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Long-period wave signatures in mesospheric OH Meinel (6,2) band intensity and rotational temperature at mid-latitudes
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Pergamon
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Adv. Space Res. Voi. 27, Nos 6-7, pp. 1171-1179, 2001 © 2001 COSPAR. Published by Elsevier Science Ltd. All fights reserved Printed in Great Britain 0273-1177/01 $20.00 + 0.00 Pit: S0273-1177(01)00153-3
LONG-PERIOD WAVE SIGNATURES IN MESOSPHERIC OH MEINEL (6,2) BAND INTENSITY AND ROTATIONAL TEMPERATURE AT MID-LATITUDES
M. J. Taylor, L.C. Gardner and W.R. Pendleton, Jr.
Space Dynamics Laboratory and Physics Department, Utah State University, Logan, Utah, USA ABSTRACT A high performance imaging system has been used to investigate the signature of long-period, -8-hr, wave-like oscillations evident in the OH Meinel (6,2) band emission (peak altitude -87 km) during the fall and early winter months. The measurements were made from two mid-latitude sites in the western USA during 1996/7. Previous investigations of the induced temperature perturbations (amplitude and phase) suggest that many of these events exhibit characteristics akin to the mid-latitude terdiurnal tide (Pendleton et al., 2000). To further investigate the origin of these waves we have performed an initial investigation using the Krassovsky ratio (rl) method, to determine the amplitude ratio of the induced perturbations in the zenith OH emission intensity and rotational temperature and to study their phase relationship (q)). A range of values for the magnitude and phase of 11 were found with a mean value of [rll = 6 + 2 (range-2-10), and ~ =-51 ° + 21 ° ( r a n g e - l l ° to -94 °) with the temperature perturbation always leading the intensity wave. These results are in good agreement with existing high-latitude studies of distinct 8-hr oscillations in the literature. However, comparison with realistic gravity wave and terdiurnal tidal model computations reveal a conflicting situation where the observed negative phase results point more towards a long-period gravity wave interpretation rather than a terdiurnal tide. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
INTRODUCTION Gravity waves are known to be a major source of mesoscale fluctuations throughout the middle atmosphere (Holton, 1979). Waves generated in the lower atmosphere by weather related disturbances and/or orographic forcing are capable of transporting large amounts of energy and momentum into the upper atmosphere which, in turn, can have a profound impact on the general circulation and temperature structure of the mesosphere and lower thermosphere (MLT) region (-80-100 km). Gravity waves induce periodic fluctuations in the background temperature and wind field that, under favorable conditions, grow significantly in amplitude with increasing height (Hines, 1960). At MLT heights, gravity waves create substantial fluctuations in the line-of-sight column brightness and rotational temperatures of several airglow emissions (by perturbing the number densities of the reacting minor species whose reaction rates are also temperature dependent) (e.g. Walterscheid and Schubert 1995). The resultant spatial and temporal variability induced by gravity waves of various horizontal and vertical scale sizes was of considerable concern for early photometric investigations of the visible and near infrared (NIR) nightglow emissions. However, in 1972 Krassovsky proposed a method of investigating adiabatic processes in the MLT region by comparing the magnitudes of the induced intensity and rotational temperature perturbations. His study focussed on the OH Meinel airglow emission which originates from a well-defined layer of half-width -8 km centered at a nominal height of -87 km (Baker and Stair, 1988) and has since been used as an excellent tracer for studying wave dynamics. Krassovsky (1972) defined the scalar quantity TI:
1171
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M. Tayloret al. n = [AI/II/[AT/T]
(1)
where the A denotes the incremental change in I and T about mean values of I and T. For sinusoidal-type variations in airglow brightness and rotational temperature, rl is defined by the complex relationship:
n
In I e
(2)
where In I = [(AI/I)/(AT/T)], qb= ~)I - ~ and the parameters [(Al/I), 01] and [(AT/T), ~ ] are the relative perturbation amplitude and phase for the band intensity and rotational temperature, respectively, associated with a quasimonochromatic wave or a partiCular Fourier component of the measured temporal variations in I and T. In this expression, the mean (i.e. unperturbed values) may be approximated by local means (Viereck and Deehr, 1989) or by longer-term averages (Zhang et al., 1993). Several notable attempts have been made to determine n for quasi-monochromatic wave events (or for time series exhibiting a range of observed wave components), with periods ranging from several minutes to a few hours (e.g. Sivjee et al., 1987; Hecht et al., 1987; Viereck and Deehr, 1989; Swenson et al., 1990; Lowe et al., 199l; Taylor et al., 1991). To date, measurements of n in the tidal period range (i.e. typically >6 hrs) are important but exceptionally few (e.g. Reisin and Scheer, 1996). Of particular interest in this study is the measurement of n for long-period waves encompassing the terdiurnal (8-hr) tidal component. This wave component is known to be a prominent (yet intermittent) feature in the MLT wind field especially during the fall and winter seasons at midlatitudes (e.g. Thayaparan, 1997; Smith, 2000). However, optical observations of long-period -8-hr waves at midlatitudes are extremely rare (e.g. Wiens et al., 1995; Pendleton et al., 2000), and currently all of the available "8-hr" n studies have been performed at high-latitudes to take advantage of the long polar night observing conditions (Viereck and Deehr, 1989; Oznovich et al., 1995; Drob, 1996). Recently, we have developed a CCD imaging system capable of investigating n for both long-and-short-period waves in the MLT region. Our studies have focussed on characterizing long-period -8-hr wave perturbations in OH rotational temperature that we have found to occur relatively frequently at mid-latitudes during the fall and early winter months (Taylor et al., 1999; Pendleton et al., 2000). In this initial study we build on this work by investigating the relationship between the induced intensity and temperature perturbations for these waves, via the magnitude and phase of n, and compare their signatures with existing gravity wave and tidal model predictions. THE CEDAR MESOSPHERIC TEMPERATURE MAPPER The CEDAR Mesospheric Temperature Mapper (MTM) is a high-performance, solid-state imaging system designed to investigate wave-induced intensity and rotational temperature perturbations in the OH Meinel (6,2) band emission with a high precision. The imager consists of a large format (6.45 cm2), 1024 x 1024 pixel CCD array coupled to a medium field (75 ° circular) telecentric lens system. The high quantum efficiency (-50% at near infrared wavelengths) and low noise characteristics (dark current -0.1 el/pix/sec at -50°C) of the CCD array help provide an exceptional capability for precise nocturnal measurements of OH mesospheric intensity (< 0.5% in I min) and rotational temperature (<1-2 K in 3 min) (Taylor et al., 1999). Furthermore, the inherent linearity and stability of the MTM have also proven to be of considerable importance for long-term, seasonal investigations of the -87-km temperatures (Taylor et al., 2001). In operation, sequential exposures are made using a temperature-stabilized filter wheel fitted with narrow-band (A~, -- 1.2 nm) filters centered on the OH M Pl(2) and Pl(4) ~-doublet emissions at 840 and 846.5 nm, respectively, followed by a background measurement at 857 nm. To enhance the signal-to-noise (S/N) ratio of the data (necessary for precise temperature determinations), the image data are 8 x 8 binned on chip to form a 128 x 128 superpixel image with a resultant zenithal footprint of about 0.9 km x 0.9 km. Rotational temperatures are computed using the ratio method, as described for the OH (8,3) band by Meriwether [1975], and adopting a value of 1.300 for the ratio of transition probabilities, A[PI(4)] / A[PI(2)]. This ratio is based on the recently-updated OH M line parameters by Goldman et al. (1998). Although the higher levels (N' >5) of the OH M (X, v'= 6) are typically not in local thermodynamic equilibrium (LTE), the levels (N' = 1 and 3) used in our temperature determinations have been shown to satisfy LTE requirements (Pendleton et al., 1993; Dodd et al., 1994). The MTM data were compared with the CSU Na lidar-derived temperatures for 87 km level smoothed over 3.7 km in height. Twelve nights of overlapping data during the period Jun-Dec 1997 (average data length 7.5 hours) yielded a correlation coefficient (r) of 0.96 and a mean nightly difference of only 0.6 K. However, detailed comparisons
Long-Period Wave Signatures in Mesospheric OH
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during the course of the night indicate that our absolute temperatures have an accuracy of + 5 K referenced to the 87 km lidar-derived temperatures. Thus, the MTM rotational-temperature data provide an excellent measurement of the local atmospheric temperature at the -87 km level (averaged over the layer width of -8 km), and have been used by many researchers on numerous prior occasions. Further details on the MTM and the method of temperature determination can be found in Pendleton et al. (2000).
