Rotational temperatures for OH and O2 airglow bands measured simultaneously from El Leoncito (31°48′S)

OOZI-9169/90$3.00+ .OO Prrgamon Press plc

.lournolo/ Abnoupherrcmrl Terrestrml Phwks, Vol. 52,No. I,pp. 47-51. 1990 Prmted I” Great Brua~n.

Rotational temperatures for OH and O2 airglow bands measured simultaneously from El Leoncito (31’48%) J~~RGEN SCHEER and ESTEBAN R. REISIN PRONARP/CAERCEM,

J. Alvarez 1218, RA-1414 Buenos Aires, Argentina

(Received in$nal,form 23 August 1989)

Abstract-Some results from 54 nights of simultaneous measurements, performed between 1984 and 1987, of rotational temperatures for the OH(c2) and O,(‘C)(@l) bands are presented. A summer enhancement by 15 K in 0, temperature has been found that has not formerly been observed in airglow measurements. At least five nights show prominent tide-like temperature oscillations with a phase shift between layers typical of upward wave propagation at about 10 km h- ‘, with up to 55 K variation. During other nights, similar oscillations are limited to the 0, layer. Data for different seasons seem to be characterized by different levels of variability. During the one equinox campaign, nocturnal temperature variations show an exceptionally stable pattern of tide-like oscillations.

1. INTRODUCTION A large body of observational data exists on the latitudinal and temporal variations of rotational temperatures of the OH Meinel bands in the terrestrial atmosphere. These data cover the time scale from minutes to years, giving testimony to the dynamic activity of the atmosphere, at the altitude of the emission layer (about 86 km). There is, however, an enormous bias, in the quantity of data, towards the northern hemisphere, and higher latitudes. The amount of data available for lower latitudes and the southern hemisphere is quite small, in comparison (TAKAHASHI and BATISTA, 1981; TAKAHASHI et al., 1986 ; ARMSTRONG, 1975, 1986). In contrast, little work has been done using the 0, atmospheric bands for probing atmospheric temperatures at the 95 km level. The usefulness of this, also in combination with OH measurements, has often been stressed (MERIWETHER, 1984; TEPLEY, 1985). As rocket and ground-based measurements show, temperature profiles near the mesopause often exhibit strong gradients where models only predict a smooth minimum (OFFERMANNet al., 1979 ; MURTAGH et al., 1987; VON ZAHN et al., 1987). Therefore, both emissions are supposed to yield independent and useful information, in spite of the rather small separation of their respective emission layers. This is according to various independent experimental determinations, most recently, e.g., by GREER et al., 1986 and TAKAHASHI et al., 1987, for 02, and by LOPEZ-MORENO et al., 1987, for OH. It might be noted that these measurements show emission profiles that are 41

approximately Gaussian with about 10 km full width at half maximum, and sometimes as narrow as 6 km. Based on a comparison of the radiative lifetime of the excited state with the inverse collision frequency it is reasonable to assume that rotational temperature for the OH(62) band agrees with the kinetic temperature at the centre of the emission layer. For the O2 band, such an assumption of local thermodynamic equilibrium is yet more convincing, since the radiative lifetime is known to be about a thousand times greater than for OH, while the collision frequency is probably not more than an order of magnitude smaller, due to the greater height of the layer. On the other hand, the measured OH rotational temperatures generally fall close to the kinetic temperature expected for the emission height, which can be taken as a certain evidence for this point, although direct comparisons are still scarce (VON ZAHN et al., 1987; GERNDT, 1986). NOXON (1978) has given the first practical demonstration of what can be learned from simultaneous measurements of OH and 0, rotational temperatures. He took advantage of the close vicinity of the OH(6-2) and the 0,(&l) atmospheric bands, and the intensity and lack of spectral contamination of this OH band, for combined observation with the same instrument. In subsequent work by other investigators, different OH bands have been used. At high latitudes, OH(83) and 0,(&l) rotational temperatures have been measured by MYRABD et al. (1984). In the southern hemisphere, other investigators (TAKAHASHI et al., 1986) have worked at latitudes below 23”S, measuring rotational temperatures of the O2 and the OH(9-5) band using Q/R-branch ratios.

