On the origin of ripple-type wave structure in the OH nightglow emission

0032-0633/90 $3.00+0,00 Pergamon Press plc

PImet Space Sci., Vol. 38, No. 11, pp. 1421-1430, 1990 Printed in Great Britain.

ON THE ORIGIN OF RIPPLE-TYPE IN THE OH NIGHTGLOW

WAVE STRUCTURE EMISSION

M. J. TAYLOR Department of Physics, The University, Southampton SO9 5NH, U.K. and M. A. HAPGOOD

Rutherford Appleton Laboratory, Chihon, Didcot, Oxfordshire, OX1 I OQX, U.K. (Received in fkat form4 June 1990) Abstract-The

origin of small-scale mesospherie wave structure with horizontal wavelengths of only 5-l 5

km and lifetimes of less than 45 min, termed “ripples”, has been investigated using video observations of the near infra-red OH nightglow emission taken overa period of several years. Previous studies in the literature indicate that ripples are associated with lower lunar transit, lunar high tide and with geomagnetic activity (as measured by KJ, Examination of our data indicated a peak in the occurrence of ripples around lower lunar transit. However, the distribution of observing time was also strongly peaked towards lower lunar transit since this occurred at night during New Moon periods, when most of the observations were made. Statistical analysis of these data revealed no significant correlation with lower lunar transit and no evidence was found to support the theory that ripples arise from the breakdown of lunar tides in the upper atmosphere. The comparison of ripple occurrence with K,, phase showed a strong correlation, with ripples tending to occur when K, was falling. However, in an analysis using the AE magnetic index, no evidence was found to support the hypothesis that ripples are associated with the breakdown at mid-latitudes of travelling ionospheric disturbances (TID) generated at polar latitudes by aurora1 processes. It appears more likely that ripples are the result of short-lived velocity shears generated in situ by the chance combination of wind and wave motions.

1. INTRODUCXION Images of the night sky at near infra-red wavelengths (7OG900 nm) frequently exhibit luminous structure (Peterson and Kieffaber, 1973 ; Moreels and Herse, 1977 ; Crawford et al., 1978 ; Armstrong, 1982 ; Taylor et al., 1987). This structure arises mainly from the airglow emissions of hydroxyl (OH) radicals in the upper mesosphere and lower thermosphere (SO-90 km) and often takes the form of coherent wave-like patterns (More&s and Herse, 1977 ; Hapgood and Taylor, 1982; Clairemidi et al., 1985). Such patterns have been interpreted as the result of short-period (< 1 h) gravity waves disturbing the OH layer (Armstrong, 1982; Taylor et al., 1987). A variety of sources for these atmospheric waves have been postulated. These include tropospheric disturbances such as weather fronts, depressions, storms and jet streams (Hines, 1974a; Krassovsky et al., 1975 ; Rottger, 1977 ; Freund and Jacka, 1979 ; Taylor and Hapgood, 1988). They also include upper atmospheric disturbances such as aurora1 processes (reviewed by Francis, 1975) and the breakdown of large-scaie waves (Tuan et af., 1979, 1983). In general the wave patterns cover extensive areas, > lo6 km2

(Moreels and Herse, 3977). The elemental wave forms are often coherent and usually display horizontal wavelengths ranging from a few tens of kilometres up to several hundred kilometres and velocities of up to 100 m s-’ (Taylor, 1986; Clairemidi et al., 1985; Taylor et al., 1987). Peterson (1979) has identified a special class of OH wave structure which he termed “ripples”. These are very small-scale wave patterns that are similar in form to the billow-type waves seen in noctilucent clouds. They contain a limited number of wavecrests (typically 3-10) and exhibit horizontal wavelengths of 515 km and lifetimes of up to 45 min (Peterson and Adams, 1983; Adams et al., 1988). Figure la shows an example of a ripple event displaying a horizontal wavelength of N 12 km. The structures were observed at an elevation of 27” (field centre) and lasted for several minutes. The restricted spatial extent of the wave forms is a primary feature of such displays. For comparison Fig. 1b shows an image of OH wave structure associated with the larger scale wave patterns which we term “stripes” (Taylor, 1986). These have longer lifetimes, appear more often than ripples, and typically exhibit horizontal wavelengths ranging from 20 to 70 km. In this example the wavelength of