OBSERVATIONS AND RESULTS Since its development in 1996/7 the MTM has been operated almost continuously (typically 20 nights/month centered on the new moon). Initial field measurements were made from Bear Lake Observatory (BLO), LIT (41.6°N, 111.6°W) during the fall 1996 and Spring 1997. Subsequently, two extended sets of measurements were made alongside two powerful Na lidar systems located at Ft Collins, CO (41°N, 105°W) (Prof. C.Y. She, • v i n i' I = n Mean = 195.6 K Colorado State University) and at the Starfire Optical 206 .BLO, Day 281: • ==~.~o~== • P Er.ie~idlud 9 =156.~ K" Range near Albuquerque, NM (35°N, 107°W) (Prof. C. WII _~= ~ l ~"K" , ~ R =-~IE af/T --=3.4% S. Gardner, University of Illinois). The Ft Collins 200 measurements spanned a full year from June 1997 to June 1998 while our Starfire studies extended from I._~ ~ 195 October 1998 to December 2000. For comparison with the lidar measurements, only the central 5 x 5 • • • %o • •~ • "r superpixels of the CCD array have been analyzed to O 190 determine the nocturnal zenith temperature variability. These data are currently being used to help characterize 185 variability in the mid-latitude seasonal temperatures at (a) the -87 km level (Taylor et al., 2000). The precision of I I I I 110 i 112 2 4 6 8 14 the individual (-3 rain) temperature determinations Time (UT) 140000 n I : u I ~ n (important for gravity wave studies) is typically < 2 K M e a n = 9 0 6 4 7 counts (depending on the prevailing airglow emission levels). ~, Period = 9.15 hr In total over 250 nights of data have been obtained ~" Amplitude = 30646 counts 120000 = ~ . * AI/I = 33.8% by the MTM many of which show clear signatures of long period (several hour) wave activity in both intensity & and rotational temperature. An example of such data is '~" 100000 given in Figure 1 which exhibits a well-defined, longN period oscillation (-9-hrs) in OH M (6,2) rotational temperature (plot a) and band intensity (plot b). The I~ 80000 data were recorded at BLO on the night of October 6-7, ,.5 • .~ 1996 (UT day 281) during a period of coordinated measurements with the Mid-Course Space Experiment "F 60000 F O " g'~'~" (b) (MSX) satellite (Pendleton et al., 2000). The I I I I I a I 2 4 6 8 10 12 14 temperature data are plotted at their full temporal Time (UT) resolution of approximately one determination every three minutes (i.e. -18 samples/hr) and the example Figure 1. Example of a long-period (-9.1-hr) error bars indicate an uncertainty of + 2 K. The bandoscillation in (a) OH rotational temperature and (b) intensity data were derived from the backgroundOH (6,2) band intensity recorded from Bear Lake corrected P~(2) emission line intensity and are plotted at Observatory, Utah on day 281, 1996. The data are the same sample rate. The total length of the data set on plotted at the full (-3 min) resolution. The solid this night was - 10 hrs. curve in (a) indicates a four-parameter least-squares The solid curve in Figure l a shows a four parameter fit to the data. The corresponding curve in (b) shows least-squares fit (mean value, wave period, amplitude, a similar-type fit to the intensity data using the wave and phase) to the raw (-3 min) temperature data period deduced from the temperature data Note the assuming a single sinusoid perturbation. The fit is distinct phase shift between the temperature and clearly very good indicating a wave period of 9.15 + 1.0 intensity waves• hr, a mean value of 196 K, a significarh wave amplitude of 6.6 + 0.4 K (perturbation amplitude AT/T = 3.4%), and the phase of maximum temperature at 4.9 + 0.2 hr (UT). In comparison, the solid curve in Figure lb shows a three parameter best fit (mean value, amplitude and phase) to
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M. Tayloret aL
the band intensity data assuming a wave perturbation of the same period (9.15- hr) as determined from the temperature data. Again the overall fit to the data is very good although in this case the intensity data also show evidence of significant shorter period oscillation prior to local midnight (07:00 UT) that is not immediately obvious in the temperature data. This fit yields a large perturbation amplitude (AI/I) of -34% and a phase of maximum intensity at 5.7 + 0.2 hr (UT). The magnitude of the Krassovsky ratio for this wave perturbation (determined from equation 2), was found to be 10 + 1. Of key importance to these measurements is the distinct phase shift ({~ = 31 ° + 11 °) between these two data sets indicating that the temperature perturbation led the intensity perturbation by 47 min (-0.5 rad) on this occasion. To further investigate the relationship between the induced amplitude and phase of the intensity and temperature perturbations for long-period (> 6-hr) waves and their variability, the MTM data have subsequently been binned into 0.5-hr sample intervals. This was done to 225 enhance the S/N ratio for the long-period wave BLO, Day 2 9 2 Mean = 209.9 K studies (without unduly compromising the T Period = 6.7 hr determination of their wave period) and to average 220 A m p l i t u d e = 6.75 K out the effects of short-period waves (typically <1 hr) that were often evident in the image data. In this initial study of the Krassovsky ratio rl, our I.._~ method of approach was as follows. The raw (-3 ~ 210 min) temperature data were first visually inspected to determine the presence of candidate long-period -r oscillations. The corresponding intensity data were 0 205 then examined, and in all of the cases studied to date, a similar but usually more structured perturbation 2O0 was evident. The 0.5-hr-averaged temperature data (a I , I = I = I , I were then analyzed assuming a simple sinusoidal 4 6 8 10 12 14 perturbation and a least-squares fit performed to Time (UT) determine the observed wave period, the fractional 140000 I i I I I wave perturbation amplitude (AT/T), and the Mean = 98587 counts Pedod = 6.7 hr "I" -Llocation of the maximum phase of the wave (@T, in Amplitude = 23965 counts UT) (as depicted in Figure 1). Since the 120000 AI/I = 24.3% o temperature data are expected to be far less affected by changing conditions in the OH emission layer during dusk and dawn twilight (see discussion for 100000 further details), the wave period derived from the -m i ! temperature data was then imposed on the leastrn 80000 squares fit to the intensity data to determine the corresponding fractional intensit2~ perturbation [AI/I] and phase (0t). In each case, the residuals were "1- 60000 examined to ensure a high-quality fit. o , , , , , , , , , , (b) Figure 2a,b shows the 0.5-hr-averaged 4 6 8 10 12 14 temperature and corresponding intensity perturbation Time (UT) data for 17-18 October, 1996 (UT day 292). The measurements were also made from BLO and form Figure 2. Second example of a long-period (-6.7-hr) part of a 10-night series of high-quality temperature wave observed from BLO. For comparison the data are measurements. The individual "error bars" indicate plotted in the same format as Figure l but have been + 1~ standard deviation of the averaged data which average into 30-min bins. (The "error bars" indicate +_ l~ is dominated by small-scale geophysical variability. standard deviation of the averaged data.) The solid curve in Figure 2a shows the best fit to the temperature data indicating a wave period of 6.7 + 1.5 hrs. The amplitude of the temperature perturbation (6.75 K) and fractional temperature perturbation (AT/T = 3.2 %), are almost identical to those derived from Figure la. However, the corresponding fit to the intensity perturbation (Figure 2b) yields a somewhat smaller perturbation value (AI/I = 24.3 %) indicating a value for Ill I = 7.6 _+3,0 and @= -66 ° _+ 26 °. These values compare reasonably well with those of Figure 1 and indicate that the temperature perturbation again led the intensity wave. i
g212
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i
I
~T
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I
I
~"'=3"2%
Long-PeriodWaveSignaturesin MesosphericOH
1175
• i , i • i i i i Figure 3 shows an identical analysis of a well24s Ft. Collins, Day 307 Mean 227.2 K defined 8.6 + 1.0 hr wave recorded at Ft. Collins, CO ~T Period = 8.8 hr 240 during the following year on 2-3 November, 1997 (UT Amplitude = 10.7 K = . % day 307). This was a particularly interesting event as 235 on this night both the M T M and the coincident Na ~. lidar measurements indicated anomalously high mean ~- 2 3 0 temperatures ( - 2 2 6 K at - 8 7 km). In this case the 225 fractional temperature perturbation amplitude (AT/T = :~ 4.7 %) was somewhat larger than in the previous two -1- 220 examples while the intensity perturbation was lower o (AI/I = 18.3 %) resulting in a significantly smaller 2t~ value for Ill I = 3.9 _+ 1.7. However, the phase (~ = 210 (a) 94 ° -+ 41 °) is again clearly negative. Ongoing 2 4 6 8 10 12 investigations of this night (S. Melo, private Time (UT) communication) have revealed the presence of an exceptionally large mesospheric inversion layer which 320o00 i i i i i may have significantly altered the OH emission profile M e a n = 226284 counts on this occasion, possibly affecting the interpretation Pemrip/:u;: "--6h ; 3 0 7 c ° u n t s X ~--~ T of this event, go 280000 AI/I = 1 8 . 3 % Figure 4 shows a somewhat different data set recorded f r o m Ft. Collins on 23-24 December, 1997 (UT day 358). In this case the raw data are plotted to ~ 240000 emphasize the presence of a strong -40-min period E oscillation superimposed on a long-period wave. The m 200000 best fit to the long period perturbation wave 9.2 + 1.5 ~" hr. This wave exhibited an exceptionally large i temperature amplitude of 13.3 K (twice that of Figures -1- ~6oo0o (b) 1 & 2) and a correspondingly large fractional o I = I i I i I i I I I temperature perturbation AT/T = 5.5 %. In 2 4 6 3 10 12 comparison, the induced fractional intensity Time (UT) perturbation (AI/I = 21.8 %) was similar to that of F i g u r e 3. T h i r d e x a m p l e o f a l o n g - p e r i o d ( - 8 . 6 Figures 2 & 3. The net effect is to drastically lower hr) w a v e , but this t i m e r e c o r d e d at Ft. C o l l i n s , C O value of = 2.2 _+ 0.5 compared with previous approximately one year after the BLO measurements• The relative phase of the intensity and m e a s u r e m e n t s . (The d a t a are a v e r a g e d in the s a m e temperature perturbations is also much less (~ = -1 1° _+ 3 ° ) but still negative within the estimated measurement w a y as F i g u r e 2.) uncertainty. For comparison, a summary of the observed wave periods and the derived Krassovsky parameters ( Ill l, d~) for these four events is given in Table 1. =
/-
Iql
T a b l e 1. Compendium of - 8 - h r wave measurements of the Krassovsky ratio parameters Iq I and ~. The BLO measurements form part of a 10-night series of high-quality measurements that exhibited a strong terdiurnal tidal signature (Pendleton et al., 2000). Site/Year
UT Day
Period (hr)
Inl
BLO/ 1996 " Ft. Collins/1997 " Eureka, C a n a d a / 1 9 9 3 " Thule, Greenland/1991 Svalbard/ 1986
281 292 307 358 01 356 Jan Dec
9.2 + 1.0 6.7 _+ i.5 8.6 + 1.0 9.2_+1.5 8.1 -+ 1-2 8.1 _+ 1-2 -8 -8
10 _+ 1 7.6 _+ 3.0 3.9 _+ 1.7 2.2_+0.5 2.8 -+ 0.7 8.8 _+ 1.7 2.5 _+0.5 -5
Reference -31 ° _+ 11 ° -66 ° _+ 26 ° -94°_+ 41 ° -11°+3 ° - 12 ° _+ 20 ° -107 ° _+ 9 ° -30 ° _+ 15 ° -0
This study " " " Oznovich et al. (1995) " Drob (1996) Viereck & Deehr (1989)
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M. Taylor etal.
