JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D24, 4755, doi: /2001jd001469, 2002

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D24, 4755, doi: /2001jd001469, 2002 Seasonal variations of medium-scale gravity wave parameters in the lower thermosphere obtained from spectral airglow temperature imager observations at Shigaraki, Japan Nikolai M. Gavrilov, 1 Kazuo Shiokawa, and Tadahiko Ogawa Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Japan Received 5 November 2001; revised 26 July 2002; accepted 28 July 2002; published 19 December [1] A method of statistical analysis is used to study parameters of the medium-scale internal gravity waves (IGWs) from the data of the spectral airglow temperature imager (SATI) observations of the OH and O 2 nightglow emissions at Shigaraki, Japan (35 N, 136 E) during April 1998 to May Average relative magnitudes of spectral components of airglow variations are % for the emission rates and 0.5 1% for the rotational temperature in different seasons. Distributions of periods, horizontal wavelengths, and phase speeds have the main maxima at hours, km, and m/s, respectively. In winter, the distributions of propagation azimuths are broad with a dominance in the northwest sector and in the southern half of the sky for OH and in the northwest and southeast sectors for O 2. In spring and summer, IGWs propagate mainly to the northern half of the sky with larger occurrence in the northeast sector for OH emission. Seasonal variations of the average and median IGW magnitudes do not exceed 20 30% and are not stable. In winter, IGWs have maximum occurrence with larger magnitudes, horizontal wavelengths, and phase speeds, particularly at azimuths of deg. These results are compared with previous studies. INDEX TERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3384 Meteorology and Atmospheric Dynamics: Waves and tides; KEYWORDS: gravity waves, night airglows, interferometry, middle atmosphere, climatology Citation: Gavrilov, N. M., K. Shiokawa, and T. Ogawa, Seasonal variations of medium-scale gravity wave parameters in the lower thermosphere obtained from spectral airglow temperature imager observations at Shigaraki, Japan, J. Geophys. Res., 107(D24), 4755, doi: /2001jd001469, Introduction [2] Many theoretical and observational studies of atmospheric internal gravity waves (IGWs) have been made recently. IGWs are recognized to transport a significant amounts of momentum and energy into the middle and upper atmosphere from their lower atmosphere sources [e.g., Lindzen, 1981; Fritts and VanZandt, 1983]. Wave momentum and energy fluxes can produce dynamical and thermal influence in the middle atmosphere, where IGWs become unstable and dissipative [e.g., Lindzen, 1981; Fritts, 1984]. A numerous observations of night airglow variations for the purpose of IGW detection have been carried out. [3] Magnitudes and directions of horizontal phase speed and other IGW parameters were optically studied using triangle correlation technique [Krassovski, 1972; Mariwether, 1975], scanning photometers [Gavrilov and Shved, 1982; Takahashi et al., 1989, 1999] and all-sky imagers 1 On leave from Department of Atmospheric Physics, Saint-Petersburg University, Saint Petersburg, Russia. Copyright 2002 by the American Geophysical Union /02/2001JD [Taylor and Hapgood, 1990; Hecht et al., 1997; Isler et al., 1997; Nakamura et al., 1998]. The spectral airglow temperature imager (SATI) has been developed by Wiens et al. [1997a] as a modification of the mesospheric oxygen rotational temperature imager (MORTI, see Wiens et al. [1991]). The MORTI measures O 2 nightglow emission only, while the SATI measures O 2 and OH emission bands. The MORTI and SATI have been widely used at Canada, Japan, Kazakhstan, and Spain during the SCOSTEP Planetary Scale Mesopause Observation (PSMOS) program. Nightglow observations with MORTI and SATI simultaneously in twelve azimuthal angles on the sky allow us to obtain periods, amplitudes, horizontal wavelengths, magnitudes and azimuths of horizontal phase speed [Wiens et al., 1997b]. [4] In November 1997 SATI has been installed in Shigaraki, Japan (35 N, 136 E) and has been continuously operated since then [Shiokawa et al., 1999; Shimomai et al., 2002]. In this study we use a method of statistical analysis developed by Gavrilov and Shved [1982] to study IGW parameters from the data of these SATI observations during years As compared with the visually observed wave structures in the all-sky imagers the SATI data allow us to obtain information about larger-scale part of IGW spectrum. The main focus of the present study is on the ACL 7-1

