon, but also with the light which stretches over the whole sky, being brightest along the ecliptic and intensified in the point opposite to the Sun (Gegenschein). Charts of the stellar light and of the zodiacal light were obtained by ELVE¥ and ROACH (1937), and published by ROACH and PETTIT (1952). It must not be overlooked that the zodiacal light is possibly subject to variations with time. (c) Galactic light. Here we have to deal with stellar light scattered in interstellar space. First noticed by ELVEY and ROACH (1937), its existence was confirmed by HENY~r and GREENSTEIN (1940). It is limited to the neighbourhood of the galactic plane. GAUZIT,
(d) Airglow of the upper atmosphere. (e) Auroral phenomena, particularly in northern latitudes. * Following a nomenclature recently proposed by ELV~Y, the emission by the atmosphere itself, excluding the aurorae, may be called the "airglow". The night sky emission can then be called the "nightglow", the twilight emission the "twilightglow", and the as yet unobserved daytime emission the "dayglow". 929
930
The airglow
The present survey is especially concerned with the study of the nightglow; in addition, a survey of the twilight glow is given. The study of the emission spectra of the terrestrial atmosphere interests geophysicists who hope to deduce from it information as to the state of the upper atmosphere. It is equally interesting to the astronomers, since the state of the upper atmosphere is conditioned by effects (ionization, dissociation, high temperature) produced by penetrating solar radiations (ultra-violet and X-rays) and also, because the excitation phenomena, as soon as these are better understood, will without doubt give valuable information about the corpuscular solar radiation, and perhaps about interplanetary matter. 1. INSTRUMENTS The intensity of the airglow is minute--it is the faintest extended light source which has been studied in astronomy; it is only very little in excess of that of a fifth magnitude star spread over 5 square degrees (DuFAY, 1928). Expressed in candle-powers per square centimetre it amounts to 4 × 10-s (HuLBURT, 1949); for comparison we may mention that the daytime blue sky has a brightness of about one candle-power per square centimetre. This means that the most powerful instruments have to be employed. Without going into the details of this subject, we mention amongst the spectrographs which have been used during the last few years the prism spectrographs and objectives calculated by COJAN (1934, 1947), the spectrographs of the Arnulf-Lyot type (ARNuLF, 1943), the grating spectrograph and the Schmidt camera of •EINEL (1948), the prism spectrograph combined with an electron image converter by KRASSOVSK¥ (1949, 1950). Quite a number of photoelectric photometers have been used, amongst which may be mentioned the instruments of ELVEY and ROACH (1937), of GRANDMONTAGNE ( 1941 ), of ABADIE, A. VASSY, and E. VAss¥ (1945), of MARLOW and PEMBERTO:N (1949), of ROACH and PETTIT (1951a), as well as an instrument recently installed at the Observatoire de Haute Provence (BAILLET, BARBIER, BOSSON, LALLEMAND, MAGUERY, 1953). The use of photometers has become much more popular since the invention of interference filters, which have a half-width of the order of 50 A. This width is scarcely greater than is obtained from spectrographs used with very wide slits in order to reduce the exposure times, or to facilitate the photographic-photometric measurements. Furthermore, the speed of photometers equipped in this way is many thousand times greater than that of spectrographs, and permits a detailed exploration of the sky using automatically recording instruments. 2. THE SPECTRUM OF THE ~IGHTGLOW The best descriptions of the various different spectral regions which are available at present are: ll,000 to 9000A (KRAssovsKY, 1949, 1950); 9000 to 7000A (MEINEL, 1950a, 1951) ; 7000 to 5800 ~ (CABA~NES, J. DUFAY, and M. DUFAY, 1950) ; 5000 to 4000 ~ (CABANNES, DUFAY, 1944); 4000 to 3000A (BARBIER, 1947). The green spectral region, between 5800 A and 5000 A, is the least well-known, and observations on it are not always in good agreement; these are due to H. BABCOCK (1939); VAINU BAPPU (1950); BARBIER, DUFAY, and WILLIAMS (1951); and M. DUFAY (1951). The following atomic lines sometimes called the green, red, and yellow lines,, have been identified with certainty and are the most easily observed radiations: [0 I] 5577, [O I] 6300-6364, and Na I 5890-5896.