J. SCHEER and E. R. RESIN

48

A rather complete survey of OH and O2 measurements is included in the recent review by FORSYTHand WRAIGHT (1987). Following Noxon’s steps, a tilting filter spectrometer has been designed for this kind of measurement, which uses a single filter for both bands (SCHEER, 1987). In this paper we present some results of a series of simultaneous measurements of OH and O2 rotational temperatures which comprises more than 50 nights. Measurements were made from El Leoncito in the Argentine Andes, at 3 l”48’S latitude. Data have been taken in several blocks of contiguous nights, in 1984, 1986, and 1987.

2. EXPERIMENTAL Measurements were made with a specially-designed tilting filter spectrometer with a spectral range from 845 to 867nm, covering the 0,(&l) atmospheric band and part of the P-branch of the OH(62) band. We should like to mention that measuring both bands simultaneously has, in principle, the additional advantage of making possible a correction of the 0, spectrum for the contribution of the P8 lines of OH(62) that coincides with the minimum between the R and P branches of the O2 band. This contribution depends, of course, on the relative intensity as well as the rotational temperature of the OH band, and may vary by almost two orders of magnitude. In our case, however, the spectral region in question is avoided, and no such correction is needed. The instrument and the method of temperature determination (based on sampling discrete spectral features and a combination of Shagaev’s coefficients and synthetic spectra) have been described in detail elsewhere (SCHEER, 1987). A new calibration has been performed verifying long-term stability and several improvements have been made regarding temperature errors, immunity against drift and variations of spectral background, etc. This has considerably improved the reliability of our data. For 02, line strength factors by WATSON (1968) are now used. They seem to be more precise than those by SCHLAPP (1937), according to the work of RITTERand WILKERSON (1987), although we found the effect on our rotational temperatures to be small (lowering by no more than 0.5K with respect to Schlapp’s asymptotic formulae, which seem to have been used mostly, and give better results than his exact ones). Relative zenith brightness is measured over a fieldof-view corresponding to about 3 km by 0.6 km at the

emission heights, with the longer side oriented eastwest. All the measurements reported here were made from the same site, the astronomical observatory ‘El Leoncito’ in the Argentine Andes, located at 3 l”48’S and 2500m above sea level. Four measuring campaigns have been made. Due to the excellent observational conditions encountered, data have been collected during practically all nights in June 22-27, 1984 (except for June 25), June 277July 13, 1986, October 26November 10, 1986 (except for November 8) and September 1430, 1987. Since 1986, measurements mostly covered complete nights (lasting from 8.5 to 11.5 h, according to season). Each night comprises, in general, about 300 to 400 pairs of (OH and 0,) temperatures. Statistical errors in rotational temperature have been improved on in several steps. Of course, statistical errors depend on airglow intensity, which was found to vary by as much as a factor of ten, for both bands. For the 1984 campaign, errors of + 13 K were typically achieved, for both bands, at 90s time resolution (SCHEER, 1987). During the two 1986 campaigns, the effective integration time was doubled (but using only two instead of four independent samples per intensity value). Thus, errors dropped to f 7 K for O2 temperature, and +4 K for OH, at times of good airglow intensity, and with a time resolution of 100s. For the last campaign, the background at 857.4 nm was sampled, instead of the P7 peak of OH, and was included in the sample sets for both OH and 0, temperature determination. Due to this, statistical errors went down to typically f 5 K (for both bands). For reasonably good but not exceptional airglow conditions (high intensity, low background), these errors decrease to f 3.5 K for 0, and If: 3 K for OH temperatures. For all the data, the systematic errors have been estimated to be less than 2 K. Time resolution was chosen to permit the study of rapid variations. Moreover, this avoids the errors in rotational temperature that might otherwise arise from spectral information being averaged over a longer period of time. These errors would be due to : (a) the temperature being quite variable, and not a linear function of spectral intensities, and (b) the presence of intensity variations, which cause the effective temperature to be biased towards periods of high airglow intensity. The explicit determination of spectral background that has been introduced for the last campaign, as mentioned, enables the direct measurement of relative brightness in both bands. Furthermore, it permits an

49

OH and Oz airglow bands measured simultaneously from El Leoncito

Table 1. Mean campaign temperatures

No.