1421

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M. J. TAYLORand M. A. HAPG~~D

the stripes is only 24 km and yet the difference in morphology between the two types of wave form is marked. There is evidence to suggest that the occurrence of ripples is related to specific upper atmospheric phenomena: lunar high tides and travelling ionospheric disturbances (TIDs) of aurora1 origin. We have examined our airglow observations made over the past 10 years with the objective of extending the investigation of the relationship between ripples, lunar tides and geomagnetic activity. In particular we have applied basic statistical techniques to the data to determine the significance of the relationships. Nightglow ripples have a similar appearance to noctilucent cloud billows and occur at similar altitudes (SO-85 km) (Taylor, 1986). Studies of billows indicate that these may be the signature of KelvinHelmhol~ instabilities induced at mesospheric heights in localized regions by large wind shears (Haurwitz and Fogle, 1969). Thus we have considered whether ripples are also caused by the KelvinHelmholtz instability. 2. PREVIOUSSTUDIESOF RIPPLES Peterson (1979) and Peterson and Adams (1983) reported that ripple-type wave structure occurred preferentially around the time of lower lunar transit (LLT). All 21 ripple events that they recorded occurred within f 3 h of lower lunar transit. As lunar high tides arise around the times of upper and lower lunar transit, they postulated that ripples result from the localized breakdown of large-scale waves generated by lunar tides in the upper atmosphere. In two later reports Peterson (1985) and Peterson and Kieffaber (1986) also associated the occurrence of ripples with geomagnetic activity. In particular, their work showed that ripples tended to occur during periods in which the three hourly k; index was decreasing. To explain this correlation they suggested that the generation of ripples might be enhanced by the interaction of travelling ionospheric disturbances (TIDs) with lunar tides, where the TIDs were generated by geomagnetic activity. The time delay between the peak in K, and the occurrence of the ripple event arose from the TID travel time from the aurora1 zone to the mid-latitude region where the ripples occurred.

10 years. Table 1 lists the site coordinates, the dates of the measurements and the number of ripple events observed during each campaign (total of 26 events). All the observations were made using a modified low light television camera capable of imaging faint structure in the near infra-red nightglow emissions with a high temporal (typically < I s) and spatial resolution. The OH nightglow emission was selected by using a Schott RG715 filter to confine the spectral response of the camera to a bandpass with half-peak limits at 715 and 810 nm (Taylor et al., 1987). Several OH Meinel bands exist within this region, the most important of which are the (9,4) and (5,l) bands. The camera was fitted with a fast (typically f/1.2) lens and had a nominal field of view of 30” horizontal by 20 vertical. Universal time was superimposed onto each image prior to storage on a time lapse video recorder. The observations were usually made at an elevation angle (field centre) of 15” to take advantage of the two- to three-fold increase in emission intensity due to the line of sight integration through the airglow layer, and to improve the contrast of the ah-glow images. Observing campaigns were always conducted around New Moon periods to ensure dark skies. For each campaign, one, sometimes two, cameras were operated on a nightly basis. A scan of the low elevation sky was made periodically (typically once an hour) to determine the direction of the most favourable airglow structure (not necessarily ripples). As ripples can occur at any elevation the sightings listed in Table 1 do not represent their abundance but rather chance observations at low elevation. However, during the 1983 campaigns a concerted effort was made to record ripple events. This is reflected in the increased number of sightings during this period. 4. DATA ANALYSIS

The video recordings of the airglow patterns were examined to determine the times when ripple events were imaged. In addition a record was kept of the time and length of the clear sky observation windows during which the ripple data were obtained. All times were measured in Universal Time (U.T.) and were converted into a time difference from LLT using the method outlined in Fig. 2. 5. RESULTS

3. ORRERVATIONS The nightglow data used in this analysis were obtained during several campaigns at various midlatitude sites in Europe and the U.S.A. over the past

