Harz, Germany

dournnlof

Atmospheric

andTerrestrial Physics, 19G5, Vol.27,pp.451to156.Pergsnlon PressLtd.Printed inNorthern Ireland

Twilight sodium observations at Lindau/Harz, Germany G. GUILINO Illstitut filr Stratosphtirenphysik am Max-Planck-Institut fiir Aeronomie,

Lindau/Harz,

Germany

and H. K. PAETZOLD Institut fiir Geophysik

und Mcteorologie Germany

(Received

3 December

der Universitiit

zu Kiiln,

1964)

Abstract--With an apparatus which discriminates strongly against the continuous background the sodium twilight glow was observed up to small angles of solar depression. After taking into account the Fraunhofer D lines and the atmospheric extinction the intensity plots agree The variations of the very satisfactorily with the results of the theory of resonance scattering. Height distributions of the sodium density were deduced resulting abundances are discussed. and thereby the data of 3 December 1963 yield a two-layered profile.

1. INTRODUCTION BECAUSE of the great difficulties

in eliminating the white-light component from the sodium response, twilight measurements are practicable in general only up to the plateau intensities corresponding to solar depression angles /I w 6” (for example HUNTEN et al., 1964). But the theory of resonance scattering (CHAMBERLAIN et al., 1958; DONAHUE and STULL, 1959) predicts a decrease of the intensity at the day side of the intensity maximum depending on the sodium abundance. Until now, only BLAMONT et al. (1958) published experimental results towards small angles of solar depression. They find an excellent agreement with the theoretical expectation by DONAHUE and STULL (1959). Further experimental data to this topic are presented in this paper. BULI,OCK and HUNTEN (1961) approached the problem of deducing the vertical sodium distribution by applying HUNTEN’S (1960) method to the averages of their measurements covering a period of three months. Similarly DONAHUE and BLAMONT (1961) pointed out that only averages over several twilights would give reasonable results regarding the width of the layer. By our method of suppressing the whitelight level the twilightglow intensities are, however, so well established that individual determinations of density distributions are justified physically. 2. TREATMENT OF OBSERVATIONS

A Fabry-Perot interferometer is operated in such a manner that the radiation flux due to the interference pattern of the examined spectral line is modulated by chopping, whilst the light of a nearby line and a continuous background contribute 451

452

G. GUILINOand H. K. P~ETZOLD

Since the interferometer is tilted only to the shot noise of the photomultiplier. 45” against the optical axis of the objective the interference pattern of monochromatic light does not consist of concentric fringes but of a system of stripes. A photographic plate was exposed to the latter. After developing and reinserting the plate in the focal plane of the objective, one gets a stop mask and by wobbling it the transmitted monochromatic radiation flux is chopped. Another wavelength gives a different interference pattern compared with the photographed one and therefore less modulation results. Continuous light produces no interferences and no modulat,ion is obtained. Contrary to the typical features of a Fabry-Perot the used instrument has a rather low resolving power, but its main advantages are the great factor of luminosity (area of the objective times solid angle of the field of view) and the capability to discriminate against the white-light level, which makes it well adapted for twilight studies. EHGARTNERet al. (1955) described the construction of t,he interferometer and determined the formula for the response of the apparatus as a function of wavelength. The entire airglow device as well as the manner of analysing the following data is discussed in detail by GUILINO (1964, 1965). For further explanation refer to these publications. The absolute calibration was realized by means of a sodium-vapour lamp. The intensity of this source is controlled to a constant value by an appropriate feedback system, and we used it as a secondary standard after having compared it with the black body radiator. The above-mentioned modulation device produces responses of the same shape and with the same sign for emission and absorption lines. If one deals with the existing superposition of these two kinds of lines and if the width of the instrumental profile is large against that of the spectral lines, then the recorded signal, 8, is proportional to the difference AI,

where I,(n), 1,(n) represent the spectral intensities of the emission and absorption The solar Fraunhofer D lines superimpose on the atmospheric line, respectively. emission lines and strongly modify the intensity of this spectral region, especially Since the instrumental profile has a towards small angles of solar depression. halfwidth of 18 A the upper equation is valid for analysing the measurements. They are plotted in Fig. 1 and it may be seen that the response according to the emission lines dominates up to solar depression angles @ 2 6+5”, whilst for /3 2 4.5” the Fraunhofer absorption lines fix the readings. Between these angles of solar depression the contributions of both spectral components are of the same order, and a marked minimum results, since a vanishing signal is not detectable because of the low signal/noise ratio. The direct current of the photomultiplier, i=, recorded together with the spectral lines is proportional to the white-light level and consequently also to

1m

I,(A) d/l.

”

0

Twilight

sodium observations

at Lindau/Harz,

Germany

Thus it is possible to deduct a correction term (DONAHUE and HUNTEN, isolate the response due to the emission line by the equation:

fm

.O

I,(a) ail = &AI+

[,&(A) .O

~57 = &AI

4.53

1958) and to

+ C,.i,.