DANIEL BARBIER
931
Molecular systems also give rise to a large number of emissions. The vibration-rotation system of OH was discovered and identified by Mm~EL (1950b), and is very intense in the infra-red. It extends with appreciable intensity into the red (CABAN~ES, J. DUFAY, and M. DUFAr, 1950), with less intensity into the green (M. DUFAY, 1951), and it can possibly be observed in the blue (HuNAERTS and NICOLET, 1950). The atmospheric system of 0 n emits the (0, 1) band which appears at 8645/~ in the form of a doublet (MEINEL, 1950d), and perhaps the (0, 2) band at 9965 A (DUFAY, 1951). The (0, 0) band cannot be observed, as it is re-absorbed in the lower atmosphere. The bands in the blue region of the spectrum have been attributed for a long time, following a suggestion of CABANNESand DUFAY (1946), to the Vegard-Kaplan system of N~, but this identification appears now doubtful (BARBIER, 1953b). The ultra-violet part of the spectrum is characterized by the Herzberg bands of 0 n (DuFAr, 1947). All these band systems are forbidden, excepting that of OH, whose observed radiations have, however, only very small transition probabilities, since they are all high overtones. A certain number of medium and faint bands which have previously remained unidentified certainly form part of the new system 1Z~--aE~- of On, recently discovered by ttERZBERG (1953). The presence of the (0, 0) band of N~+ in the spectrum of the nightglow has been confirmed by M. DUFAY (1953) and MEINEL (1953). According to B~BIER (1953) the profile of the band sometimes resembles that of the N2+ band, but often differs from it. Spectra of the nightglow also show a continuum. This is partly due to the light from the stars, and it shows the presence of the Fraunhofer lines F, G, h, H, K. These, however, do not have the same relative intensities as in the Sun, because of the presence of A-type stars in the stellar population (I~¢IEINEL,1953). The discontinuity of the continuum at about 4000 A recalls spectra of stars of the solar type, with a depression at 3883 A produced by the CN band. It is now certain that the continuum is not completely explained by the light from the stars. Its altitude may be determined, as will be discussed further below, and one does not find an infinite value, but something like 10,000 km in the neighbourhood of the D-line (BARBIER, 1944), 1600 km for 4500 A, and 500 km for 3500 ~ (BARBIER, 1947). This shows that a part of this spectrum is produced in our atmosphere. An hypothesis which attempts to explain the continuum as due to the wings of very intense lines (BA~BIER, 1947) appears to be applicable in the blue and ultra-violet but cannot be supported in the green where bands are very weak (BARBIER, DUFAr, WILLIAMS, 1951). There exists therefore a true continuous spectrum of atmospheric origin in the light of the night sky. 3. HEIGHTS OF AIRGLOW EMISSIONS
The usual method, known as VAN RHIJN'S method, is based on the comparison of two intensity measurements performed at the zenith and at a rather large zenith distance ~. These are related by the equation: 1(0=I(o).
where R is the radius of the Earth.
1
(R+H),J
'
932
The airglow
This formula assumes: (a) That the emission takes place in a thin layer at height H. It is easy to show that if the emission layer is thick this formula remains valid up to zenith distances of the order of 80 °, and H is then a kind of mean value for the height of the emitting atoms (BARBIER, DUFAY, WILLIAMS, 1951). (b) It also assumes that the observations have been corrected for the whole extraterrestrial radiation, for atmospheric extinction and scattering, and for the geographical non-uniformity of the layer. When using filters or spectrographs of small resolution, it is most important to eliminate the effect of extra-terrestrial radiation; this has been emphasized by the analysis of ROACH and PETTIT (1952). In order to do so one must make a detailed analysis of numerous quantities covering an observing period of about one year (BARRIER, DUFAY, WILLIAMS, 1951); or, alternatively, a comparison can be made between the radiation concerned and that from a neighbouring spectral region (RoAcH, BARBIER, 1950). The problem of atmospheric scattering is extremely complicated; a simple solution has been obtained for the case of small scattering coefficients (BARRIER, 1944). A more exact solution which is valid for the ultra-violet spectrum has also been made (BARBIER, 1952a). The non-uniformity of the layers has been made the subject of detailed studies (ROACH and BARBIER, 1950; BARBIER, DUFAY, WILLIAMS, 1951); we shall return to this question below. This effect can be eliminated by using the mean of a large number of observations. A general discussion of height