5.1. Correlation with LLT The distribution of ripple events relative to LLT is shown in Fig. 3a. All 26 events occurred within an interval of 9 h centred on LLT. However, the dis-

Origin

of small-scale

nightglow

structures

FIG. 1. PHOTOGRAPHS SHOWING (a) AIRGLOW KIPPLES WITH HORIZONTAL WAVELENOTW OF 12 km AND (b) FOK COMI’AKI~N A TYPlt.tL IMAGE OF A~RC;I.OW WAVE STKUCTURE WlTbl A HORIZONTAL WAVELENGTH OF 24 km. The ripples were observed at a central elevation of 27” whilst the stripes were imaged at 15’ elevation. Note the difl’ering spatial extent oftbe two types of structure.The field of view of both images was 30- horizontal by 20 vertical.

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Origin of small-scale nightglow structures TABLE1.LISTOFTHESlTECOORDINATESAND

THEDATESOFAIRGLOWCAMPAIGNSDIJRING DATAWBREOBTAINED

WHICHTHERIPPLE

Observing period

No. of ripple events

22 March-4 April 1979 5-16 August 1980

1 2

111.42”W

8-18 May 1983

9

34.70”N

106.4”W

4-8 June 1983

5

White Sands New Mexico Sacremento Peak New Mexico

32.40”N

106.5”W

9-llJune1983

3

32.78”N

105.8”W

13-17 June 1983

1

Mt Res. Station Colorado

40.00”N

105.6”W

ll-22May1988 7-22 June 1988 9-18 July 1988

1 2 2

Site

Latitude

Longitude

Gomergrat Switzerland

45.98”N

7.78”E

Bear Lake Utah

41.93”N

Capilla Peak New Mexico

Total number of ripple events : 26

of total observing time (Fig. 3b) shows that our observing window was also restricted to a limited range of about 12 h centred on LLT. To examine the possibility that the peak in the ttibution

_LLT

Greenwich

__ ________

-Ripple Event----

I

--__

--

occurrence of ripples in our data resulted from a natural observational bias we made a statistical analysis. This used the simple hypothesis that the occurrence of ripples is evenly distributed relative to LLT. Consequently, if ripples do occur preferentially around LLT, the analysis should lead us to reject the

__

1

5

No. of

4

Events 3 2 1

At

“y!r’

0 -6

t

-Local LLT-----{J--

-4

-2

0

2

4

6

Time from lower lunar transit (hrs)

t-U ‘- 360

300

-LLT Greenwich__ I _____________J___

Obs. 250 Time 200 (minsj 150

FIG. 2. SKETCHSHOWINGTHE CALCULATIONOFTHE ARIPPLEEVENTRELATIVETO LLT.

TIMEOF

The times of LLT on the Greenwich Meridian, before and after each event, were found from the Nautical Almanac (HMSO, 1987, etc.). The difference between these times gives I~. t, was then calculated using to and 1 (the longitude of the observing site in degrees, measured positive to the West). This gives the time of local LLT. The time of the ripple event relative to LLT was then found by calculating At.

100 50 0 -6

-4

-2

0

2

4

6

Time from lower lunar transit (hrs) FIG. ~.HIST~GRAMPL~TS SHOWING(~)TI~EDISTRIBUTIONOF RIPPLEEVENT~@OTAL 26 EVENT$RELATIVETO LLT, AND (b) THEDISTRIBIJTTONOFOBSERVINGTIMERELATIVBTO LLT.

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M.

J.

TAYLOR

and M. A.