2 ACL 7-2 GAVRILOV ET AL.: GRAVITY WAVES FROM SATI OBSERVATIONS seasonal variations of statistical IGW characteristics observed from variations of the OH and O 2 emissions at altitudes km. 2. Method of Analysis [5] The data analyzed in the present study were obtained with SATI instrument at Shigaraki, Japan (35 N, 136 E) between October 1998 and March Experimental Technique [6] The SATI instrument and its application to the study of IGWs in the lower thermosphere have been described by Wiens et al. [1997a]. The device measures the column emission rate (called intensity hereafter) and vertically averaged temperature of both the O 2 atmospheric (0 1) band nightglow layer centered at altitude 94 km and the OH Meinel (6 2) layer centered at 87 km [Zhang and Shepherd, 1999]. [7] SATI uses an interference filter and optical system that produce a spectrum having a circular shape with the radial coordinate proportional to wavenumber of transmitted light. A coded CCD (charge-coupled device) placed in the image plane records the spectral pattern and send it to the computer for storage and processing [Wiens et al., 1997a; Shiokawa et al., 1999]. Thermo-electric cooling of the CCD detector minimizes noise. [8] Two interference filters are switched using a filter wheel to get information about the O 2 (b 1 g + X 3 g ) (0 1) P branch and the Q branch of the OH(X 2, n) (6 2) band nightglow emissions. A comparison of measured spectra with calculated model samples corresponding to different rotation temperatures gives estimations of average column emission rates and brightness-weighted rotational temperatures for each of 12 sectors of the image representing 12 azimuths at the sky at a zenith angle of 30 ±3.5 [Wiens et al., 1997a]. Exposure time is 2 min for both O 2 and OH emissions Determination of Wave Parameters [9] IGWs propagating in the middle and upper atmosphere enforce oscillations of temperature, T, density, pressure, and air velocity. These oscillations may produce changes in photochemical reaction rates and result in variations of night airglow intensity, I, and other characteristics. Gavrilov and Shved [1982] developed statistical method, which allows to obtain period, t, amplitude, horizontal length, l h, and azimuth, j, of IGW propagation. This method is used in this study. [10] As described above, SATI gives the values of airglow intensity, I i (t k ), and rotational temperature, T i (t k ), at sky points with azimuths j i (i =1,2,..., M), which are located along the sky circle with the zenith angle q. Times of the observations t k 2 [ t/2, t/2], where t is the measurement duration. For every sky point we calculate the second order polynomial fits, I 0i (t k ) and T 0i (t k ), which may be considered as background values. To study wave variations we estimate absolute deviations Ii(t 0 k )=I i (t k ) I 0i (t k ) and Ti(t 0 k )=T i (t k ) T 0i (t k ) and respective relative values d Ii (t k )=Ii(t 0 k )/I 0i (t k ) and d Ti (t k )=Ti(t 0 k )/T 0i (t k ). The functions d Ii (t k ) and d Ti (t k ) for each sky point are subjects for spectral frequency analysis. Fourier cosine- and sine-transformations of relative intensity variations are calculated as follows: ) X i ðsþ ¼ 2 Y i ðsþ t Z t=2 t=2 ( ) cos st Gt ðþd Ii ðþ t dt; sin st where s =2p/t is the observed frequency; G(t) = sin(2pt/ t)/(3.7t/t) is the normalized Gibbs smoothing window factor introduced to reduce the influence of the adjacent frequency components caused due to a finite t value [Gavrilov and Shved, 1982]. Functions X i (s) and Y i (s) allow us to estimate the Fourier transform module, V i (s), argument (phase), f i (s), and power spectral density, S i (s), for each spectral component: 1=2; V i ðsþ ¼ Xi 2 ðsþþyi 2 ðsþ f i ðsþ ¼ arccos½x i ðsþ=v i ðsþš; ð2þ S i ðsþ ¼ V 2 ðsþ=2b; where B = 16.4/t is the bandwidth of the frequency filter corresponding to the transformation (1) (in case of G(t) = 1 we have B =4p/t). The transformations (1) and (2) are equivalent to the standard Fourier transformation of autocorrelation function [Serebrenikov and Pervozvansky, 1965]. Also the transformations (1) and (2) are equivalent to a narrow numerical frequency filter having effective transmission bandwidth equal to B. Therefore, we will call below the results of transformation (1), (2) as narrow frequency band signals (NBS). To obtain the horizontal wavenumbers and azimuths of the NBS propagation we use a plane wave approximation, when the phase has to vary along the circle of radius r on the sky as follows: i ð1þ ~f i ðsþ ¼ f 0 kr cosðj i j 0 Þ; ð3þ where f 0 is the phase at zenith; k and r are the horizontal wavenumber and radius of observation circle on the sky, respectively; j i and j 0 are the azimuths of observation points and wave propagation, respectively. For given r = htanq (where h and q are the height and zenith angle of observations) parameters f 0, k and j 0 can be estimated for each NBS with the least-square fitting of (3) to the observed phases f i (s) [see Gavrilov and Shved, 1982]. After calculation of the approximation (3), we may estimate the average spectra including the phase correlated, ~S(s) and random, S 0 (s), parts and their total, S(s): where ~S ðsþ ¼ S 0 ðsþ ¼ SðsÞ ¼ ~X 2 ðsþþ ~Y 2 =2B; ½X 02 ðsþþy 02 ðsþš=2b; ~S ðsþþs 0 ðsþ; ) ( ) ~X ¼ 1 P ~X i M ; i ~Y ~Y i ~X i ¼ V i cos f i f ~ i ; ~Y i ¼ V i sin f i f ~ i ; ) 8 X ¼ 1 P < X i ~X = M i : 2 ; Y i ~Y ; Y 02 ð4þ ð5þ

3 GAVRILOV ET AL.: GRAVITY WAVES FROM SATI OBSERVATIONS ACL 7-3 Figure 1. SATI measured OH emission rate (top) and its relative variations (bottom) on November 20, The dashed line on the top plot shows quadratic trend, which is subtracted to obtain the relative intensity variations in the bottom panel. Figure 2. Fourier Transform Modules of OH emission rate variation from bottom plot of Figure 1 (thin curve) and that averaged over 12 sky points (thick curve) on November 20, [11] For a signal with purely random phases the mathematical expectation M( ~X )=M(~Y) = 0. Therefore, the value ~S(s) could estimate a contribution of the component having quasi-linear phase variation in the horizontal plane Experimental Examples [12] As an illustration of the possibilities of the statistical method described above, a typical observation of OH emission intensity with SATI on November 20, 1998 is considered below. Figure 1 shows an example of initial SATI data, low-frequency square trend, and relative deviations of OH airglow intensity obtained by subtracting the low-frequency square trend. Shiokawa et al. [2002] showed that Shigaraki SATI gives underestimated values of OH rotational temperature. They obtained also regression formula for the correction of OH rotation temperature obtained with SATI. Using the relative temperature deviations in our analysis diminish the related errors. An analysis based on the regression formula by Shiokawa et al. [2002] shows that the underestimation of rotational OH temperature may result in overestimation of its relative deviations not exceeding 10 15%, which can not influence significantly the results of our analysis. [13] Figure 2 presents the spectrum of relative variations from Figure 1. One can see several spectral maxima. The thin curve in Figure 2 shows the spectrum obtained for the sector No.1 (azimuth 102 ), while the thick curve represents the spectrum averaged over entire 12 sectors on the sky circle observed with SATI. A comparison of these curves reveals the repetition of the most thin curve maxima in the thick curve of Figure 2, which is more noticeable at periods larger than 1 hour. Therefore, atmospheric variations corresponding to these spectral maxima should have horizontal scales comparable or larger than the diameter of the observation circles ( km at altitudes km). Such variations should be suspected to have contribution from atmospheric IGWs. [14] Figure 3 shows the spectral density S(s) (thick curve) averaged over all 12 sectors and the results of its decomposing according to (4) and (5) into phase correlated, ~S(s) (thin curve) and random S 0 (s) (dashes curve) parts. Longdashes line in Figure 3 shows the spectral slope (s 5/3 ), which is frequently observed in the spectra of atmospheric variations. One can see that ~S(s) decreases faster in Figure 3 and becomes smaller than S 0 (s) at high frequencies. Therefore, at shorter periods (t < 0.5 hours in Figure 3) the information