measurements has been given by BARBIER (1952b). This should be supplemented by the following recently measured altitudes: Herzberg system of 0 2 : 2 0 0 kin, very uncertain, according to BARBIER (1953a). Green line: 250 km (DuFAY, BERTHIER, MORIGNAT, 1953). OH bands and the atmospheric system of O2:140-150 km (BERTHIER, 1953). The present opinion of the author of this survey is that it is most likely that all radiation comprising the nightglow is emitted from the region near 200 km. This view is strengthened by the observed correlations between the intensities of the various bands. We shall see below that the light of the night sky shows irregularities. It is hoped that it will be possible in future to measure these heights by simple triangulation. 4. TEMPERATURES OF THE EMITTING LAYERS
(a) Kinetic temperature. The kinetic temperature may be deduced from the Doppler broadening of a monochromatic line. H. D. BABCOCK (1923) obtained "sharp" interference fringes of the order of 85,000. According to the significance placed on the word "sharp", this can mean that the maximum temperature for the green emission line lies anywhere between 680°K and 1700°K (SPITZER, 1949). It is desirable for interferometrie measurements to be repeated with higher orders of interference. (b) Rotational temperatures. MEINEL (1950C) obtained for the (0,1) atmospheric band of 02 a reasonably consistent temperature of 150 ° 4- 20°K, although in one night of anomalously great intensity he measured 200 ° 4- 10°K. The OH bands give good determinations since they are resolved into lines. MEINEL (1950d) obtained a temperature of 260 ° 4- 50K.
DANIEL BARBIER
933
The derivation of rotational temperatures is only justified if collisions are frequent enough for the molecules to distribute themselves between the different rotational levels according to BOLTZMANN'S law. This point has been treated b y BRANSCOMB (1950). (C) Vibrational temperature. From the ratio of intensities of components due to the doublet structure of the ground state of OH, MEINEL (1950d) has derived a temperature of 172 ° ~- 10°K, which does not agree with the rotational temperature. However, I~¢[EINEL can explain the discrepancy if he assumes that the maximum excitation energy is 3.242 eV. 5. THE ABSOLUTE INTENSITIES OF THE NIGHTGLOW RADIATIONS Intensities are expressed b y the number of transitions per second in a vertical column of cross-section 1 cm 2. It is only possible to give the order of magnitude of this quantity, since these intensities are essentially variable. Furthermore, for certain radiations we must rely on estimates as measurements have not yet been made. The red and green lines of [O I] and the lines of Na I have intensities of the order of l0 s, according to the work of several authors. The total intensity of all the three infra-red bands of OH near l~u is 4.5 x 101° (ROACH, PETTIT, WILLIAMS,
1950). The intensity of the continuum at about 5200/k is of the order of 8 × l05 per Angstrom unit (BARBIER, DUFAY, WILLIAMS, 1951). Other emissions have also been investigated: the (4),1) band of 02 gives 2 × 109; the observable parts of the Herzberg system give l0 s. 6. POLARIZATION OF THE NIGHTGLOW LINES BRICARD and KASTLER (1950) have shown that the red and green oxygen lines are not polarized (or b y less than 1.5 per cent), and that if the yellow sodium lines are polarized, their polarization must also be weak. These workers have used a SAVARTLYOT polariscope; they have shown that this instrument can also be used as a spectral filter. 7. SEASONAL VARIATIONS OF NIGHTGLOW INTENSITY Nearly all extensive series of intensity measurements have been carried out in the Northern hemisphere between 35 ° and 50 ° latitude. These different series are in good agreement. Fig. 1 illustrates the mean of the observed variations. These are based on the work of DUFAY and TCHENa (1947), ROACH and PETTIT (1941b), and BARBIER,DUFAY,and WILLIAMS (1951). The measurements have not been corrected for the effect of the adjacent continuum, nor for the presence of weaker OH line which blend with the [0 I] and Na I lines. The figure shows the four well-established maxima for the green line found at the Observatoire de Haute Provence, but not observed at Cactus Peak in Southern California (RoAcH, HELEN PETTIT, WILLIAMS, ST. AMAND, and DOROTHY DAVIS, 1953). Furthermore, we note the similarity which exists between the green line and the continuum on the one hand, and between the red and yellow lines on the other hand. The seasonal variation of the Herzberg system is very similar to that of the green line (BARBIER, 1953a). Some still unpublished results show that the bands in the blue behave in the same way; the seasonal variation of the other bands is still unknown.