HAPGOOD

TABLE 2. CALCULATIONOF CHI-SQUAREDFOR THE DIS~BU~ON OBSERVING TIMESEIOWNIN FIG. 3a,b* Number of ripple events Time from LLT

Observed

Expected

+6

0

t5 t4 f3 f2 fl 0 -1 -2 -3 -4 -5

1 4 3 3 1 4 5 3 2 0 0

0.78 1.36 1.71 3.03 2.87 2.75 3.26 2.26 3.26 2.69

Totals :

26

OF RIPPLEEVENTSAND

Merged values

Observed

Expected

(0 - Ej2/E

-3to3h

21

20.12

0.038

5.87

0.129

Other hours

5

1.46 0.56 25.99

Chi-squared = 0.167 1 degree of freedom

*Note that the merging of the bins smooths out the minimum in our data at -I-1 h relative to LLT (Fig. 3a), which clearly has no statistical significance.

hypothesis. This task was performed using the chisquared test (Till, 1974). The expected number of ripple events E(i) for time interval i was calculated using : J%I = JG”NITo~s,

(1)

where N, was the total number of ripple events, T(;f was the observing time in the interval i (as shown in Fig. 3b), and robs was the total clear sky observing time. The values of E(i), the expected, and O(z), the observed number of ripple events for each interval, were first binned in kIteN& of 1 h centred on the hour, e.g. bin+ 1 contains data between +0:30 and + 1:30. Although we had 26 events, our data set is clearly insufficient to perform an hour by hour statistical analysis as the expected values were all numerically lower than five (the minimum required by the chi-squared test). However, the data are sufficient to examine the occurrence of ripples inside and outside the f? h window, centred on LLT, which was identified by Peterson and Adams (1983). Table 2 shows how the data have been merged. They are contained in two intervals giving one degree of freedom for the chi-squared test. In this case, the probability of obtaining the observed &i-squared value of 0.167 from an even distribution by chance is about 68%. This is much greater than the accepted significance level of 5% and so we are not able to reject the hypothesis. Thus the peak in our distribution of ripple events around LLT is of no statistical significance.

5.2. Correlation with geomagnetic activity To investigate the relationship with geomagnetic activity we first examined the rCp value for the 3 h interval containing each ripple event and the value for the previous 3 h interval. Multiple ripple events occurring within a single 3 h interval have necessarily been treated as a single observation since there is only one independent Kp value. This reduces our number of independent events to 14, of which nine occurred when K,, was falling, three when it was rising and two when Kp was steady. These values are consistent with the results of Peterson (1985) who in a similar type of analysis of 58 ripple events determined that 39 occurred when K, was falling, nine when it was rising and 10 with Kp steady. To provide control data for this study we have analysed all Kp values, irrespective of magnitude, between January 1975 and September 1989 (a period which spans all of our airglow observations). There were 18,345 cases of Kp falling, 17,603 cases of Kp rising and 7146 cases for which K, was steady. Thus falling and rising cases occur with roughly equal frequency. Given this result there appears to be a natural tendency for ripples to occur when Kp is falling. Using a combination of our observations with those of Peterson, we investigated this conclusion statistically. We used the hypothesis that ripples were independent of the phase of Kp change and so the distribution with this phase follows the proportions set by the control data. The analysis is shown in Table 3. The probability of obtaining the calculated chi-

Origin of small-scale nightglow structures

1427

T~LE~.~AL~LATIONOFCHI-SQUA~FOR~DISTRIB~IONOF~PP~ ~~ASA~CT~ONOF~GEOMAo~~CACTIV~~l~EX~~*

Number of ripple events K, state

Observed

Expected

Failing Rising Steady

48 12 12

30.65 29.41 11.94

9.82 10.31 0.003

Totals :

12

72.00

C&i-squared = 20.13 2 degrees of freedom

(O--W/E

*Note in this case our observations have been combined with the data set of Peterson (1978).

squared of 20.13 by chance is less than 0.1%. The association of ripples with falling K, values is therefore highly si~~~nt. 6. DISCUSSION