about plane waves is lost, probably, due to instrumental and turbulent noise. [15] Figure 4 represents an example of phase variations versus propagation azimuth and their least-square cosine approximation for narrow frequency band signals corresponding to the maxima No.2, 3, and 5 from Figures 2 and 3. Estimations of the IGW parameters according to (2) and (3) are presented in Table 1 (the numbers of waves in Table 1 and peaks in Figures 2 and 3 coincide). [16] The examples of Figures 1 4 refer to the OH emission rate. For emission rate of O 2 and rotation temper- Figure 3. Power spectral densities S(s) (thick curve), ~S(s) (thin curve), and S 0 (s) (dashed curve) of OH emission rate variations on November 20, The long-dashed straight line shows power law s 5/3.

4 ACL 7-4 GAVRILOV ET AL.: GRAVITY WAVES FROM SATI OBSERVATIONS horizontal speed being smaller than a certain limit [see Gavrilov et al., 1996]. Most of NBS obtained from the variations of airglow characteristics in our study have substantial horizontal phase speeds and low frequencies, so that c h 2 u 0 2, and f 2 s N 2, where u 0 is the mean speed velocity; f is the Coriolis parameter at latitude of observation and N is the Brünt-Väisälä frequency. For such IGWs the criteria of real m obtained by Gavrilov et al. [1996] may be simplified to the form of jc h j ¼ s= jk h j < 2HN; ð6þ Figure 4. Azimuthal variations of measured phases (dots) and their least squares cosine approximation (dashed curves) for the spectral components corresponding to the maxima labeled 2, 3 and 5 in Figures 2 and 3 (plots from top to bottom, respectively). atures the results of the analysis are similar except for the larger random noise spectral density S 0 (s), especially for both O 2 and OH rotation temperatures. According to Figure 3 larger S 0 (s) leads to smaller IGWs numbers that can be distinguished from the spectra. Therefore, in this paper we present mainly IGW parameters estimated from the data on the OH and O 2 emission rates Selection Criteria [17] After calculating the described parameters of NBS, we can separate different groups of NBS, which represent different types of atmospheric motions and waves. In the present study we are interested in NBS having properties usually described by linear theory of IGWs. In particular, such waves have real values of vertical and horizontal wave number m and k h. Spectra of the upper atmosphere variations may also contain, in addition to IGW harmonics, contributions from other types of motion (turbulence, convection, stationary and trapped waves, etc.) and from random errors of measurements. Gavrilov et al. [1996] applied some criteria to select NBS that satisfy the anticipated properties of quasi-linear IGWs from the spectra of the middle atmosphere wind measurements with the MU radar. These criteria are based on the statistical properties and theory of atmospheric IGWs. [18] First, we select narrow frequency band signals having local maxima of Fourier transform modules ji 0 /I 0 j or jt 0 / T 0 j. Second, the cosine fit to the variation of the spectral component phase versus azimuth (see Figure 4) has to hold with more than 90% statistical confidence. The third criterion is the excess of coherent power spectral density ~S(s) over the random one S 0 (s) as estimated with (4) and shown in Figure 3. [19] Another criterion arises from the requirement of vertical wave number m to be real, which corresponds to propagating gravity waves. The IGW dispersion equation gives real m values only in case of the value of intrinsic where H is the scale height of the atmosphere. The last criterion arises from the requirement that the horizontal length of the IGW harmonic to be larger than the horizontal distances between the sectors of the sky circle observed by different SATI beams (about km at altitudes km). Otherwise, we may get non-unique cosine approximation of the observed phase variations. [20] To verify the applicability of the statistical analysis technique and IGW selection criteria described above to the SATI airglow measurements, we used test sets of data having the experimental values of time in each 12 azimuths of observations but with model airglow intensities and temperatures representing superposition of 24-, 12-, 8-, 6- hour tides and four IGW harmonics with different frequencies, wavelengths