934
The airglow 200
IS°L
~ ~ , ~
[
ioo
o,]
SOt ~ O t J
5577~,
i i h , F M A M J
,
, , i ~ i J AS ON
i D
i J
Ioo 5o 0
,
,
,
,
,
F M A M J
,
i
L
i
i
J
A S O N D J
,
L L i t A S O N D J
t
i
,
i
200
t "--/
tOO 01
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J
CONTI S2OONUUNP o o
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i J
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0
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FM
,
i
AMJ
i
,
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Fig. l. Seasonal variation of the different components of the nightglow; tile mean intensity of each radiation is taken to be 100
8. DAILY VARIATIONS--STRuCTURE OF THE LAYERS
Up to the present only the three atomic lines and the continuum have been studied from the point of view of finding daily variations. These radiations in general undergo changes in the course of the night, and these intensity variations are different for different parts of the sky, and also are very different from one hour to another. From the observations of ELVEY and FARNSWORTH (1942), ELYEY (1948), DUFAY and TCHENG MAO-LIN (1946), BARBIER, DUFAY, and WILLIAMS (1951), ROACH and PETTIT (1951a), BARBIER (1953a), and of HELEN PETTIT, WILLIAMS, ST. AMAND, DOROTHY DAVIS (1953), we can deduce that on the average: (1) the green ray passes through a maximum somewhat after 04 local time; and (2) the intensity of the red and yellow lines is almost constant throughout the night, although at the beginning of the night they are in generM more intense towards the west, and at the end of the night towards the east (this is referring to the night proper, i.e. excluding twilight phenomena). Luminous patches and regions have frequently been observed in the sky (see, for example, ELVEY, SWINGS, and LINKE, 1941). These phenomena have now been opened to investigation through the introduction of automatic photometers. It is possible to draw charts of the upper atmosphere for a given radiation and a given moment, using observations covering the whole horizon at a constant zenith distance (ROACH and BARBIER, 1950; BARBIER, DUFAY, WILLIAMS, 1951). These charts can be amplified by using observations made at different zenith distances; according to ROACH and PETTIT (1951a, 1951b, 1952) and DAVIS (1951), the modifications of these charts in the course of one night can frequently be explained by imagining the Earth to rotate under a layer fixed in space, which, for the red, yellow, and green radiations, does not vary with time. This would evidently indicate a fixed spaeial pattern for the excitation. BARBIER, DU~AY, and WILLIAMS (1951) find similar results, but frequently also motions from north to south, or south to north, and sometimes the intensity increases or decreases all over the sky without distortion of the pattern. ROACH, HELEN PETTIT, WILLIAMS, ST. AMAND, and DOROTHY DAVIS (1953) have
DANIEL BARBIER
935
recently discussed a series of intensity measurements of the green line extending over four years. MASAAKIHURUHATA,working on the OH infra-red bands, using a nonautomatic photometer, also arrived at the conclusion that there are movements in the north to south direction. 9. VARIOUS CORRELATIOI~S According to published results the intensities of the red [O I] line and the D line of sodium are strongly correlated. There is almost no correlation between the intensities of the red and green [O i] lines (DUFAYand TCHENG,1944), while the intensity of the continuum is strongly correlated with that of the green line (BARBIER,DUFAY, WILLIAMS, 1951). Observations in progress with an eight-colour photometer (BARRIER) show that the radiation of the night sky falls into two groups: (a) 5577, Herzberg bands, blue bands, continuum. (b) 6300, D-lines, OH bands. Within each group the intensities of the radiation varies proportionally during the same night, or (more often still) during the whole of a moonless observation period: the proportionality factor changes from month to month. The radiation in these two groups (a) and (b) can vary independently, but it also happens frequently that these two groups are "coupled", varying in a parallel way during several hours. Numerous authors have looked for a correlation between the intensity of the principal lines of the night sky light and solar activity, magnetic activity, and ionization of the E and F layers. These investigations, however, still present a rather confused picture (see DUFAY, 1943, 1952). 10. THEORETICAL INTERPRETATION
It is outside the scope of this article to make a complete study of the problems involved in the interpretation of the luminous emission of the night sky. On this subject the reader will wish to consult the papers by BATES (1948) and NICOLET (1951). The proposed hypotheses can be divided into three classes: (A) The solar energy is stored during the day in the atmosphere as a result of the dissociation of molecules and the ionization of the atoms and molecules; it is then re-emitted during the night by various mechanisms: (Aa) Radiative recombination, e.g. : 0 ÷ 0 = O~ t - hr.