C, means a factor of proportionality. After the application of this correction regarding the influence of the Fraunhofer D lines the deep minimum at the plots of Fig. 1 is replaced by a flat intensity decrease for @ < 5-5”. 30-

SOLAR

Al, ,Ki,or~yiy(~,#u i

“I

I

‘--I

DEPRESSION

Fig. 1. Recorded twilight signals of the sodium I) lines (A =: 5893 A) at Lindarl, converted int-0 rayleighs *, vs. angle of solar depression in the emitt,ing layer. The indie&ed t3rr0rs of the data are due to nncertainties in chart reading.

a severe and not always thoroughl~r regarded restri&ion in the interpretation of airglow measurements causes the atmospheric extinction, especially for observing stations at low altitudes above sea level (Lindau 142 m). Therefore the data from Fig. 1 were carefully reduced to intensities outside the atmosphere by taking into account the extinction by Rayleigh scattering, by ozone absorption and by haze. 3. RESULTS AND Drscuss~ors The results of the measurements are shown in Fig. 2 after being reduced according to the facts described in Section 2. The intensity decrease at the day side of the maximum becomes steeper for increasing intensities. This correlation allows for * A rayleigh

=

lo6qtzantn , cm-2 (column)

. SAC-‘.

45.4

(3. CUILINO

and H. K.

PAETZ~LD

resonance absorption of the exciting radiation and is in very satisfactory agreement with the theory of the plateau intensities by CHAMBERLAIN et a-t. (1958). The sodium abundance was determined by interpolating the observed intensity maximum and the theoretical intensity maximum resuIting from the above plateau intensities and by taking into account the shadowing of the incident sunlight by the atmosphere. EO 3, IS.57

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Fig. 2. Total zenith intensities corresponding to the measurements of Fig. 1 after correcting for Fr&unhofer I) lines and atmospheric extinction. The mean intensities, I?,, of the sodium nightglow a,re subtracted. From the theory of CHAMBERLAIN et al. (1958) the sodium abnndsnce, N, is derived.

For a statistical treatment the observations are not numerous enough but they harmonize with the well-known seasonal variation of the sodium abundance in the Northern hemisphere. In detail the meas~ements at Lindau (51-7’ N, 191” E) with high abundances

in December,

small ones at the end of February

and somewhat

higher ones in March/April are matched better to the results from the Observatoire de Haute Provence (43.9’ N, 5-7” E) (BLAMONT et al., 1958 ; DONAHUE and BLAMONT, 1961); NGUYEN-HUU-DOAN, 1964) than to those from Saskatoon (52-l” N, 106.7” W)

Twilight sodium observations at Lindau/Hurz, Germany

455

(CHAMBERLAINet al., 1958; BULLOCK and HUNTEN, 1961). DONAHUE and BLAMONT (1961) explained the difference between the annual variation at 44” N and 52” N by the conception of a NS moving belt of high sodium concentration. This implies that the seasonal variation at Lindau and Saskatoon should coincide, since both stations have nearly the same latitude. Presumably the motions are more complicated. The morning/evening ratio of the intensities of 213 April 1963 is 0.92 and this value is expected from the calculations of BLAMONTet al. (1958) regarding the Doppler shift between the solar and terrestrial absorption lines due to the rotational and orbital speed of the earth. Thus the lower intensity in the morning may be caused by the changed excitation conditions and must not indicate a diminished sodium abundance.

/I

28 Frb Mar

63rv 63rv

Fig 3. Density distributions deducted from the derivatives of the total intensity curves of Fig. 2. The dash-dotted plot represents a distorted profile in arbitrary unit’s. Vertical distributions of the sodium density were worked out for those twilight curves of Fig. 2 which contain sufficient measured points at the night side of the intensity maximum. Thereby the difference method of HUNTEN (1960) including the Bracewell treatment to unfold the distribution was applied. The transmission function of the atmosphere was taken from HUNTEN (1962) for an average ozone amount of 0.33 cm, and the standard distribution needed for HUNTEN’S difference method, was assumed to be a Gaussian. In Fig. 3 the resulting distributions are shown. The peak heights and their small variations from 88 km to 92.5 km agree well wit,h the averaged profiles of BULLOCK and HUNTEN (1961) and HUNTEN (1962). In contrast to these results, the extraordinary course of the intensity on 3 December 1963 gives a double-peaked distribution, represented by the dash-dotted curve in %ig. 3. We only plotted the smeared version of the distribution as the relatively

G. GUILINO and H. K. PAETZOLD

456

small number of measured intensity values does not allow an exact fixation of such a distinct profile. However, a sharp peak near 64 km and a broader one near 96 km may easily be distinguished. Physically speaking this means the existence of two sodium layers one above the other on that day. Previously DONAHUE and BLAMONT (1961)detected a similar phenomenon at Tamanrasset. Acknowledgements-G. GUILINO is indebted to Professor J. BARTELS, director of the Tnstitut fur Stratospharenphysik, Lindau, for putting at his disposal the scientific facilities of this institute. He is equally indebted to Professor A. EHMERT for his kind help and advice and wishes to express his sincere thanks for helpful discussions with Dr. G. PFOTZER. REFERENCES DONAHUET. JI. and

1958

Ann GBophys. 14, 253.

I~IJLLOCE IV. R. and HUBTEN D. 31. CHAMBERLAIS J. W., HIJP~TEN D. M. and MACK J. E. I)ONAHUE T. M. and HUNTEN D. N. DONAHUE T. M. and STIJLL IT. R. DONAHUE T. M. and BLAMOXT J. E. EHGARTNE~~ G., PIEPENBRINI~ W. und XA~ER-LEIBXITZ H., zum Teil nach Messungen van MATER K. GIJILINO G.

1961 1958

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HLAEIONT

HTELL

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1961 1955

13,165.

1965 1960 1962 1964

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1964

Ann

1964

Gdophys.

20, 1.