and propagation azimuths. In all test runs we obtained good agreement between the initially specified characteristics of IGW harmonics and those reconstructed with the statistical analysis. The analysis of the dispersion of the model IGW parameters recovered from the test sets representing different observation nights reveals that the usual measurement duration of 2 10 hours allows to select effectively NBS corresponding to the waves with periods hours. [21] Test calculations showed also that the errors of NBS characteristics are larger, when obtained from the variations of rotation temperatures than from nightglow emission rates due to larger noise level. Therefore, all results of the present paper about frequencies, wave numbers, phase speeds and azimuths of selected NBS are presented only for the OH and O 2 nightglow intensities. 3. Results of Statistical Analysis [22] The procedure of calculation and selection of NBS attributed to IGWs described in the previous section is applied to the measurements of the intensities and rotation temperatures of the OH and O 2 nightglows with SATI device at Shigaraki (35 N, 136 E) during October 1998 to May The SATI has made regular observations of the nightglow emissions during moonless periods since November Table 1. Parameters of Narrow Spectral Band Signals Corresponding to the Maxima in Figures 2 and 3 t, hours ji 0 /I 0 j l h,km c h, m/s j 0, deg P ± 7 > ± 4 > ± 4 > ± 6 > ± 9 > ± 9 >0.95

5 GAVRILOV ET AL.: GRAVITY WAVES FROM SATI OBSERVATIONS ACL 7-5 Figure 5. Rose diagrams of wave propagation azimuths (clockwise from north) at different seasons for OH (top) and O 2 (bottom) emissions. Radius represents the number, n, of narrow frequency band signals (NBS) attributed to IGWs For the analysis we select only the nights, when there were no visible clouds in the sky, which were checked by the sky images obtained by a collocated CCD nightglow imager from October Also we select only the data with the background spectral emission intensity between the emission bands less than 8 R/Å. This is an additional criteria for the selection of clean sky data. The durations of used nighttime measurement intervals are between 2 and 12 hours Seasonal Variations of IGW Characteristics [23] To study seasonal variations of statistical characteristics, all data are subdivided into four groups: winter (from November to February), summer (from May to August), spring, and autumn (months between the above). The total numbers of clean sky nights of measurements are 82, 27, 15 and 16 for winter, spring, summer, and autumn, respectively. [24] Figure 5 represents the rose histograms of propagation azimuths of NBS attributed to IGWs obtained from variations of the OH and O 2 intensities in different seasons. One can see that in winter, distributions of NBS azimuths are broad with some domination in the northwest sector and the southern half of the sky for OH and in the northwest and southeast sectors for O 2 emission. In summer, NBS mainly propagate to the northern half of the sky, more concentrating to the northeast sector for the OH emission. In spring, the directions are mainly to the northeast sector, similar to the summer distribution for the OH emission. In autumn NBS azimuths have broader distributions mainly to the northern hemisphere. [25] Figure 6 shows the histograms of different NBS characteristics in different seasons. Histograms of NBS frequencies, s, are more or less similar for winter, spring and autumn in Figure 6. In summer, distribution of s has a maximum at larger values than that in other seasons. Distributions of horizontal wavelength, l h, and phase speed, c h, in Figure 6 have the main maxima at km and m/s, respectively, in all seasons. Histograms of NBS relative magnitudes have maxima in the ranges of 0 2% and 0 1% for OH emission rate and rotational temperature, respectively, in all seasons. [26] Figures 7 and 8 present individual values of NBS characteristics plotted versus the day number within a year for the OH and O 2 emissions, respectively. All character- Figure 6. Histograms of parameters (I 0 /I 0 and T 0 /T 0 m, emission rate and rotation temperature deviations; s, observable frequency; l h and c h, horizontal wavelength and phase speed, respectively) of narrow frequency band signals (NBS) attributed to IGWs at different seasons.