1 2 3 4

Campaign June 22ZJune 27, 1984t June 27-July 13, 1986 October 26-November 10, 19861Septemberl4-September 30, 1987

No. of nights

Approx. No. of data pairs

5 17 15 17

1000 6000 4500 5000

*The values eiven earlier BCHEER. 19871 have been corrected t Except for ;he night mentioned in the previous section.

additional correction (to the first-order elimination already achieved by the Shagaev scheme) to be applied so that our rotational temperatures should be unaffected even by exceptionally unfavourable background conditions (as large as twice the highest airglow signal, as can occur with the moon close to the field-of-view). In the past, however, such conditions have been rarely encountered, so that our data for the earlier campaigns remained unaffected. For the data of the earlier campaigns, the intensities have been approximated using the smallest peak instead of the correct background. This leads to a slight underestimation of the intensities that is, however, of no importance for the following discussion. In this paper, we refer to intensities not in the sense of integrated band intensities, but only with respect to the spectral region observed.

3.

RESULTS

3.1. Campaign averages First, let us consider campaign averages. These are our most precise results in that they are affected only by systematic errors and the true variance of the data. The average temperatures observed during the four measuring periods, together with their statistical errors, are shown in Table 1. With the exception of the first campaign, each average given in this table represents more than fourteen practically contiguous and nearly complete nights. So there is no doubt as to the representative nature of our data, for the periods of observation. We stress this because, otherwise, many missing nights might distort the averages derived, due to the large variation of night averages that we and other investigators (e.g., OFFERMANet al., 1983 ; GERNDT, 1986) have found. For clarity, these data are also shown in Fig. 1, as a function of season. Note that average temperatures for the same (or nearly the same) seasons nearly coincide. This helps to confirm that the temperatures are reliable, i.e. the instrument performance is indeed stable and the measurements are comparable between

Mean temperatures OH OZ 197.91 1.4K 200.8i 1.5 K 193.7* 1.5K 190.4+0.7K

189.2*3.6K* 190.3&0.8 K 203.6& 1.8K 205.9+0.8K

here for a slight error (a few K).

different campaigns (in spite of moving the instrument, and the changes made to improve hardware and software). As shown in Fig. 1, OH temperatures are higher by 9%lOK than O2 temperatures in winter (June/July), but are lower (by 10-15 K) than 0, temperatures in spring (September/November). This is the reason why the total averages for both emissions (over all four campaigns) are rather similar. In fact, they differ by only 1.5 K (195.7K for OH versus 197.2K for 0,). This coincidence is reasonable, in the light of recent climatological work (FLEMING et al., 1988), which shows, at 32”S, an annual mean for 87 km that is indeed close to 195 K, and, for 90-106 km, close to (but less than) 190 K. For OH, the highest temperatures occur during winter (about 8K above summer values), in agreement with the usual behaviour in the upper mesosphere, and as has been thoroughly documented recently, for OH temperatures at medium and high latitudes (GERNDT, 1986). For O,, somewhat surprisingly, we find the opposite behaviour: the temperatures are lowest in winter, and rise by about 15K towards spring. This is not likely to be associated with short-term perturbations, as these should be expected to cancel out during the averaging involved here. Thus, our finding is in contrast to the (northern hemisphere) obser-

JFMAMJJASOND

MONTH Fig. 1. Campaign averages of rotational temperatures for 0, (rectangles) and OH (triangles) showing seasonal variations.

50

J. SCHEER and

vations of TEPLEY (1985), at low latitudes, and SHEFOV (1971), at mid-latitudes. The observed behaviour is, however, understandable in general terms as the effect of the competition between adiabatic heating due to meridional circulation, dominant in the upper mesosphere and at high latitudes, and direct solar heating, dominant at higher altitudes. This implies a phase jump in the annual temperature variation at a certain altitude, increasing with latitude. Our data suggest that, for our latitude, the phase jump occurs in between the emission layers. As an illustration, we consider the climatology of FLEMING et al. (198Q which clearly shows such a transition layer where the phase of the annual variation makes a jump of 180” over a height of only about 3 km, and where the altitude increases with latitude, but exceeding the probable height of the 0, layer. At 30”s the phase jump is shown at about 103 km. This seems to be 10 km too high to explain our data. On the other hand, the amplitude given in the model is quite similar to what we observe. At any rate, the apparent discrepancy with Shefov’s results disappears in this context when the latitudinal variation of the phase jump altitude is taken into account. In the case of Tepley’s data, however, we find it difficult to explain the discrepancy between our data and his big winter warming (by 38 K), which is nearly six times greater than the value given by the model. By interpreting the seasonal variation observed in terms of the annual variation, we certainly do not want to exclude the possibility of other periods being involved. There is, indeed, an indication that the observed variation might rather be semiannual, but our data do not cover the annual cycle sufficiently to be conclusive in this respect. When looking for an alternative way to explain the seasonal variations observed, one might want to speculate on the possibility of vertical motions of the emission layers, especially the 0, layer. Let us notice, first, that the annual averages mentioned above indicate that, on the average, both emission layers are close to a mesopause surrounded by a region exhibiting only small temperature gradients. In order that the O2 layer might experience the observed summer increase in temperature, due to vertical motion alone, it must be located in a region of positive temperature gradient (i.e., above a mesopause level), and rise in summer. For a temperature profile according to the CIRA 72 model, the first condition is feasible, and a vertical movement of the 0, layer of not more than 10 km would be sufficient to generate the observed variation. Although direct measurements of the height