DISCUSSION Surprisingly, there are relatively few measurements of "q in the literature for long period (several hour) gravity waves. Reisin and Scheer (1996) have performed the most comprehensive mid-latitude study to date using OH (6,2) and O2(0,1) band spectrometric data obtained from two sites in opposite hemispheres located at 32 ° S and 37 ° N. Their analyses spanned a wide range of wave periods but focussed on quasi-monochromatic events with observed periods centered on 12 + 2 hrs and yielded a mean value for It I I = 5.0 + 0.6 and ~ = -70.3 ° + 6.5 ° for the semidiurnal period range. In comparison, our initial results listed in Table 1 indicate a mean amplitude for ITI I = 6 + 2 (range -2-10) and ~ = -51 o + 21 ° both of which are in satisfactory agreement with the findings of Reisin and Scheer. However, our study has focussed on a significantly different part of the wave spectrum encompassing waves with periodicities close to the 8-hr terdiurual tide (within our measurement accuracy). Of particular importance to both of these studies is the finding that the phase of r I was consistently negative indicating that the induced temperature perturbation always led the intensity wave. We are not aware of any other mid-latitude OH 240 measurements of rl in the literature covering this period Mean = 210.4 K Ft. Collins, D a y 3 5 8 range. For a more direct comparison of our intensity Period = 9.2 hr 230 A m p l i t u d e = 13.3 K • '~ L 'I and temperature data for -8-hr waves we turn to a 220 ttT/T = 5 . 5 % ~ limited set of very high-latitude measurements that have been conducted during the long polar nights by Viereck and Deehr (1989), Oznovich et al., (1995), and Drob (1996). For convenience, the results of these three studies are also listed in Table 1. The measurements of Oznovich et al., (1995) are the most detailed and will be described first. Their data were obtained from Eureka, Canada (80°N) during December/January 1993/94 18o (a) using a Michelson interferometer. On two occasions = I i I = I i I = I (UT day 356 and day 01) separated by 10 days, they 0 2 4 6 8 10 observed marked -8-hr oscillations consisting of 3 and Time (kiT) 5 cycles respectively. Measurements of the OH M 180000 i i i i i (3,1) band intensity and rotational temperature during M e a n = 1 2 8 1 8 0 counts Period = 9.2 hr 't these two extended periods yielded corresponding 160000 A m p l i t u d e = 18083 counts A values for ITII = 8.8 + 1.7, ~ = -107 ° + 9 °, and = ~,1/I = . ** 2.8 + 0.7, ~ = -12 o + 20 ° with temperature leading o intensity. On examination these apparently disparate 140000 results are in good agreement with our mid-latitude observations. In particular, our data and that of 120000 Oznovich et al. both contain examples of large I1"1I and corresponding large -¢ (e.g. days 292 and 356), and ~" 100000 small associated limited (or close to zero) phase shift (e.g. days 358 and 01). Whether these are -r O (b) signatures of two different types of wave disturbance 80000 I I i I I i I = I 2 4 6 8 10 remains uncertain at this time. However, our data also Time (UT) indicate intermediate values for I'q I and ¢. Figure 4. Another example of Ft. Collins OH data The second high-latitude data set was reported b y showing a strong - 4 0 min oscillation superimposed on Drob (1996) and consisted of a most unusual wave train a long period (~9.2-hr) wave. In this case the data are comprising approximately ten complete 8-hr plotted at the full ~3-min resolution to emphasize the oscillations (i.e - 8 0 hrs of data). The data were shorter period wave. recorded from Thule Air Base, Greenland (76.3°N) during late December 1991, and measurements of the OH (3,1) M band (again using a Michelson interferometer) indicated values for I1"1I = 2.5 + 0.5 and ¢ = -30 ° + 15 °. These values are remarkably similar to our day 358 results and the day 01 measurements of Oznovich et al. (1995). Finally, Viereck and Deehr (1989) also reported large-scale oscillations with a period of about 8-hr in their data set recorded at Longyearbyen, Svalbard (78°N) during 28-29 December 1986. Measurements of the OH(6,2) band emission using an Ebert-Fastie spectrometer yielded rough estimates for the magnitude - 5 and phase ~9 ° of these waves. i
i
i
i
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- -
Iol
I~lwith
,
i
Long-Period Wave Signatures in Mesospheric OH
1177
To place these results in perspective, 12 I v I v I I I I I I - - Sch el al '91 Figure 5 compares the magnitude (plot a) (a) ........ Sch et al. '91 and phase (plot b) of the results listed in 0 w~n & Sch '95 10 • MTM (This Study) IT i Table 1 with model calculations assuming a • viel~lck & Deehr '89 • Oznovich el ill '95 gravity wave perturbation (Schubert et al., • Drob'~ 1991) or a terdiurnal tide (Walterscheid and Schubert, 1995). The gravity wave model data are for a realistic, extended, dissipative .__= emission region and are plotted versus cY ...........1 .................--..-..T .................................... observed wave period for two assumed ........................ L9 I , horizontal wavelengths of 500 km and 1000km. The individual data points in plot lO00km k~ TI ~(a) appear to fall into two groups: one with B T high |111- 8-10 and the other exhibiting low I111- 2-4. The gravity wave model falls I I I I q i I I I I in between theses two groups but it is clear 4 5 6 7 8 9 10 11 12 that waves o f longer and ,shorter period Observed Period (hr) would intersect these clusters. In I I I I ~ I I I comparison, the open circles indicate 4O (b) o computed values for I11[ assuming an 8-hr O 2O migrating tide (Walterscheid and Schubert, O I000 krn 0 T 1995). Several modes are predicted (each 0 A exhibiting different e~uivalent depths) and -20 covering the range Ir I | = 1.4 - 4.3. These ............ " T T ........ ............... ' ..... modes fall into the lower cluster only. " ~ -40 Similar computations of the zonally "O v -60 -esymmetric 8-hr tide (not shown) indicate a similar clustering around t111 -2 - 4 but -80 also a mode at 9 (which coincides o w., ~ s,=,,95 I I -100 • MTM(ThisStudy) [1 with the upper clustering). However, this • Vieruek & D e e h r ' 8 9 I~1 • O z n o v i e h et al. '95 --4 mode has a relatively large equivalent depth -120 • Orob '96 l (-27 km) and may be evanescent. As the I I I I i I i I I -140 zonally symmetric tide is expected to be 4 5 6 7 8 9 !0 11 12 prominent at very high latitudes, it is Observed Period (hr) therefore applicable to the polar region data listed in Table 1, but probably not to our Figure 5. Summary plots showing (a) the distribution in the mid-latitude data. Thus, it is tempting to magnitudes I rl 1, and (b) phases ~ of 1"1for the mid- and highsuggest that the lower cluster may well be a latitude data listed in Table 1. For comparison the results of a signature of tidally induced perturbation. realistic gravity wave model (Schubert et al., 1991) and the However, as the gravity wave model can be modal computations for the 8-hr migrating tide (Walterscheid adjusted to predict values of I1] I similar to and Schubert, 1995) are plotted. those observed, comparison of the magnitudes of rl alone cannot readily be used to discriminate between these two types of wave perturbations. Examination of the expected phase relationship for long-period gravity waves (Figure 5b) and the migrating terdiurnal tide with the data indicate a different situation. Again two clusters of data are evident one centered around -80 ° to -100 ° and the other around -10 ° to -20 °. In this case the gravity wave model for horizontal wavelengths of 500-1000 km fit well with the small phase cluster. Ironically, this phase cluster corresponds to the small 11"1] group that fitted best with a tidal interpretation. More importantly, calculations for both the migrating 8-hr tide and the zonally symmetric tide indicate positive values for the phase of 11 in the range -7 - 43 ° (Walterscheid and Schubert, 1995). Although some of our mid-latitude data (and the tabulated high-latitude data) indicate quite small values for ~, the ensemble of data clearly shows that the temperature perturbation invariably led the intensity wave. With our current knowledge of the expected tidal phase relationship, this would indicate that none of the waves were tidal in origin.