This mechanism would lead to the emission of the Herzberg bands and of the atmospheric bands of 02. (Ab) Recombination by triple collisions, e.g. : 0÷0÷0=0~-?0'. The excited oxygen atom would then emit the green and red lines while returning to its normal state (CHAPMAN). (Ac) Chemical reaction, e.g. : O~ ÷ H = OH' ÷ 0 2.
936
The airglow
This reaction, proposed b y BATES and NICOLET, could explain the bands of OH. CHAPMAN has also proposed the reaction: N a 0 ÷ 0 ---- Na' ÷ 02. (Ad) Electronic, radiative, dissociative, and ionic recombinations. All mechanisms of class (A) encounter difficulties, since they would be particularly effective at altitudes of about 100 km, which is very much below those established by observation. Furthermore, they suffer from the drawback that they require a specific behaviour for each of the groups, in contradiction to the observed correlations. (B) Excitation due to the impact of interplanetary particles, particularly electrons or cosmic dust, upon the atmosphere. Proposals of this kind have been made by ELVEY, BIERMANN, and TE~ BRUGGENCATE. (C) Excitation b y electronic collisions, after atmospheric electrons have been accelerated b y electric fields. The difficulty is to maintain the existence of an electric field within a highly conducting medium. WULF (see the article by ROAC~ and PETTIT, 1952) suggests that such a field could produce itself in streams of ionized gas under the influence of the magnetic field of the Earth. One can further imagine that the separation of the interplanetary particles by the terrestrial magnetic field, on account of their different sizes and masses, or through differences in their depths of penetration into the atmosphere, may give rise to an electric field. Theories of type B or C have not yet been properly developed. It appears probable, however, that theories of these types will be required in order to explain emissions from great heights. I 1. THE TWILIGHT GLOW The emission spectrum of the twilight glow consists of the D lines of sodium, the forbidden lines of [0 ~] 6300-6364 and 5577 (this line much the weakest of the three), the forbidden line of [N I] at 5199 and the negative system of N2+. The D lines. The observations of BRICARD and KASTLER (1944) leave no doubt that these lines are emitted by optical resonance: they are narrow like those produced in absorption in a tube of sodium vapour; they are weakly polarized perpendicularly to the Sun (BRIcAI~D and KASTLER, 1950). These authors also explain how other observers could have arrived at different results (see BRICARD and KASTLER, 1952). The total number of sodium atoms in a column of unit cross-section is of the order of 5 X l0 s to 109 (BRICARDand KASTLER, 1950; BARBIER,1948), and varies from day to day. One can account in a somewhat schematic manner for the observations in terms of an exponential decrease with height of the number of sodium atoms with a scale height of 8 km, i.e. approximately the same as that for the total number of molecules. It must, however, be assumed that there are no sodium atoms below 70 km (BARBIER, 1948). Because of their very great intensity, the D lines in the twilight glow have been recorded up to 800 km (BARRIER and ROACH, 1950). At these altitudes the scale height is large (250 km). KVIFTE (1953) also adopts the fluorescence mechanism for the emission of the sodium lines, but thinks that the observations by the zenith-horizon method favour an absorbing layer at 40 km altitude.