E. R.

REISIN

June/July

1986

octob~9~~mber

September

1987

DATE Fig. 2. Night averages of intensities and temperatures of both bands for the last three campaigns. Bars show the nocturnal variability.

of the 0, layer show no evidence of a seasonal variation by even such an amount, the seasonal and latitudinal coverage of these measurements may be insufficient to discard this possibility. On the other hand, if the mesopause is indeed higher than the 0, layer, as shown by more recent models (e.g., FLEMING et al., 1988) and rocket data (see, e.g., VON ZAHN et al., 1987), the vertical displacement would have to be even greater, and so more unlikely. As to seasonal variations of intensity, and with the same reservation as that made with respect to temperature, our data do not support any of the different and contradictory claims made in the literature (see review by FORSYTHand WRAIGHT, 1987). Instead, for 02, there is no apparent seasonal pattern at all, while for OH we have a winter enhancement (see Fig. 2). 3.2. Night averages The rather strong quasi-tidal oscillations that were observed during many nights (see below) make night averages sensitive to the choice of the time interval considered. It is therefore advantageous that the measurements mostly covered complete nights (last-

OH and 0, airglow bands measured ing more than eleven hours, for the second, June/July 1986, campaign). Figure 2 shows the night averages of rotational temperatures during the last three campaigns (the first campaign being omitted because of its short duration). The bars indicate the variation during each night (the variance is calculated after periods of less than 15 min have been suppressed by digital filtering). Relative intensities are also shown, for comparison. It can be seen in this figure that the average intensity varies by as much as a factor of two, from nightto-night. This is definitely less than the maximum variation during a single night, as discussed below. The maximum deviation of night mean temperatures for each campaign was between 10 and 20K, except for the first campaign where it reached 4OK, for 02. A quantitative comparison of relative (root-meansquare) variations yielded the following results :

(I) The levels of variability (expressed in terms of variance) are different for each campaign. (2) The two emission layers show a correlation in dynamic activity, for temperatures as well as for intensities, in that night-to-night variabilities assume minimum values during the last (spring equinox) campaign. Maximum values are attained during the third (October~November 1986) campaign, for all cases except 0, intensity. (3) During the last campaign, nocturnal variability was from 1.8 to 3.4 times greater than night-to-night variability, for intensities and temperatures of both layers. (4) This uniformity does not appear for the other campaigns, where intensities and temperatures behave differently: only in the second campaign do the temperatures nearly behave as mentioned. Nocturnal and night-to-night variabilities can also be nearly equal, as for the intensities during the same campaign, and temperatures during the third campaign. The slow variations might even dominate, as in the case of the intensities in the third campaign, where the variance ratio was 0.6 for OH, and 0.8 for Oz. That is to say, the variability in the band of periods shorter than about 24 h does not generally exceed the variability for longer periods. The special behaviour for the spring equinox campaign is due to a combination of minimum variance of long periods, as mentioned, with a high variance at short periods. The short period variance is mainly caused by tide-like oscillations, as shown in the following section. The unusually high O2 intensity of one night (June 28, 1986 ; see Fig. 2) has relatively little impact on the

simultaneously

from El Leoncito

51

behaviour derived from this campaign, in the context of this comparison of variability at different time scales, because nocturnai variance is also enhanced. On the other hand, this night is a good example of how strong night-to-night variations can be, without any obvious explanation. Observational conditions were equally excellent for this, and the neighbouring nights.

3.3.