I J-,
I111-
,
!
I 178
M. Tayloret al.
In their discussion of their high-latitude data Oznovich et al., (1995) also considered the possibility that the waves were the signature of a high-latitude non-migrating tide (zero zonal wave number) or a long-period gravity wave. The persistence of each event (-24 and -40 hours) and the derived long vertical wavelength of -80 km supported a tidal-type perturbation. However, the negative phase results were indicative of a gravity wave-type perturbation. Likewise, the long duration (-80 hours) of the -8-hr wave observed by Drob (1996) is strongly suggestive of a tidal-type disturbance. Recently Pendleton et al., (2000) have investigated the origin of the longperiod waves evident in our mid-latitude data. BLO data averaged over a 10 day period indicated a distinct -8-hr periodicity whose amplitude and phase were found to be consistent with those determined from 24-hr Na lidar measurements of the terdiurnal tidal component (States and Gardner, 2000). A similar result was also found for a 4 day average of data obtained from Ft. Collins. The two BLO data sets presented here were drawn for the 10-night ensemble suggesting that they too were tidal in origin. However, this conclusion is currently not consistent with our r I studies described above. SUMMARY This initial investigation into the Krassovsky ratio has yielded new information on the induced intensity and temperature perturbations for ~8-hr period waves present in the mid-latitude MLT region around the fall and early winter months. Determination of rl at mid-latitudes for such long-period waves is limited by the duration of the night-time measurements (up to 12 hours during winter months) and by photochemical effects around dusk and dawn twilight that can substantially alter the mean height and thickness of the OH emission layer, and hence the shape of its zenith emission profile. For this reason the OH temperature data were used to determine the period of the wave as the intensity ratio P~(4)/Pt(2) used to compute the rotational temperatures is expected to be far less affected by changing conditions in the OH emission layer during twilight. Two clusterings of ]111 and ¢ were found that exhibited similar characteristics to previous measurements at high-latitudes. In general the low Ill I and small ¢ cluster are in best agreement with the current tidal and gravity wave models. However, there are strong evidence (at mid-and high latitudes) for much larger values of I rl I -8-10 that exhibit substantial negative phases - -80 ° to -100 °. These wave perturbations are not readily accommodated using either the existing gravity wave or tidal models. Additional data relating to the long duration of these events points to a tidal-type source for many of the wave perturbations discussed here. However, their negative Krassovsky phase values suggest a gravity wave-type interpretation. Other data, currently under analysis, support this conclusion and it is hoped that these results will stimulate further modeling studies to help understand the apparently conflicting phase relationships. ACKNOWLEDGEMENTS Financial support for the development and operation of the CEDAR Mesospheric Temperature Mapper was provided by NSF grants ATM-9403474 and ATM-9612810. We are most grateful to C.Y. She, M.A. White and S.S. Chen (Colorado State University) for their considerable help with the MTM measurements at Ft, Collins, CO. One of us (LCG) was supported in part on NSF grant ATM-9525815 during the data analysis. REFERENCES Baker, D.J., Stair Jr., A.T., 1988. Rocket experiments of the altitude distributions of the hydroxyl airglow. Phys. Scr., 37, 611-622. Dodd, J.A., Lipson, S.J., Lowell, J.R., Armstrong, P.S., Blumberg, W.A.M., Nadile, R.M., Adler-Golden, S.M., Marinelli, W.J., Holtzclaw, K.W., Green, B.D., 1994. Analysis of hydroxyl earthlimb airglow emissions: kinetic model for state-to-state dynamics of OH (v, N). J. Geophys. Res., 99, D2, 3559-3585. Drob, D.P., 1996. Ground-based optical detection of atmospheric waves in the upper mesosphere and lower thermosphere. Ph.D. Thesis, University of Michigan. Goldman; A., Schoenfeld, W.G., Goorvitch, D., Chackerian Jr., C., Dothe, H., M61en, F., Abrams, M.C., Selby, J.E.A., 1998. Updated line parameters for the OH X2H-X21-I (v",v') transitions. J. Quant. Spectrosc. Radiat. Transfer, 59, 453-469. Hecht, J.H., Walterscheid, R.L., Sivjee, G.G., Christensen, A.B., Pranke, J.B., 1987. Observations of wave-driven fluctuations of OH nightglow emission from Sondre Stromfjord, Greenland. J. Geophys. Res., 92, 6091-6099. Hickey, M.P., Schubert, G., Walterscheid, R.L., 1992. Seasonal and latitudinal variations of gravity wave-driven fluctuations in OH nightglow. J. Geophys. Res., 97, A10, 14,911-14,922.
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Hines, C.O., 1960. Internal atmospheric gravity waves at ionospheric heights. Can. J. Phys., 38, 1441-1481. Holton, J., 1979. An introduction to dynamic meteorology. International Geophysical series, 23, Academic Press. Krassovsky, V.I., 1972. Infrasonic variations of the OH emission in the upper atmosphere. Ann. Geophys. 28, 739746. Lowe, R.P., Gilbert, K.L., Turnbull, D.N., 1991. High latitude summer observations of the hydroxyl airglow. Planet. Space Sci., 39, 1263-1270. Meriwether, J. W., 1975. High latitude airglow observations of correlated short-term fluctuations in the hydroxyl Meinel 8-3 band intensity and rotational temperature. Planet. Space Sci., 43, 1211-1221. Oznovich, I., McEwen, D.J., Sivjee, G.G., 1995. Temperature and airglow brightness oscillations in the polar mesosphere and lower thermosphere. Planet. Space Sci., 43, No 9, 1121-1130. Pendleton, W.R. Jr., Espy, P.J., Hammond, M.R., 1993. Evidence for non-local-thermodynamic-equilibrium rotation in the OH nightglow. J. Geophys. Res., 98, A7, 11567-11579. Pendleton, W.R. Jr., Taylor, M.J., Gardner, L.C., 2000. Terdiurnal oscillations in OH Meinel rotational temperatures for fall conditions at northern mid-latitude sites. Geophys. Res. Lett., 27, 1799-1802. Reisin, E.R., Scheer, J., 1996. Characteristics of atmospheric waves in the tidal period range derived from zenith observations of O~ (0,1) Atmospheric and OH(6,2) airglow at lower midlatitudes. J. Geophys. Res., 101, D16, 21,223-21232. Schubert, G., Walterscheid, R.L., Hickey, M.P., 1991. Gravity wave-driven fluctuations in OH nightglow from an extended, dissipative emission region. J. Geophys. Res., 96, No A8, 13,869-13,880. Sivjee, G.G., Walterscheid, R.L., Hecht, J.H., Hamwey, R.M., Schubert, G., Christensen, A.B., 1987. Effects of atmospheric disturbances on polar mesopause airglow OH emissions. J. Geophys. Res., 92, 7651-7656. Smith, A. K., 2000. Structure of the terdiurnal tide at 95 km. Geophys. Res Lett., 27, 177-180. States, R. J., Gardner, C .S., 2000. Thermal structure of the mesopause region (80-105 km) at 40°N Latitude. Part II: Diurnal variations. J. Atmos. Sci., 57, 78-92. Swenson, G.R., Mende, S.B., Geller, S.P., 1990. Fabry-Perot imaging of OH(8-3): rotational temperatures and gravity waves. J. Geophys. Res., 95, 12,251-12,263. Tarasick, D.W., Hines, C.O., 1992. The observable effects of gravity waves on airglow emissions. Planet. Space Sci., 38, 1105-1119. Taylor, M.J., Espy, P.J., Baker, D.J., Sica, R.J., Neal, P.C., Pendleton, W.R. Jr., 1991. Simultaneous intensity, temperature and imaging measurements of short period wave structure in the OH nightglow emission. Planet. Space Sci., 39, 1171-1188. Thayaparan, T., 1997. The terdiurnal tide in the mesosphere and lower thermosphere over London, Canada (43 ° N, 81°W). J. Geophys. Res., 102, D18, 21,695-21,708. Taylor, M. J., Pendleton, W.R. Jr., Gardner, C.S., States, R.J., 1999. Comparison of terdiurnal tidal oscillations in mesospheric OH rotational temperatures and Na lidar temperature measurements at mid-latitudes for fall / spring conditions. Earth, Planets, and Space, 51,877-885. Taylor, M.J., Pendleton, W.R. Jr., Liu, H.-L., She, C.Y., Gardner, L.C., Roble, R.G. and Vasoli, V., 2001, Large amplitude perturbations in mesospheic OH Meinel and 87-km Na lidar temperatures around the autunmal equinox, Geophys. Res. Lett., in press. Viereck, R.A., Deehr, C.S., 1989. On the interaction between gravity waves and the OH Meinei (6,2) and the 02 Atmospheric (0,1) bands in the polar night airglow. J. Geophys. Res., 94, 5397-5404. Walterscheid, R.L., Schubert, G., 1995. Dynamical-chemical model of fluctuations in the OH airglow driven by migrating tides, stationary tides, and planetary waves. J. Geophys. Res., 100, A9, 17,443-17,449. Wiens, R.H., Zhang, S.P., Peterson, R., Shepherd, G.G., 1995. Tides in emission rate and temperature from 02 nightglow over Bear Lake Observatory. Geophys. Res. Lett., 22, 2637-2640. Zhang, S., Wiens, R.H., Shepherd, G.G., 1993. Gravity waves from 02 nightglow during the AIDA'89 campaign, II, Numerical modeling of the emission rate/temperature ratio, rl. J. Atmos. Terr. Phys., 54, 377-395.