The LLT analysis of our data indicates that the apparent peak in the distribution of ripples arose from a natural bias to make airglow observations around the time of lower lunar transit. Moonlight increases the sky brightness and illuminates thin meteorological clouds which can then resemble patches of airglow structure. In the absence of moonlight these clouds are easily distinguishable in the video recordings as they stand out in silhouette against the bright airglow background. Indeed, as Peterson and Adams (1983) noted, observations of airglow are normally impossible at upper lunar transit because the Moon is at its brightest in the night sky. Observations of the airglow emissions are therefore naturally restricted to times around LLT. This point is illustrated in Fig. 4 which shows how the times of dark skies, free of both twilight and moonlight (shaded area), are distributed with respect to LLT. The plot uses data for a mid-latitude site (Sacremento Peak, New Mexico) for the New Moon period in June 1983. For comparison the times when ripples were observed on four nights during this observing period are also indicated on the figure. The dark skies, and hence the observations, were restricted to a period of about 6 h either side of lower lunar transit. At other mid-latitude sites and other times of year the range of times available for observations will be similar to that shown in the figure, though the total observing time will vary with the seasons. Thus there is no evidence in our data associating the occurrence of ripples with high lunar tides. Recent radar studies of the winds induced by the lunar tides (Meek and Manson, 1987) cast further doubt on the postulate that ripples are generated

by the breakdown of such tides. They have shown that, at OH airglow altitudes (around 85 km), the lunar tide rapidly changes phase with altitude but has only a small amplitude of 1-2 m s-‘. Although the rapid phase change indicates the presence of a wind shear the amplitude of the lunar tide is far too small to generate the observed ripples (a further discussion of this point is given later). The semi-diurnal solar tide in the upper atmosphere is much stronger than the lunar tide with amplitudes around 20 m s- ’ at airglow altitudes (Manson and Meek, 1986). To investigate any association between the occurrence of ripples and this tide we performed a statistical analysis, similar to that shown in Table 2, but using local (i.e. solar) time in place of time relative to LLT. In this case a chi-squared value of 0.045 was obtained with two intervals (one degree of freedom). The probability of obtaining this result by chance is more than 83%. Thus, there is no evidence in our data set to link the occurrence of ripples with the solar semi-diurnal tide. In contrast to the tidal analyses, the comparison of ripple occurrence with Kp phase indicates an association with ripples tending to occur when Kp is falling, in good agreement with the results of Peterson (1985). Statistical analysis of the combined data set shows that this correlation is highly significant. However, our data do not support the hypothesis that ripples arise from an interaction between lunar tides and aurorally generated TIDs since there was no significant correlation between lower lunar transit and ripples. If ripples are generated by TTDs of aurora1 origin interacting with other large-scale disturbances in the upper atmosphere, or if they are produced by the spontaneous breakdown of aurora1 TIDs at mid-latitude (Tuan et al., 1979), it is reasonable also to expect a strong correlation between ripple occurrence and the magnitude of the aurora1 electrojet index AE. This index is a measure of the power dissipated by the

M. J. TAYLORand M. A. HAPGOOD

1428

Nautical

?2? f. c I? E! I-

Sib~35~N

Twilight Starts

6-

!kYi

5 O-I k z -6-I

I=op%Tq .=Rk@e Event

iii

E e IL

E” -12i=

I.

I

23 -

27

CD ,

31

l I’

4

I.

8

May-:

FIG. 4. DIAGRAM SHOWING

I

12

@

’

I.

16

O-Moods

I.

20

I.

24

Phase

I

20

June 1983

LIMITS FORDARK SKYAT 35” NORTHLATITUDE IN JUNE1983 (SHAoEnAREA). The vertical bars denote actual observing periods whilst the solid circles denote the occurrence times of several ripple events. THE OBSERVING

aurora1 electrojet and is available at fine time resolution (1 min) compared with the coarse (3 h) measure of geomagnetic activity given by Kp We have examined the behaviour of AE for the nine nights on which ripples were observed from 1979 to 1983. (AE is not yet available for 1988.) Of these events, four occurred during or immediately following periods of high AE (> 1000 nT), three when activity had been very low (AE < 100 nT) for at least the previous 8 h, and two events occurred within 6 h of weak activity (AE N 300 nT). This indicates that these ripple events were not predominantly associated with high magnetic activity. Thus the AE analysis does not support a link between ripples and TIDs of aurora1 origin. This result is not inconsistent with our K, phase analysis as close inspection of the data reveals that the majority of Kp changes were associated with low levels of geomagnetic activity which in turn indicate no significant TID production. One possible explanation of the K, correlation may be found in the work of Hines (1974b) who proposed that gravity wave activity in the upper atmosphere could actually contribute to the K, index by modulating the E-region electric currents. He determined that an induced magnetic fluctuation of only 6 nT would contribute N 1 unit to the threehourly Kp index. Thus, during relatively quiet geomagnetic conditions there could exist a correlation