DANIEL BARBIER
937
BLAMONT and KASTLER (1951), using a photoelectric polarization photometer, were able to follow the twilight emission up to 200 km, where the scale height was about 100 km. The origin of the atmospheric sodium is perhaps terrestrial, from sea spray or volcanic dust (according to the ideas of DEJARDIN and BERNARD, 1938); or cosmical, from meteorites or cosmic dust (according to CABANNES, DUFAr, and GAVZIT, 1938). The presence of free sodium atoms is prevented at low altitudes by recombination with other atoms to form molecules, for instance, NaO. Also the distribution of neutral atoms will be partly conditioned by the ease with which they are ionized. The [0 I] radiation. The emission of the red lines has been followed up to a height of 1000 km b y CABANNESand GARRIGUE (1936) ; by ELVEY and FARNSWORTH(1942) ; and by ELVEY (1948). The apparent scale height is very great; it is probably controlled at low altitudes b y collisional de-excitation. The weak enhancement of the green line in the twilight glow is difficult to establish, but according to DUFAY and GAUZIT (1947) it certainly takes place. It requires the absorption of a quantum of 2972 A, after which the excited atom can return to its ground state b y the successive emission of 5577 and of 6300-6364. The 5199 line of [N I]. Recently discovered b y CouaT~s (1950), this line has been studied b y M. DUFAY (1952). It is particularly intense in summer; its intensity is of the order of one-tenth of the intensity of the green line in the airglow. It will be emitted when an atom returns to the ground state after absorbing a quantum of 3466/k and emitting one of 10,406 A. This line has a|so been observed by NICOLET and PASTIELS (1952). Bands ofN~ +. The most recent data are due to M. DVFAr (1949). The intensity of these bands is very variable and the altitude of their emission is 100 km. These results are confirmed b y SwI~os and NICOLET (1949). They also show that the bands of N~+ behave in the same way at dawn and dusk twilights, and that there is no correlation between the intensities of Ne + and Na. BATES (1949) has shown that a simple fluorescence mechanism is more efficient for producing these bands than one by which neutral molecules are simultaneously ionized and excited. The intensity of the 3914 band can be as great as 109 transitions per second in a slant column of unit cross-section at 75 ° zenith distance; this requires about 2 × l01° ions in a vertical column of unit cross-section. It is of interest to compare the twilight radiation with that from comets, the light of which is also excited by fluorescence (BARBIER, 1953). This comparison brings out the difference between the chemical composition of the comets and the terrestrial atmosphere, and removes a difficulty concerning the explanation of the existence of the N2+ ions in the comets. 12. CONCLUSIONS
We already understand in its main outline the radiation of the twilight; many details still remain obscure but we may hope that their elucidation will throw more light on the state of the upper atmosphere. The origin of the nightglow, at least as far as the radiations in high altitudes are concerned, is still unknown. We must ask which development should be aimed at in the observing programmes, in order to give the maximum assistance to the theoreticians. It is possible to think of qualitative improvements of the observations, for example,
938
The airglow
by increasing the resolution or the light gathering power of the spectrographs, or by improving the spectral purity of the radiation recorded by photoelectric photom e t e r s . H e r e p r o g r e s s is s t i l l p o s s i b l e ; w e h a v e n o t y e t r e a c h e d t h e l i m i t o f t h e technical possibilities, but it would not be right to overlook the fact that relatively minor gains will be extremely costly. Q u a n t i t a t i v e i m p r o v e m e n t o f t h e o b s e r v a t i o n s will c e r t a i n l y g i v e g r e a t r e t u r n s for some years to come: a large number of radiations have not yet been observed regularly; the observing stations are not yet evenly distributed over the Earth, and t r i a n g u l a t i o n s b e t w e e n s t a t i o n s s o m e 500 k m a p a r t s h o u l d b e p o s s i b l e . The introduction of new techniques remains a possibility. Here we can hope for instruments to be sent up into the upper atmosphere on rockets; or for direct e x p e r i m e n t s t o b e c o n d u c t e d o n t h e a t m o s p h e r e (for e x a m p l e , w e m a y q u o t e t h e p r o p o s a l o f BATES, 1950, t o "SOW" s o d i u m a t o m s i n t o t h e u p p e r a t m o s p h e r e ) . T h e r e s u l t s will l e a d t o a n e x t e n s i o n o f o u r k n o w l e d g e o f g r e a t a l t i t u d e s , a n d o f all aspects of the intensity variation, the structure of the emission layers, and the correlation of the intensities of the radiations between themselves or with cosmic or geophysical phenomena.
kEFEREN('ES
ABAD:~, P., VASSY, A. and VASSY, E . ARNULY, A . . . . . . . . . . BASGOCK, H. D . . . . . . . . . BABCOCK, H . . . . . . . . . .