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Adv. Space Res. Voi. 27, Nos 6-7, pp. 1171-1179, 2001 © 2001 COSPAR. Published by Elsevier Science Ltd. All fights reserved Printed in Great Britain 0273-1177/01 $20.00 + 0.00 Pit: S0273-1177(01)00153-3
LONG-PERIOD WAVE SIGNATURES IN MESOSPHERIC OH MEINEL (6,2) BAND INTENSITY AND ROTATIONAL TEMPERATURE AT MID-LATITUDES
M. J. Taylor, L.C. Gardner and W.R. Pendleton, Jr.
Space Dynamics Laboratory and Physics Department, Utah State University, Logan, Utah, USA ABSTRACT A high performance imaging system has been used to investigate the signature of long-period, -8-hr, wave-like oscillations evident in the OH Meinel (6,2) band emission (peak altitude -87 km) during the fall and early winter months. The measurements were made from two mid-latitude sites in the western USA during 1996/7. Previous investigations of the induced temperature perturbations (amplitude and phase) suggest that many of these events exhibit characteristics akin to the mid-latitude terdiurnal tide (Pendleton et al., 2000). To further investigate the origin of these waves we have performed an initial investigation using the Krassovsky ratio (rl) method, to determine the amplitude ratio of the induced perturbations in the zenith OH emission intensity and rotational temperature and to study their phase relationship (q)). A range of values for the magnitude and phase of 11 were found with a mean value of [rll = 6 + 2 (range-2-10), and ~ =-51 ° + 21 ° ( r a n g e - l l ° to -94 °) with the temperature perturbation always leading the intensity wave. These results are in good agreement with existing high-latitude studies of distinct 8-hr oscillations in the literature. However, comparison with realistic gravity wave and terdiurnal tidal model computations reveal a conflicting situation where the observed negative phase results point more towards a long-period gravity wave interpretation rather than a terdiurnal tide. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
INTRODUCTION Gravity waves are known to be a major source of mesoscale fluctuations throughout the middle atmosphere (Holton, 1979). Waves generated in the lower atmosphere by weather related disturbances and/or orographic forcing are capable of transporting large amounts of energy and momentum into the upper atmosphere which, in turn, can have a profound impact on the general circulation and temperature structure of the mesosphere and lower thermosphere (MLT) region (-80-100 km). Gravity waves induce periodic fluctuations in the background temperature and wind field that, under favorable conditions, grow significantly in amplitude with increasing height (Hines, 1960). At MLT heights, gravity waves create substantial fluctuations in the line-of-sight column brightness and rotational temperatures of several airglow emissions (by perturbing the number densities of the reacting minor species whose reaction rates are also temperature dependent) (e.g. Walterscheid and Schubert 1995). The resultant spatial and temporal variability induced by gravity waves of various horizontal and vertical scale sizes was of considerable concern for early photometric investigations of the visible and near infrared (NIR) nightglow emissions. However, in 1972 Krassovsky proposed a method of investigating adiabatic processes in the MLT region by comparing the magnitudes of the induced intensity and rotational temperature perturbations. His study focussed on the OH Meinel airglow emission which originates from a well-defined layer of half-width -8 km centered at a nominal height of -87 km (Baker and Stair, 1988) and has since been used as an excellent tracer for studying wave dynamics. Krassovsky (1972) defined the scalar quantity TI:
1171
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M. Tayloret al. n = [AI/II/[AT/T]
(1)
where the A denotes the incremental change in I and T about mean values of I and T. For sinusoidal-type variations in airglow brightness and rotational temperature, rl is defined by the complex relationship:
n
In I e
(2)
where In I = [(AI/I)/(AT/T)], qb= ~)I - ~ and the parameters [(Al/I), 01] and [(AT/T), ~ ] are the relative perturbation amplitude and phase for the band intensity and rotational temperature, respectively, associated with a quasimonochromatic wave or a partiCular Fourier component of the measured temporal variations in I and T. In this expression, the mean (i.e. unperturbed values) may be approximated by local means (Viereck and Deehr, 1989) or by longer-term averages (Zhang et al., 1993). Several notable attempts have been made to determine n for quasi-monochromatic wave events (or for time series exhibiting a range of observed wave components), with periods ranging from several minutes to a few hours (e.g. Sivjee et al., 1987; Hecht et al., 1987; Viereck and Deehr, 1989; Swenson et al., 1990; Lowe et al., 199l; Taylor et al., 1991). To date, measurements of n in the tidal period range (i.e. typically >6 hrs) are important but exceptionally few (e.g. Reisin and Scheer, 1996). Of particular interest in this study is the measurement of n for long-period waves encompassing the terdiurnal (8-hr) tidal component. This wave component is known to be a prominent (yet intermittent) feature in the MLT wind field especially during the fall and winter seasons at midlatitudes (e.g. Thayaparan, 1997; Smith, 2000). However, optical observations of long-period -8-hr waves at midlatitudes are extremely rare (e.g. Wiens et al., 1995; Pendleton et al., 2000), and currently all of the available "8-hr" n studies have been performed at high-latitudes to take advantage of the long polar night observing conditions (Viereck and Deehr, 1989; Oznovich et al., 1995; Drob, 1996). Recently, we have developed a CCD imaging system capable of investigating n for both long-and-short-period waves in the MLT region. Our studies have focussed on characterizing long-period -8-hr wave perturbations in OH rotational temperature that we have found to occur relatively frequently at mid-latitudes during the fall and early winter months (Taylor et al., 1999; Pendleton et al., 2000). In this initial study we build on this work by investigating the relationship between the induced intensity and temperature perturbations for these waves, via the magnitude and phase of n, and compare their signatures with existing gravity wave and tidal model predictions. THE CEDAR MESOSPHERIC TEMPERATURE MAPPER The CEDAR Mesospheric Temperature Mapper (MTM) is a high-performance, solid-state imaging system designed to investigate wave-induced intensity and rotational temperature perturbations in the OH Meinel (6,2) band emission with a high precision. The imager consists of a large format (6.45 cm2), 1024 x 1024 pixel CCD array coupled to a medium field (75 ° circular) telecentric lens system. The high quantum efficiency (-50% at near infrared wavelengths) and low noise characteristics (dark current -0.1 el/pix/sec at -50°C) of the CCD array help provide an exceptional capability for precise nocturnal measurements of OH mesospheric intensity (< 0.5% in I min) and rotational temperature (<1-2 K in 3 min) (Taylor et al., 1999). Furthermore, the inherent linearity and stability of the MTM have also proven to be of considerable importance for long-term, seasonal investigations of the -87-km temperatures (Taylor et al., 2001). In operation, sequential exposures are made using a temperature-stabilized filter wheel fitted with narrow-band (A~, -- 1.2 nm) filters centered on the OH M Pl(2) and Pl(4) ~-doublet emissions at 840 and 846.5 nm, respectively, followed by a background measurement at 857 nm. To enhance the signal-to-noise (S/N) ratio of the data (necessary for precise temperature determinations), the image data are 8 x 8 binned on chip to form a 128 x 128 superpixel image with a resultant zenithal footprint of about 0.9 km x 0.9 km. Rotational temperatures are computed using the ratio method, as described for the OH (8,3) band by Meriwether [1975], and adopting a value of 1.300 for the ratio of transition probabilities, A[PI(4)] / A[PI(2)]. This ratio is based on the recently-updated OH M line parameters by Goldman et al. (1998). Although the higher levels (N' >5) of the OH M (X, v'= 6) are typically not in local thermodynamic equilibrium (LTE), the levels (N' = 1 and 3) used in our temperature determinations have been shown to satisfy LTE requirements (Pendleton et al., 1993; Dodd et al., 1994). The MTM data were compared with the CSU Na lidar-derived temperatures for 87 km level smoothed over 3.7 km in height. Twelve nights of overlapping data during the period Jun-Dec 1997 (average data length 7.5 hours) yielded a correlation coefficient (r) of 0.96 and a mean nightly difference of only 0.6 K. However, detailed comparisons
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during the course of the night indicate that our absolute temperatures have an accuracy of + 5 K referenced to the 87 km lidar-derived temperatures. Thus, the MTM rotational-temperature data provide an excellent measurement of the local atmospheric temperature at the -87 km level (averaged over the layer width of -8 km), and have been used by many researchers on numerous prior occasions. Further details on the MTM and the method of temperature determination can be found in Pendleton et al. (2000).