Diurnal

cariations

In the present discussion we will mainly be concerned with variations with periods of some hours. In general, we found that the intensities vary by up to one order of magnitude; for a single night, the 0, intensity was found to change by a factor of five (September 30, 1987), and the OH intensity to change by a factor of four (September 17, 1987). These important changes are not likely to be caused by atmospheric absorption or instrumental effects, because they do not occur simultaneously for both emissions. For 0,. intensity variations have been reported recently by TAKAHASHI et al., (1986), for measurements made in 1983 and 1984, which equally span a factor of five. Although, during many nights, the intensities and the corresponding temperatures seem clearly correlated (as far as periods of several hours are concerned), there are also many examples where this definitely is not the case. For O,, it seems to be that cases of correlation are somewhat more frequent than the opposite. For OH, the absence of correlation seems to be more frequent. Our data suggest that correlations between intensities and temperatures are of no permanent nature. The largest temperature variations by night have been observed during the spring equinox campaign. Moreover, the pattern of temperature variations (especially for Ox) was rather stable, from night to night, as shown in Fig. 3a. A similar behaviour is visible in the O2 intensities (Fig. 3b). This has generally not been the case during the other campaigns, and seems to be related to a dominance of the semidiurnal tide over other slow variations. During various nights, there is a strong (sometimes near-sinusoidal) variation of long period (6-12 h) in both 0, and OH temperatures (by 55 K, for 02, on September 14, 19X7, and by 40K, for OH, on September 30, 1987). Some of our results look similar to recently published OH temperatures, measured at high latitude, during 24 h (WALTERSCHEID ebal., 1986). The two most prominent examples are shown in Fig. 4, for July 5, 1986 and September 14, 1987. Although our data comprise 1I h, at most, the wave shape is

52

J. SCHEER and

quite pronounced and the dominant period can be easily identified. The facts that the same wave shape appears at two different layers, and the observation of a phase shift, as discussed below, seem to justify the interpretation of this phenomenon in terms of a propagating wave, possibly of tidal origin. The presence of the same long-period waveforms at two different altitudes, but with the higher layer leading in

E. R. REISIN

phase, has also been observed in rotational temperatures by TAKAHASHI et al. (1986). Both nights suffer from a high level of noise in the 0, temperatures for two hours before midnight, caused by very low oxygen band intensities then. This is not a coincidence, because both nights exhibit intensity waves with shapes similar to those seen in temperature. So, we have a correlation between intensity

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r ‘7 \

r ‘1 -i 160’1

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2122230

I

I 1 2

3

4 5

6

212223

0

1 2 3

4 5

6

LOCAL STANOARO TIME [hl Fig. 3a. Nocturnal variations of rotational temperatures measured from September 14 to 26, 1987, for O2 (left-hand side) and OH (right). Curves are shifted upward by 30 K with respect to the previous night. The time scale is Argentine standard time (3 h before UT). Local midnight is at 01.37 Local Standard Time. Fluctuations of 5 min period and below are eliminated by a five-point digital filter with weights 0.1 I, 0.24, 0.33, 0.24, and 0.11. The marked drop in the OH temperature for the last data points of some nights is an artefact caused by the steep background rise at dawn. These points have not been removed to demonstrate that this effect cannot be confounded with normal atmospheric behaviour and its onset can be easily localized.

OH and O? airglow bands measured simultaneously from El Leoncito

II’,

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I

212223D

I

I

I1

Ii

I

1 2 3 4 5 6

‘I

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2122230

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53

I

1 2 3 4 5 6

LOCAL .STANDARO TIME Cl-11 Fig. 3b. Same as Fig. 3a, but for intensities. The o&et between the neighbouring curves is 6 relative units. Data are not filtered here. (Note that no dawn enhancements are visible, even for the nights where the last OH temperatures are affected (especially September 15, 20, 22, and 29, which also shows that the background is perfectly eliminated from our data).

as well. Fig. 5 shows three further examples to apply to our low latitude. We have found that the of this tide-like behaviour for July 9, and 10, 1986, waves of smaller amplitude, at least, can be approxiand September 26,1987. mated quite well by a superposition of a diurnal and The amplitudes at the OH layer are close to, if not a semi-diurnal wave, which is indeed a necessary coneven greater than, those reported by WALTERSCHEIDdition for identification as a tide, but this is not the et a/. (1986). Although one might be tempted to think only possible fit. We believe the tidal interpretation to of a relation with the semi-diurnal or ter-diurnal tide, be correct ; this is also strongly suggested by the fact current tidal theory cannot explain such a high amplithat maximum phase occurs at nearly the same time variations,

tude (3. M. Forbes, private communications. Neither does the mechanism proposed by Walterscheid seem

each night, as evident from Figs 3a, 4, and 5. However, we are conscious that an unambiguous identification

--I

-

-7

-

02 ROT. TEMP. [ K 1

-.