. . . . . . . . . .
1945 1943 1923 1939
Ann. Gdophys., 1, 189. Ann. Astrophys., 6, 2l. Ap. J., 57, 209. Publ. Astron. Soc. Pac~ific, 51, 47.
1953 1950 1944 1947 1948 1952a 1952b
Ann. Gdophys., 9, 309. Ap. J., 111, 201. Ann. Gdophys., 1, 144. Ann. Astrophys., 10, 47 and 141. Ann. Gdophys., 4, 193. Ann. Astrophys., 15, 247. ~tude opti9ue de la haute atmosphere.
1953
La Physique des Com~tes, Colloque de Liege; 263. Ann. Astrophys., 16, 96. C.R. Acad.Sci. (Paris), 287, 599. Contrib.
BAILLET, i . , BARBIER, D., BOSSON, F., LALLEMAND,
A. and MAOI3ErCY,J . . . . . . . . . BAPPU, V . . . . . . . . . . . . . BARBIER, D . . . . . . . . . . . .
Colloque de Liege; 43. 1953a 1953b BARBIER, D., DUFAY, J. and WILLIAMS, D. BARBIER, D. and ROACH, F. E . . . . . . . BATES, D. R . . . . . . . . . . . .
BERTHIER, P . . . . . . . . . . . . BLAMONT, J. S. a n d KASTLER. A . . . . . . BRANSCOMB, L. M . . . . . . . . . . BRICARD, J. a n d KASTLER, A . . . . . . .
1951 1950 1948 1949 1950 1953 1951 1950 1944
1950 1952 CABA~ES, J. and DUFAY, J . . . . . . .
1944 1946 1947
CABA~ES, J., DUFXY, J. and DUFAY, M . . CABANNES,J., DUFAY, J. and GAUZIT,J. C-~B~-~.S, J. and G)~tRIOUE, H . . . . . . C~VOSTIKOV, I. A . . . . . . . . . .
1950 1938 1936 1948
Inst. Astrophys. A, No. 151. Ann. Astrophys., 14, 399. Trans. Amer. Geophys. Un., 31, 13. The Emission Spectra of the Night S k y and Aurorae, London; 21. Proc. Roy. Soc., 196, 562. J. gcophys. Res., 55, 347. C.R. Acad. Sci. (Paris), 287, 928. Ann. Gdophys., 7, 73. Phys. Rev., 79, 619. Ann. Gdophys., 1, 53. Ann. Gdophys., 6, 286. L'dtudc optique de l'atmosph~re terrestre,
Liege, 87.
Ann. Gdophys., 1, 1. Ann. Gdophys., 2, 290. Relations entre lee phdnom~nes solaires et gdophysiques, Lyon, 217. C.R. Acad. Sci. (Paris), 280, 1233. C.R. Acad. Sci. (Paris), 206, 1525. C.R. Acad. Sci. (Paris), 208, 484. Lumi~re du ciel nocturne. Acad. Sci.
U.S.S.R., Moscow.
DANIEL BARBIER
939
1934
I~tude lumi~re ciel nocturne, E d i t . R e v u e
1947 1950 1951 1936 1938 1928 1943 1947 1951 1952
Ann. Astrophys., 10, 33. C.R. Acad. Sci. (Paris}, ~,~1, 62. J. geophys. Res., 56, 567. Rev. rood. Phys., 8, 1. C.R. Acad. Sci. (Paris}, 207, 81. Bull. Obs. Lyon, 10, (9); Thesis. Ann,. Univ. de Lyon, lr Ann,. Gdophys., 8, 1. Ann,. Gdophys., 7, 1. lntern. Astron. Union Draft Report, R o m e ,
DUFAY, J., BERTHIER, P. a n d MORIGNAT, B. DUFAY, J. a n d GAUZIT, J . . . . . . . .
1953 1947
DUFAY, J. a n d TCHENG MAo-LIN
.
1946 1947 1949 1951 1952
C.R. Acad. Sci. (Paris}, 237, 828. Relations entre les phdnom~nes solaires et ge~ophysiques, L y o n ; 245. Ann. G~ophys., 2, 189. Ann. Gdophys., 8, 153. Ann. Gdophys., 5, 2. C.R. Acad. Sci. (Paris}, 232, 2344. L'dtude optique de l'atmosph~re terrestre,
.