OBSERVATIONS AND RESULTS Since its development in 1996/7 the MTM has been operated almost continuously (typically 20 nights/month centered on the new moon). Initial field measurements were made from Bear Lake Observatory (BLO), LIT (41.6°N, 111.6°W) during the fall 1996 and Spring 1997. Subsequently, two extended sets of measurements were made alongside two powerful Na lidar systems located at Ft Collins, CO (41°N, 105°W) (Prof. C.Y. She, • v i n i' I = n Mean = 195.6 K Colorado State University) and at the Starfire Optical 206 .BLO, Day 281: • ==~.~o~== • P Er.ie~idlud 9 =156.~ K" Range near Albuquerque, NM (35°N, 107°W) (Prof. C. WII _~= ~ l ~"K" , ~ R =-~IE af/T --=3.4% S. Gardner, University of Illinois). The Ft Collins 200 measurements spanned a full year from June 1997 to June 1998 while our Starfire studies extended from I._~ ~ 195 October 1998 to December 2000. For comparison with the lidar measurements, only the central 5 x 5 • • • %o • •~ • "r superpixels of the CCD array have been analyzed to O 190 determine the nocturnal zenith temperature variability. These data are currently being used to help characterize 185 variability in the mid-latitude seasonal temperatures at (a) the -87 km level (Taylor et al., 2000). The precision of I I I I 110 i 112 2 4 6 8 14 the individual (-3 rain) temperature determinations Time (UT) 140000 n I : u I ~ n (important for gravity wave studies) is typically < 2 K M e a n = 9 0 6 4 7 counts (depending on the prevailing airglow emission levels). ~, Period = 9.15 hr In total over 250 nights of data have been obtained ~" Amplitude = 30646 counts 120000 = ~ . * AI/I = 33.8% by the MTM many of which show clear signatures of long period (several hour) wave activity in both intensity & and rotational temperature. An example of such data is '~" 100000 given in Figure 1 which exhibits a well-defined, longN period oscillation (-9-hrs) in OH M (6,2) rotational temperature (plot a) and band intensity (plot b). The I~ 80000 data were recorded at BLO on the night of October 6-7, ,.5 • .~ 1996 (UT day 281) during a period of coordinated measurements with the Mid-Course Space Experiment "F 60000 F O " g'~'~" (b) (MSX) satellite (Pendleton et al., 2000). The I I I I I a I 2 4 6 8 10 12 14 temperature data are plotted at their full temporal Time (UT) resolution of approximately one determination every three minutes (i.e. -18 samples/hr) and the example Figure 1. Example of a long-period (-9.1-hr) error bars indicate an uncertainty of + 2 K. The bandoscillation in (a) OH rotational temperature and (b) intensity data were derived from the backgroundOH (6,2) band intensity recorded from Bear Lake corrected P~(2) emission line intensity and are plotted at Observatory, Utah on day 281, 1996. The data are the same sample rate. The total length of the data set on plotted at the full (-3 min) resolution. The solid this night was - 10 hrs. curve in (a) indicates a four-parameter least-squares The solid curve in Figure l a shows a four parameter fit to the data. The corresponding curve in (b) shows least-squares fit (mean value, wave period, amplitude, a similar-type fit to the intensity data using the wave and phase) to the raw (-3 min) temperature data period deduced from the temperature data Note the assuming a single sinusoid perturbation. The fit is distinct phase shift between the temperature and clearly very good indicating a wave period of 9.15 + 1.0 intensity waves• hr, a mean value of 196 K, a significarh wave amplitude of 6.6 + 0.4 K (perturbation amplitude AT/T = 3.4%), and the phase of maximum temperature at 4.9 + 0.2 hr (UT). In comparison, the solid curve in Figure lb shows a three parameter best fit (mean value, amplitude and phase) to
i 174
M. Tayloret aL
the band intensity data assuming a wave perturbation of the same period (9.15- hr) as determined from the temperature data. Again the overall fit to the data is very good although in this case the intensity data also show evidence of significant shorter period oscillation prior to local midnight (07:00 UT) that is not immediately obvious in the temperature data. This fit yields a large perturbation amplitude (AI/I) of -34% and a phase of maximum intensity at 5.7 + 0.2 hr (UT). The magnitude of the Krassovsky ratio for this wave perturbation (determined from equation 2), was found to be 10 + 1. Of key importance to these measurements is the distinct phase shift ({~ = 31 ° + 11 °) between these two data sets indicating that the temperature perturbation led the intensity perturbation by 47 min (-0.5 rad) on this occasion. To further investigate the relationship between the induced amplitude and phase of the intensity and temperature perturbations for long-period (> 6-hr) waves and their variability, the MTM data have subsequently been binned into 0.5-hr sample intervals. This was done to 225 enhance the S/N ratio for the long-period wave BLO, Day 2 9 2 Mean = 209.9 K studies (without unduly compromising the T Period = 6.7 hr determination of their wave period) and to average 220 A m p l i t u d e = 6.75 K out the effects of short-period waves (typically <1 hr) that were often evident in the image data. In this initial study of the Krassovsky ratio rl, our I.._~ method of approach was as follows. The raw (-3 ~ 210 min) temperature data were first visually inspected to determine the presence of candidate long-period -r oscillations. The corresponding intensity data were 0 205 then examined, and in all of the cases studied to date, a similar but usually more structured perturbation 2O0 was evident. The 0.5-hr-averaged temperature data (a I , I = I = I , I were then analyzed assuming a simple sinusoidal 4 6 8 10 12 14 perturbation and a least-squares fit performed to Time (UT) determine the observed wave period, the fractional 140000 I i I I I wave perturbation amplitude (AT/T), and the Mean = 98587 counts Pedod = 6.7 hr "I" -Llocation of the maximum phase of the wave (@T, in Amplitude = 23965 counts UT) (as depicted in Figure 1). Since the 120000 AI/I = 24.3% o temperature data are expected to be far less affected by changing conditions in the OH emission layer during dusk and dawn twilight (see discussion for 100000 further details), the wave period derived from the -m i ! temperature data was then imposed on the leastrn 80000 squares fit to the intensity data to determine the corresponding fractional intensit2~ perturbation [AI/I] and phase (0t). In each case, the residuals were "1- 60000 examined to ensure a high-quality fit. o , , , , , , , , , , (b) Figure 2a,b shows the 0.5-hr-averaged 4 6 8 10 12 14 temperature and corresponding intensity perturbation Time (UT) data for 17-18 October, 1996 (UT day 292). The measurements were also made from BLO and form Figure 2. Second example of a long-period (-6.7-hr) part of a 10-night series of high-quality temperature wave observed from BLO. For comparison the data are measurements. The individual "error bars" indicate plotted in the same format as Figure l but have been + 1~ standard deviation of the averaged data which average into 30-min bins. (The "error bars" indicate +_ l~ is dominated by small-scale geophysical variability. standard deviation of the averaged data.) The solid curve in Figure 2a shows the best fit to the temperature data indicating a wave period of 6.7 + 1.5 hrs. The amplitude of the temperature perturbation (6.75 K) and fractional temperature perturbation (AT/T = 3.2 %), are almost identical to those derived from Figure la. However, the corresponding fit to the intensity perturbation (Figure 2b) yields a somewhat smaller perturbation value (AI/I = 24.3 %) indicating a value for Ill I = 7.6 _+3,0 and @= -66 ° _+ 26 °. These values compare reasonably well with those of Figure 1 and indicate that the temperature perturbation again led the intensity wave. i
g212
•
i
I
~T
/\
I
I
~"'=3"2%
Long-PeriodWaveSignaturesin MesosphericOH
1175
• i , i • i i i i Figure 3 shows an identical analysis of a well24s Ft. Collins, Day 307 Mean 227.2 K defined 8.6 + 1.0 hr wave recorded at Ft. Collins, CO ~T Period = 8.8 hr 240 during the following year on 2-3 November, 1997 (UT Amplitude = 10.7 K = . % day 307). This was a particularly interesting event as 235 on this night both the M T M and the coincident Na ~. lidar measurements indicated anomalously high mean ~- 2 3 0 temperatures ( - 2 2 6 K at - 8 7 km). In this case the 225 fractional temperature perturbation amplitude (AT/T = :~ 4.7 %) was somewhat larger than in the previous two -1- 220 examples while the intensity perturbation was lower o (AI/I = 18.3 %) resulting in a significantly smaller 2t~ value for Ill I = 3.9 _+ 1.7. However, the phase (~ = 210 (a) 94 ° -+ 41 °) is again clearly negative. Ongoing 2 4 6 8 10 12 investigations of this night (S. Melo, private Time (UT) communication) have revealed the presence of an exceptionally large mesospheric inversion layer which 320o00 i i i i i may have significantly altered the OH emission profile M e a n = 226284 counts on this occasion, possibly affecting the interpretation Pemrip/:u;: "--6h ; 3 0 7 c ° u n t s X ~--~ T of this event, go 280000 AI/I = 1 8 . 3 % Figure 4 shows a somewhat different data set recorded f r o m Ft. Collins on 23-24 December, 1997 (UT day 358). In this case the raw data are plotted to ~ 240000 emphasize the presence of a strong -40-min period E oscillation superimposed on a long-period wave. The m 200000 best fit to the long period perturbation wave 9.2 + 1.5 ~" hr. This wave exhibited an exceptionally large i temperature amplitude of 13.3 K (twice that of Figures -1- ~6oo0o (b) 1 & 2) and a correspondingly large fractional o I = I i I i I i I I I temperature perturbation AT/T = 5.5 %. In 2 4 6 3 10 12 comparison, the induced fractional intensity Time (UT) perturbation (AI/I = 21.8 %) was similar to that of F i g u r e 3. T h i r d e x a m p l e o f a l o n g - p e r i o d ( - 8 . 6 Figures 2 & 3. The net effect is to drastically lower hr) w a v e , but this t i m e r e c o r d e d at Ft. C o l l i n s , C O value of = 2.2 _+ 0.5 compared with previous approximately one year after the BLO measurements• The relative phase of the intensity and m e a s u r e m e n t s . (The d a t a are a v e r a g e d in the s a m e temperature perturbations is also much less (~ = -1 1° _+ 3 ° ) but still negative within the estimated measurement w a y as F i g u r e 2.) uncertainty. For comparison, a summary of the observed wave periods and the derived Krassovsky parameters ( Ill l, d~) for these four events is given in Table 1. =