between Kp and the occurrence of mid-latitude gravity waves. Under the right conditions these waves may also give rise to ripple events (see later). However, in this case it is not clear how the observed phase relationship, with maximum ripple occurrence when Kp was falling, fits with this idea. Tuan et al. (1979) proposed that ripples may be generated by a large-scale wave (of aurora1 or other origin) exciting a natural oscillation (around the local Brunt-ViiisZill period) by virtue of its passage through the upper atmosphere. If this were a significant mechanism then we might expect ripples to appear periodically and to be associated often with other larger scale airglow features. The evidence for this is mixed. There are examples in the literature of photometer data showing small-scale oscillations that are clearly superimposed on much larger amplitude, longer period waves. Furthermore, on nights when ripples occurred there was a clear tendency for them to reappear several times (but at various elevations and azimuths). However, our image data also show that the ripples primarily occurred in isolated regions of sky and exhibited no obvious association with the other larger scale airglow structures. Thus, this mechanism, although plausible, does not appear to be a primary source of airglow ripples. As mentioned in the Introduction, ripples have similar properties (wavelength, lifetime and spatial

1429

Origin of small-scale nightglow structures extent) to the billow-type structures seen in noctilucent clouds (Taylor, 1986). Thus we have investigated the possibility that ripples are caused by the breakdown of large wind shears through the KelvinHelmholtz instability, as has been suggested for NLC billows. The large shears can arise from the growth in amplitude of large-scale waves as they propagate energy upwards (Tuan et al., 1983). This breakdown occurs over a limited altitude range where the local Richardson number Ri is Iess than about I,l4 (Landahl and Mollo-Ch~stensen, 1986). Hirota et al. (1983) have shown that, at 85 km, this situation is reached when the vertical wind shear exceeds 40 m s- ’ km- ‘. In exceptional circumstances this condition may be satisfied by a single wave motion but in many cases the atmosphere will be subject to a combination of waves. In the fatter case the critical shear may be exceeded in localized regions through constructive interference of several waves [as outlined by Hirota et al. (1983) and Peterson and Kieffaber ( 1986)]. Radar measurements have been used to determine the magnitude of the wind shears in the upper atmosphere (70-100 km). By averaging the data, periodic components such as the tides and planetary waves can be extracted from the total wind. The average differential shear induced by the lunar tide at 85-90 km is typically l-2 m s- ’ km- I (Meek and Manson, 1987) and that of the solar tide is 10-15 m s- ’ km-’ (Manson and Meek, 1986). These are too small to satisfy the Richardson criterion for instability and so it seems unlikely that the tides alone can generate the shearing required to produce ripples. Indeed, if this were the situation then ripples would be a common, nightly occurrence which, as our data and that of Peterson and Adams (1983) indicate, is not so. However, it is possible that combinations of propagating waves may generate localized regions where the differential velocity shears are in excess of 40 m s- ’ km- ‘. Evidence in support of this mechanism can be found in the Arecibo radar observations of Hirota et al. (1983). They observed that marked oscillations of period about 20 min were sometimes present at 85 km altitude for intervals of up to 1 h. These oscillations occurred only when the wind shear had a value around the expected critical magnitude of 40 m s-’ km-‘. Another possible origin for ripple-type structure is the free propagation of very small-scale gravity waves generated in the lower regions of the atmosphere by tropospheric disturbances. However, due to the filtering effects of the middle atmosphere it is extremely unlikely that any such waves will reach the upper mesosphere (Hines, 1960; Taylor and Hapgood, 1988).