1953 1942 1948
CoJA~, J .
.
.
.
.
.
.
.
.
.
.
.
.
O p t i q u e , P a r i s ; 43. COURT~S, G . DAvIs, D. N . DEJA~tDIN, G . DEJARDIN, G. D UFAY, J . .
.
.
. . . . . . . . . . . . . . . a n d BERNARD, R . . . . . . . . .
.
.
. . . . .
.
. . . . . . . . . . .
. . . . . .
138.
DUFAY, M .
.
ELVE¥, C. T .
.
.
.
.
.
.
.
.
.
.
.
. . . . .
.
.
.
.
.
.
.
.
.
Liege, 141. Thesis, L y o n .
Rev. Mod. Phys., 14, 140. The Emission Spectra of the Night Sky and Aurorae, L o n d o n ; 16. Ap. J., 96, 451. Ap. J., 85, 213. Ap. J . , 93, 337. Ann. Phys. (Paris}, S~r. 11, 16, 253. Ann. Astrophys., 3, 117 (also Ap. J., 1941,
ELVEY, C. T. a n d FARNSWORTH,A. H . . . . . ELVEY, C. T. a n d ROACH, F. E . . . . . . . ELVEY, C. T., SWINGS, P. a n d LINKE, % V . . GRANDMONTAGNE, R . . . . . . . . . . HENYEY, L. G. a n d GREENSTEIN, J . L .
1942 1937 1941 1941 1940
HERZBERG, G . . . . . . HULBURT, E. O . . . . . HUNAERTS, J. a n d NICOL~T, KRASSOVSKY, V. J . . . .
NICOLET, M . . . . . . . . . . . .
1953 1949 1950 1949 1950 1953 1949 1948 1950a 1950b 1950c 1950d 1951 1953 1951
NICOLE% M. a n d PASTIELS, R . . . . . . .
1952
Canad. J. Phys., 31, 657. J. Opt. Soc. Amer., 89, 211. Ciel et Terre, 66, 213. Dokl. Acad. Sci. U.S.S.R., 66, No. 1. Dokl. Acad. Sci. U.S.S.R., 70, No. 6. Univ. Fys. Inst. Oslo, Meddel. No. 205. Rev. sci. Instrum., 20, 724. Publ. Astron. Soc. Pacific, 60, 317. Trans. Amer. Geophys. Union, 31, 21. Ap. J., 111, 206. Ap. J., 112, 120. Ap. J., 112, 464. Reports Progr. Physics, 14, 121. Ap. J., 118, 200. Relations entre les phdnom~nes solaires et terrestres ; rapport 7, Paris. l~tude optique de la haute atmosphere,
ROACH, F. E. a n d BARBIER, D . . . . . . . ROACH, F. E. a n d PETTIT, H . B . . . . . .
1950 1951a 1951b 1952
Trans. Amer. Geophys. Union, 31, 7. J. geophys. Res., 56, 325. Ann. Astrophys., 14, 399. ~tude optique de la haute atmosphere,
ROACH, F. E., PETTIT, H . a n d WILLIAMS,D . R . . ROACH, F. E., PETTIT, H . B., WILLIAMS, D. R.,
1950
J. geophys. Res., 55, 183.
SPITZER, L., J r . . . . . . . . . . .
1953 1949
Ann. Astrophys., 16, 185. Atmospheres of the Earth and Planets.
SWINGS, P .
1949
Atmospheres of the Earth and Planets.
1949
Ap. J., 109, 327.
. . . . M. . . .
. . . . . . . .
. . . . . . . . .
KVIFTE, G . . • . . . . . . . . . . MARLOW, D. a n d PEMBERTON, J. C . . . . . MEINEL, A. B . . . . . . . . . . . .
98, 70).
Colloque de Liege, p. 147.
Colloque de Liege. ST. AMAND, P., DAVIS, D. N . . . . . . .
( E d i t e d b y G. P. KUIPER), Chicago. .
.
.
.
.
.
.
.
.
.
.
.
( E d i t e d b y G. P. KUIPER), Chicago. SWINGS, P. a n d NICOLET, M .
.
.
.
.
.
.