/-
Iql
T a b l e 1. Compendium of - 8 - h r wave measurements of the Krassovsky ratio parameters Iq I and ~. The BLO measurements form part of a 10-night series of high-quality measurements that exhibited a strong terdiurnal tidal signature (Pendleton et al., 2000). Site/Year
UT Day
Period (hr)
Inl
BLO/ 1996 " Ft. Collins/1997 " Eureka, C a n a d a / 1 9 9 3 " Thule, Greenland/1991 Svalbard/ 1986
281 292 307 358 01 356 Jan Dec
9.2 + 1.0 6.7 _+ i.5 8.6 + 1.0 9.2_+1.5 8.1 -+ 1-2 8.1 _+ 1-2 -8 -8
10 _+ 1 7.6 _+ 3.0 3.9 _+ 1.7 2.2_+0.5 2.8 -+ 0.7 8.8 _+ 1.7 2.5 _+0.5 -5
Reference -31 ° _+ 11 ° -66 ° _+ 26 ° -94°_+ 41 ° -11°+3 ° - 12 ° _+ 20 ° -107 ° _+ 9 ° -30 ° _+ 15 ° -0
This study " " " Oznovich et al. (1995) " Drob (1996) Viereck & Deehr (1989)
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DISCUSSION Surprisingly, there are relatively few measurements of "q in the literature for long period (several hour) gravity waves. Reisin and Scheer (1996) have performed the most comprehensive mid-latitude study to date using OH (6,2) and O2(0,1) band spectrometric data obtained from two sites in opposite hemispheres located at 32 ° S and 37 ° N. Their analyses spanned a wide range of wave periods but focussed on quasi-monochromatic events with observed periods centered on 12 + 2 hrs and yielded a mean value for It I I = 5.0 + 0.6 and ~ = -70.3 ° + 6.5 ° for the semidiurnal period range. In comparison, our initial results listed in Table 1 indicate a mean amplitude for ITI I = 6 + 2 (range -2-10) and ~ = -51 o + 21 ° both of which are in satisfactory agreement with the findings of Reisin and Scheer. However, our study has focussed on a significantly different part of the wave spectrum encompassing waves with periodicities close to the 8-hr terdiurual tide (within our measurement accuracy). Of particular importance to both of these studies is the finding that the phase of r I was consistently negative indicating that the induced temperature perturbation always led the intensity wave. We are not aware of any other mid-latitude OH 240 measurements of rl in the literature covering this period Mean = 210.4 K Ft. Collins, D a y 3 5 8 range. For a more direct comparison of our intensity Period = 9.2 hr 230 A m p l i t u d e = 13.3 K • '~ L 'I and temperature data for -8-hr waves we turn to a 220 ttT/T = 5 . 5 % ~ limited set of very high-latitude measurements that have been conducted during the long polar nights by Viereck and Deehr (1989), Oznovich et al., (1995), and Drob (1996). For convenience, the results of these three studies are also listed in Table 1. The measurements of Oznovich et al., (1995) are the most detailed and will be described first. Their data were obtained from Eureka, Canada (80°N) during December/January 1993/94 18o (a) using a Michelson interferometer. On two occasions = I i I = I i I = I (UT day 356 and day 01) separated by 10 days, they 0 2 4 6 8 10 observed marked -8-hr oscillations consisting of 3 and Time (kiT) 5 cycles respectively. Measurements of the OH M 180000 i i i i i (3,1) band intensity and rotational temperature during M e a n = 1 2 8 1 8 0 counts Period = 9.2 hr 't these two extended periods yielded corresponding 160000 A m p l i t u d e = 18083 counts A values for ITII = 8.8 + 1.7, ~ = -107 ° + 9 °, and = ~,1/I = . ** 2.8 + 0.7, ~ = -12 o + 20 ° with temperature leading o intensity. On examination these apparently disparate 140000 results are in good agreement with our mid-latitude observations. In particular, our data and that of 120000 Oznovich et al. both contain examples of large I1"1I and corresponding large -¢ (e.g. days 292 and 356), and ~" 100000 small associated limited (or close to zero) phase shift (e.g. days 358 and 01). Whether these are -r O (b) signatures of two different types of wave disturbance 80000 I I i I I i I = I 2 4 6 8 10 remains uncertain at this time. However, our data also Time (UT) indicate intermediate values for I'q I and ¢. Figure 4. Another example of Ft. Collins OH data The second high-latitude data set was reported b y showing a strong - 4 0 min oscillation superimposed on Drob (1996) and consisted of a most unusual wave train a long period (~9.2-hr) wave. In this case the data are comprising approximately ten complete 8-hr plotted at the full ~3-min resolution to emphasize the oscillations (i.e - 8 0 hrs of data). The data were shorter period wave. recorded from Thule Air Base, Greenland (76.3°N) during late December 1991, and measurements of the OH (3,1) M band (again using a Michelson interferometer) indicated values for I1"1I = 2.5 + 0.5 and ¢ = -30 ° + 15 °. These values are remarkably similar to our day 358 results and the day 01 measurements of Oznovich et al. (1995). Finally, Viereck and Deehr (1989) also reported large-scale oscillations with a period of about 8-hr in their data set recorded at Longyearbyen, Svalbard (78°N) during 28-29 December 1986. Measurements of the OH(6,2) band emission using an Ebert-Fastie spectrometer yielded rough estimates for the magnitude - 5 and phase ~9 ° of these waves. i
i
i
i
•
- -
Iol
I~lwith
,
i
Long-Period Wave Signatures in Mesospheric OH
1177
To place these results in perspective, 12 I v I v I I I I I I - - Sch el al '91 Figure 5 compares the magnitude (plot a) (a) ........ Sch et al. '91 and phase (plot b) of the results listed in 0 w~n & Sch '95 10 • MTM (This Study) IT i Table 1 with model calculations assuming a • viel~lck & Deehr '89 • Oznovich el ill '95 gravity wave perturbation (Schubert et al., • Drob'~ 1991) or a terdiurnal tide (Walterscheid and Schubert, 1995). The gravity wave model data are for a realistic, extended, dissipative .__= emission region and are plotted versus cY ...........1 .................--..-..T .................................... observed wave period for two assumed ........................ L9 I , horizontal wavelengths of 500 km and 1000km. The individual data points in plot lO00km k~ TI ~(a) appear to fall into two groups: one with B T high |111- 8-10 and the other exhibiting low I111- 2-4. The gravity wave model falls I I I I q i I I I I in between theses two groups but it is clear 4 5 6 7 8 9 10 11 12 that waves o f longer and ,shorter period Observed Period (hr) would intersect these clusters. In I I I I ~ I I I comparison, the open circles indicate 4O (b) o computed values for I11[ assuming an 8-hr O 2O migrating tide (Walterscheid and Schubert, O I000 krn 0 T 1995). Several modes are predicted (each 0 A exhibiting different e~uivalent depths) and -20 covering the range Ir I | = 1.4 - 4.3. These ............ " T T ........ ............... ' ..... modes fall into the lower cluster only. " ~ -40 Similar computations of the zonally "O v -60 -esymmetric 8-hr tide (not shown) indicate a similar clustering around t111 -2 - 4 but -80 also a mode at 9 (which coincides o w., ~ s,=,,95 I I -100 • MTM(ThisStudy) [1 with the upper clustering). However, this • Vieruek & D e e h r ' 8 9 I~1 • O z n o v i e h et al. '95 --4 mode has a relatively large equivalent depth -120 • Orob '96 l (-27 km) and may be evanescent. As the I I I I i I i I I -140 zonally symmetric tide is expected to be 4 5 6 7 8 9 !0 11 12 prominent at very high latitudes, it is Observed Period (hr) therefore applicable to the polar region data listed in Table 1, but probably not to our Figure 5. Summary plots showing (a) the distribution in the mid-latitude data. Thus, it is tempting to magnitudes I rl 1, and (b) phases ~ of 1"1for the mid- and highsuggest that the lower cluster may well be a latitude data listed in Table 1. For comparison the results of a signature of tidally induced perturbation. realistic gravity wave model (Schubert et al., 1991) and the However, as the gravity wave model can be modal computations for the 8-hr migrating tide (Walterscheid adjusted to predict values of I1] I similar to and Schubert, 1995) are plotted. those observed, comparison of the magnitudes of rl alone cannot readily be used to discriminate between these two types of wave perturbations. Examination of the expected phase relationship for long-period gravity waves (Figure 5b) and the migrating terdiurnal tide with the data indicate a different situation. Again two clusters of data are evident one centered around -80 ° to -100 ° and the other around -10 ° to -20 °. In this case the gravity wave model for horizontal wavelengths of 500-1000 km fit well with the small phase cluster. Ironically, this phase cluster corresponds to the small 11"1] group that fitted best with a tidal interpretation. More importantly, calculations for both the migrating 8-hr tide and the zonally symmetric tide indicate positive values for the phase of 11 in the range -7 - 43 ° (Walterscheid and Schubert, 1995). Although some of our mid-latitude data (and the tabulated high-latitude data) indicate quite small values for ~, the ensemble of data clearly shows that the temperature perturbation invariably led the intensity wave. With our current knowledge of the expected tidal phase relationship, this would indicate that none of the waves were tidal in origin.
I J-,
I111-
,
!