7. SUMMARY

We have shown that a peak in the occurrence of ripples around the time of lower lunar transit is the result of natural observational selection. Statistical analysis of our data revealed no significant correlation between the occurrence of ripples with either the lunar or the solar tides. A similar analysis applied to the occurrence of ripples with the phase of I$ yielded a correlation of high statistical significance. One inte~retation of this correlation is that the generation of ripples is linked with TIDs of aurora1 origin. However, an examination of the corresponding values of the AE index does not support this interpretation. A more general mechanism whereby ripples arise from a fortuitous interaction of wind and wave motions is indicated. This combination of disturbances would provide a natural explanation for several features of ripples. For example, a shear existing over a wide area (e.g. from planetary waves) might be reinforced in localized areas by a shear associated with smaller scale waves (e.g. freely propagating gravity waves). The tendency for ripples to occur more frequently on certain nights could be explained by there being a high, but sub-critical, value of the shear on those nights. Thus the atmosphere would be predisposed towards instability and there would be a greater probability that some smaller scale gravity waves could push the shear above the critical level and thus generate ripples. The occurrence of ripples in small regions of sky and for limited durations would be explained by those being the areas/times in which the smaller scale waves were in phase with the largescale wave and thus reinforced the shear. In other parts of the sky the smaller scale waves would be out of phase and would reduce the total shear. The similarity between the short period oscillations in zonal wind shear measured by Hirota et al. (1983), and the airglow ripples observed by Peterson and colleagues and by ourselves strongly supports this mechanism and leads us to speculate that such variations are the radar signature of optical ripples. Coincident airglow observations in conjunction with mesospheric radar measurements are needed to investigate this postulate. It appears that ripples may provide a useful tool for remote sensing the micro-scale dynamics of the upper atmosphere in the vicinity of the mesopause. Acknowledgements-We thank the directors and staff of all the sites listed in Table 1 for allowing us the use of their facilities and Dr A. W. Peterson for many interesting discussions. We also thank the Space Dynamics Laboratory of Utah State University for logistical support during the 1983 measurements. The observations in 1979 and 1983 were

1430

M. J. TAYLORand M. A. HAP~XXZD

funded by the U.K. Science and Engineering Research Council (SERC). Recent observations at the Mountain Research Station, Colorado, and some of the data analysis were supported by the U.S. Air Force Office of Scientific Research (AFOSR) as part of the MAPSTAR programme. Values of the aurora1 electrojet indices were taken from the databooks published by World Data Center C2 for Geomagnetism, Kyoto, Japan. Values of the K, index were retrieved from the on-line database service provided by World Data Centre Cl for Solar-Terrestrial Physics, Chilton, U.K.

REFERENCES

Adams, G. PI., Peterson, A. W., Brosnahan, J. W. and Neuschaefer, 3. W. (1988) Radar and optical observations of mesospheric wave activity during the lunar eclipse of 6 July 1982. J. atmos. terr. Phys. 50, l I. Armstrong, E. B. (I 982) The association of visible airglow features with a gravity wave. J. atmos. terr. Phys. 44,325. Clairemidi, J., Herse, M. and Moreels;‘G. (1985) Bi-dimensional observation of waves near the mesopause at amoral latitudes. Planet. Space Sci. 33, 1013. Crawford, J., Rothwell, P. and Taylor, M. J. (1978) ASSESS 2 : a simulated mission of Spacei& Review article. Nature 275, 17. Francis, S. H. (1975) Global propagation of atmospheric gravity waves : a review. J. utmos. zerr. Phys. 37, 1011. Freund, J. T and Jacka, F. (1979) Structure in the 557.7 nm (01) airglow. .J. trtmos. terr. Phys. 41,25. Hapgood, M. A. and Taylor, M. J. (1982) Analysis of airglow image data. Ann. Geiphys. 38, Sb5. Haurwitz. B. (1961) Wave formations in noctilucent clouds. Piartet..Space Sci. 592.