OH TEMP. ROT. [ K I e

0)

m

T

T

r-4

REL.OH INTENSITY

- -. - ,.

REL.02 INTENSITY

OH and O2 airglow

bands measured

July9,1986

simultaneously

Ju\yIO,1986

from El Leoncito

55

September26,1987

LOCAL STANDARD TlME[ hI Fig. 5. Three further

examples

of slow temperature oscillations observable that 0, always leads in phase.

(several days), and an explanation in terms of standing waves has been suggested (OFFERMANNet cd., 1987). A similar phenomenon might be at work in our case, although this topic seems not to have been taken into account in current gravity wave models. There were also nights with very little oscillation in temperature, at least at one altitude (especially for October28 ,I986

OH).

in both emission layers. Note

For example,

for OH, the oscillation was as with a 14 h period (July 8, 1986), or 4.5 K peak-to-peak with a 6 h period, for O? (July 6, 1986). This means that any tide-like effects can be almost completely absent, occasionally. It should be noted that one of the nights when OH temperatures remained quite stable (July 8, 1986) had

low as 1.5 K peak-to-peak

September17,1987

LOCAL STANDARD TIME[ h1 Fig. 6. Two nights

(October

28, 1986, and September 17, 1987) where a strong present only in the Oz layer.

temperature

oscillation

is

56

J. SCHEERand E. R.

variable cloudiness. This is in agreement with our observation that clouds leave our rotational temperatures unaffected, even when the absorption is rather strong. The same is true for 02. Finally, with respect to oscillations with periods of less than one hour, we have not observed any dominant, short-period ‘monochromatic’ oscillations in temperature that can be clearly distinguished from noise, similar to those that have been described in the past (e.g., ARMSTRONG, 1975 ; MYRABB et al., 1983). Similar conclusions have been drawn in a recent paper by ARMSTRONG(1986).

4. CONCLUSIONS Our results can be summarized

as follows :

(i) Information obtained from observations of one airglow layer is not trivially related to information derived from another, but is rather complementary to it. (ii) For both OH and O2 layers, we find average temperatures close to 195 K. (iii) The OH layer shows the usual winter maximum, whereas for the temperature in the O2 layer we find a rise by 15 K, towards summer. This behaviour of the 0, layer can possibly be explained by direct solar heating, or by vertical movements of the layer. (iv) We find seasonal variations of intensities that are different from those presented in the literature, with no variation for O,, and a summer decrease for OH. (v) In this paper, we have also discussed the phenomenology of nocturnal and night-to-night

blSlN

variability for the different campaigns, finding that dynamic activity levels are correlated, for both layers. Towards summer, nocturnal variability does not exceed variability with longer periods, whereas during the equinox this is strongly the case. (vi) Variability of the airglow intensities is very high (by a factor of ten). (vii) For the nocturnal variation, we find that temperatures and intensities are not persistently correlated. During the September 1987 campaign, temperature variations show a consistent pattern from night-to-night, which seems to be closely related to tidal behaviour. This may be typical of equinox conditions, but more data are clearly required for confirmation. (viii) Some nights show prominent tide-like oscillations in both layers, with a phase shift compatible with upward wave propagation. We estimate the vertical phase velocity to be 10 km h- ‘. The large amplitude observed in these cases (up to 55 K peak-to-peak) cannot presently be explained by tidal models. (ix) Occasionally, a strong oscillation in O2 temperature is not accompanied by a variation in OH, which is reminiscent of the ‘quiet layers’ that have been described by other investigators. (x) In our data, no monochromatic wave events of short period are evident. Acknowledgements-The authors express their thanks to the staff of ‘Felix Aguilar’ and CASLEO astronomical observatories at El Leoncito and members of Centro de Investigaciones Regionales de San Juan for their logistical support during the measuring campaigns. Helpful discussions with Prof. OFFERMANN are also gratefully acknowledged. This work was part of the Argentine Programa National de Radiopropagacion.

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