I 178
M. Tayloret al.
In their discussion of their high-latitude data Oznovich et al., (1995) also considered the possibility that the waves were the signature of a high-latitude non-migrating tide (zero zonal wave number) or a long-period gravity wave. The persistence of each event (-24 and -40 hours) and the derived long vertical wavelength of -80 km supported a tidal-type perturbation. However, the negative phase results were indicative of a gravity wave-type perturbation. Likewise, the long duration (-80 hours) of the -8-hr wave observed by Drob (1996) is strongly suggestive of a tidal-type disturbance. Recently Pendleton et al., (2000) have investigated the origin of the longperiod waves evident in our mid-latitude data. BLO data averaged over a 10 day period indicated a distinct -8-hr periodicity whose amplitude and phase were found to be consistent with those determined from 24-hr Na lidar measurements of the terdiurnal tidal component (States and Gardner, 2000). A similar result was also found for a 4 day average of data obtained from Ft. Collins. The two BLO data sets presented here were drawn for the 10-night ensemble suggesting that they too were tidal in origin. However, this conclusion is currently not consistent with our r I studies described above. SUMMARY This initial investigation into the Krassovsky ratio has yielded new information on the induced intensity and temperature perturbations for ~8-hr period waves present in the mid-latitude MLT region around the fall and early winter months. Determination of rl at mid-latitudes for such long-period waves is limited by the duration of the night-time measurements (up to 12 hours during winter months) and by photochemical effects around dusk and dawn twilight that can substantially alter the mean height and thickness of the OH emission layer, and hence the shape of its zenith emission profile. For this reason the OH temperature data were used to determine the period of the wave as the intensity ratio P~(4)/Pt(2) used to compute the rotational temperatures is expected to be far less affected by changing conditions in the OH emission layer during twilight. Two clusterings of ]111 and ¢ were found that exhibited similar characteristics to previous measurements at high-latitudes. In general the low Ill I and small ¢ cluster are in best agreement with the current tidal and gravity wave models. However, there are strong evidence (at mid-and high latitudes) for much larger values of I rl I -8-10 that exhibit substantial negative phases - -80 ° to -100 °. These wave perturbations are not readily accommodated using either the existing gravity wave or tidal models. Additional data relating to the long duration of these events points to a tidal-type source for many of the wave perturbations discussed here. However, their negative Krassovsky phase values suggest a gravity wave-type interpretation. Other data, currently under analysis, support this conclusion and it is hoped that these results will stimulate further modeling studies to help understand the apparently conflicting phase relationships. ACKNOWLEDGEMENTS Financial support for the development and operation of the CEDAR Mesospheric Temperature Mapper was provided by NSF grants ATM-9403474 and ATM-9612810. We are most grateful to C.Y. She, M.A. White and S.S. Chen (Colorado State University) for their considerable help with the MTM measurements at Ft, Collins, CO. One of us (LCG) was supported in part on NSF grant ATM-9525815 during the data analysis. REFERENCES Baker, D.J., Stair Jr., A.T., 1988. Rocket experiments of the altitude distributions of the hydroxyl airglow. Phys. Scr., 37, 611-622. Dodd, J.A., Lipson, S.J., Lowell, J.R., Armstrong, P.S., Blumberg, W.A.M., Nadile, R.M., Adler-Golden, S.M., Marinelli, W.J., Holtzclaw, K.W., Green, B.D., 1994. Analysis of hydroxyl earthlimb airglow emissions: kinetic model for state-to-state dynamics of OH (v, N). J. Geophys. Res., 99, D2, 3559-3585. Drob, D.P., 1996. Ground-based optical detection of atmospheric waves in the upper mesosphere and lower thermosphere. Ph.D. Thesis, University of Michigan. Goldman; A., Schoenfeld, W.G., Goorvitch, D., Chackerian Jr., C., Dothe, H., M61en, F., Abrams, M.C., Selby, J.E.A., 1998. Updated line parameters for the OH X2H-X21-I (v",v') transitions. J. Quant. Spectrosc. Radiat. Transfer, 59, 453-469. Hecht, J.H., Walterscheid, R.L., Sivjee, G.G., Christensen, A.B., Pranke, J.B., 1987. Observations of wave-driven fluctuations of OH nightglow emission from Sondre Stromfjord, Greenland. J. Geophys. Res., 92, 6091-6099. Hickey, M.P., Schubert, G., Walterscheid, R.L., 1992. Seasonal and latitudinal variations of gravity wave-driven fluctuations in OH nightglow. J. Geophys. Res., 97, A10, 14,911-14,922.
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Hines, C.O., 1960. Internal atmospheric gravity waves at ionospheric heights. Can. J. Phys., 38, 1441-1481. Holton, J., 1979. An introduction to dynamic meteorology. International Geophysical series, 23, Academic Press. Krassovsky, V.I., 1972. Infrasonic variations of the OH emission in the upper atmosphere. Ann. Geophys. 28, 739746. Lowe, R.P., Gilbert, K.L., Turnbull, D.N., 1991. High latitude summer observations of the hydroxyl airglow. Planet. Space Sci., 39, 1263-1270. Meriwether, J. W., 1975. High latitude airglow observations of correlated short-term fluctuations in the hydroxyl Meinel 8-3 band intensity and rotational temperature. Planet. Space Sci., 43, 1211-1221. Oznovich, I., McEwen, D.J., Sivjee, G.G., 1995. Temperature and airglow brightness oscillations in the polar mesosphere and lower thermosphere. Planet. Space Sci., 43, No 9, 1121-1130. Pendleton, W.R. Jr., Espy, P.J., Hammond, M.R., 1993. Evidence for non-local-thermodynamic-equilibrium rotation in the OH nightglow. J. Geophys. Res., 98, A7, 11567-11579. Pendleton, W.R. Jr., Taylor, M.J., Gardner, L.C., 2000. Terdiurnal oscillations in OH Meinel rotational temperatures for fall conditions at northern mid-latitude sites. Geophys. Res. Lett., 27, 1799-1802. Reisin, E.R., Scheer, J., 1996. Characteristics of atmospheric waves in the tidal period range derived from zenith observations of O~ (0,1) Atmospheric and OH(6,2) airglow at lower midlatitudes. J. Geophys. Res., 101, D16, 21,223-21232. Schubert, G., Walterscheid, R.L., Hickey, M.P., 1991. Gravity wave-driven fluctuations in OH nightglow from an extended, dissipative emission region. J. Geophys. Res., 96, No A8, 13,869-13,880. Sivjee, G.G., Walterscheid, R.L., Hecht, J.H., Hamwey, R.M., Schubert, G., Christensen, A.B., 1987. Effects of atmospheric disturbances on polar mesopause airglow OH emissions. J. Geophys. Res., 92, 7651-7656. Smith, A. K., 2000. Structure of the terdiurnal tide at 95 km. Geophys. Res Lett., 27, 177-180. States, R. J., Gardner, C .S., 2000. Thermal structure of the mesopause region (80-105 km) at 40°N Latitude. Part II: Diurnal variations. J. Atmos. Sci., 57, 78-92. Swenson, G.R., Mende, S.B., Geller, S.P., 1990. Fabry-Perot imaging of OH(8-3): rotational temperatures and gravity waves. J. Geophys. Res., 95, 12,251-12,263. Tarasick, D.W., Hines, C.O., 1992. The observable effects of gravity waves on airglow emissions. Planet. Space Sci., 38, 1105-1119. Taylor, M.J., Espy, P.J., Baker, D.J., Sica, R.J., Neal, P.C., Pendleton, W.R. Jr., 1991. Simultaneous intensity, temperature and imaging measurements of short period wave structure in the OH nightglow emission. Planet. Space Sci., 39, 1171-1188. Thayaparan, T., 1997. The terdiurnal tide in the mesosphere and lower thermosphere over London, Canada (43 ° N, 81°W). J. Geophys. Res., 102, D18, 21,695-21,708. Taylor, M. J., Pendleton, W.R. Jr., Gardner, C.S., States, R.J., 1999. Comparison of terdiurnal tidal oscillations in mesospheric OH rotational temperatures and Na lidar temperature measurements at mid-latitudes for fall / spring conditions. Earth, Planets, and Space, 51,877-885. Taylor, M.J., Pendleton, W.R. Jr., Liu, H.-L., She, C.Y., Gardner, L.C., Roble, R.G. and Vasoli, V., 2001, Large amplitude perturbations in mesospheic OH Meinel and 87-km Na lidar temperatures around the autunmal equinox, Geophys. Res. Lett., in press. Viereck, R.A., Deehr, C.S., 1989. On the interaction between gravity waves and the OH Meinei (6,2) and the 02 Atmospheric (0,1) bands in the polar night airglow. J. Geophys. Res., 94, 5397-5404. Walterscheid, R.L., Schubert, G., 1995. Dynamical-chemical model of fluctuations in the OH airglow driven by migrating tides, stationary tides, and planetary waves. J. Geophys. Res., 100, A9, 17,443-17,449. Wiens, R.H., Zhang, S.P., Peterson, R., Shepherd, G.G., 1995. Tides in emission rate and temperature from 02 nightglow over Bear Lake Observatory. Geophys. Res. Lett., 22, 2637-2640. Zhang, S., Wiens, R.H., Shepherd, G.G., 1993. Gravity waves from 02 nightglow during the AIDA'89 campaign, II, Numerical modeling of the emission rate/temperature ratio, rl. J. Atmos. Terr. Phys., 54, 377-395.