Haurwitz, B. and Fogle, B. (1969) Wave forms in noctilucent clouds. Deep-Sea Rex 16, 85. Hines, C. 0. (1960) Internal atmospheric gravity waves at ionospheric heights. Can. J. Pkys. 38, 1441. Hines, C. 0. (1974a) A possible source of waves in noctilucent clouds, in The Upper Atmosphere in Motion, p. 661. Geophysical Monograph 18, American Geophysical Union, Washington, D.C. Hines, C. 0. (1974b) Wind-induced magnetic fluctuations, in The Upper Atmosphere in Motion, p. 837. Geophysical Monograph 18, American Geophysical Union, Washington, D. C. Hirota, I., Maekawa, Y., Fukao, S., Fukuyama, K., Sulzer, M. P., Fellous, J. L.,Tsuda, T. and Kato, S. (1983) Fifteenday observation of mesospheric and lower thermospheric motions with the aid of the Arecibo UHF radar. J. geophys. Res. 88,6835.

HMSb (Her Majesty’s Stationary Office) (1987) The Nauticni Aimunac 1989. HMSO, London (Published annually). Krassovsky, V. I., Kuzmin, K. I., Piterskaya, N. A.,

Semenov, A. I., Shagaev, M. V., Shefov, N. N. and Toroshelidze, T. I. (1975) Results of some airglow observations of internal gravitational waves. Planet. Spuce Sci. 23,896. Landahl, M. T. and Mollo-Christensen, E. (1986) Turbulence and Random Processes in FluidMechanics, p. 9. Cambridge University Press, Cambridge. Manson, A. H. and Meek, C. E. (1986) The dynamics of the mesosphere and lower thermosphere at Saskatoon (52 N). J. atmos. Sci. 43,276. Meek, C. E. and Manson, A. H. (1987) Middle atmosphere lunar tides at Saskatoon (52 N, 107 W). Planer. Space Sci. 35,445. Moreels, G. and Herse, M. (1977) Photographic evidence of waves around the 85 km level. Planet. &ace Sci. 25.265. Papadopo~os, D., Tuan, T. F., Peterson, A. W. and Nadille, R. M. (1983) A genera&ration of the Hines’ dispersion relation. Adu. Space Res. 3,37. Peterson, A. W. (1979) Airglow events visible to the naked eye. A#. Optics l&3390. Peterson, A. W. (1985) EOS66,997. Peterson, A. W. and Adams, G. W. (1983) OH airglow phenomena during the 5-6 July 1982 total lunar eclipse. Appl. Optics 22, 2682.

Peterson, A. W. and Kieffaber, L. M. (1973) Infrared photography of OH airglow structures. Nature 244, 92. Peterson, A. W. and Kieffaber, L. M. (1986) The correlation of lunar tidal ripples in the OH airglow with recurrent magnetic storms. Proc. of rhe International Symposium on Sauce Physics, Beijirw, China. 10-14 November 1986, _ - -: . pp. 8-20. Academica Smtca, Beijing, China. Rcttger, J. (1977) Travelling disturbances in the equatorial ionosphere and their association with penetrative cumulus convection. J. atmos. terr. Phys. 39,987. Taylor, M. 1. (1986) TV observations of mesospheric wave structure, in Collection of Works of the International Workshop of Noctiiucent Clouds, TaNinn, Estonian SSR, USSR, August 1984, p. 153. Valgus, Tallinn, Estonian

S.S.R. Taylor, M. J. and Hapgood, M. A. (1988) Identification of a thunderstorm as a source of short period gravity waves in the upper atmospheric nightglow emissions. Planet. Space Sci. 36, 975. Taylor, M. J., Hapgood, M. A. and Rothwell, P. (1987) Observations of gravity wave propagation in the OI(557.7 nm), Na (589.2 nm) and the near infra-red OH nightglow emissions. Planet. Space Sci. 35,413. Till, R. (1974) S~u~~st~c~Methods for the Earth Scientist, Chap. 4. Macmillan, London. Tuan, T. F., Hedinger, R., Silverman, S. M. and Okuda, M. (1979) On gravity wave induced Brunt-Vaisala oscillations. J. aeouhys. Res. 84. 393. Tuan, T. F., Papadopoulos, D., Peterson, A. W. and Nadile, R. M. (1983) Analvsis of aravitv-wave induced instabilities and turbulence vi&osit;parameters from optical emissions. Adv. Space Res. 2, 137.