6 ACL 7-6 GAVRILOV ET AL.: GRAVITY WAVES FROM SATI OBSERVATIONS temperature, N temp, data for both OH and O 2 emissions. The average NBS parameters from Table 2 are also plotted as black dots in Figures 7 and 8. Table 2 shows largest average and median values of horizontal number, k h, and smallest phase speeds, c h, in summer for both the OH and O 2 emissions. From Table 2 and Figures 7 and 8 one can see that despite nonuniform distribution of the maximum bars, seasonal variations of the average and median NBS magnitudes are not very large (of about 20 30%). For OH emission there is some difference between the summer maxima of ji 0 =I 0 j and ji 0 /I 0 j med and the summer minima of jt 0 =T 0 j and jt 0 /T 0 j med in Table 2. One should keep in mind that the summer values in Table 2 are based only on the data for May and August, because of practical lack of nights with appropriate for observations weather in summer. The average magnitudes for the O 2 emission are more consistent, having the main maxima in winter and summer-autumn and a minimum in spring. The average and median values of horizontal wavelengths and phase speeds in Figures 7 and 8 obtained from Table 2 have maxima in winter and minima in summer for both OH and O 2 emissions Dependence of IGW Characteristics on Azimuth [28] Figure 9 shows NBS parameters for the OH emission versus the azimuth of propagation direction in horizontal Figure 7. NBS parameters and histogram of their numbers (bottom) versus the day of year for the OH emission. Vertical bars show individual measured NBS parameter values. Black dots reveal the average values for different seasons from Table 2. istics except jt 0 /T 0 j are obtained from the emission rates data. Figures 7 and 8 may give information about seasonal variations of NBS and IGW characteristics. Bottom plots in Figures 7 and 8 show histograms of selected NBS numbers. One can see practical lack of data between the middle of June and middle of August due to almost constant cloudy weather at Shigaraki at that time. This gap makes it difficult to obtain complete picture of seasonal variations of NBS and IGW characteristics. Form of bars in Figures 7 and 8 emphasizes the largest values of IGW characteristics. From Figures 7 and 8 one can distinguish more frequent maximum magnitudes of emission rates and rotational temperature NBS in November December and their decrease in September October for both the OH and O 2 emissions. Large NBS magnitudes for the O 2 emission are also more frequent in January February in Figure 8. There are also more frequent large values of horizontal wavelengths and phase speeds in November December in Figures 7 and 8. [27] Average and median NBS parameters are given in Table 2 together with the total numbers of NBS selected for different seasons from the emission rate, N int, and rotational Figure 8. Same as Figure 7 but for the O 2 emission.

7 GAVRILOV ET AL.: GRAVITY WAVES FROM SATI OBSERVATIONS ACL 7-7 Table 2. Parameters of NBS Attributed to IGWs OH Emission O 2 Emission Parameter Winter Spring Summer Autumn Winter Spring Summer Autumn Average Parameters s 10 3, rad/s ji 0 =I 0 j,% 1.8 ± ± ± ± ± ± ± ± 0.2 jt 0 =T 0 j,% 0.54 ± ± ± ± ± ± ± ±.09 k h 10 2,km ± ± ± ± ± ± ± ± 0.1 c h, m/s 104 ± 4 99 ± 5 86 ± 8 99 ± ± ± 7 85 ± 6 92 ± 7 Median Parameters s med 10 3, rad/s ji 0 /I 0 j med % jt 0 /T 0 j med % k hmed 10 2,km c hmed, m/s j 0med, deg Numbers of Narrow Spectral Band Signals N int N temp plane for NBS having periods longer and shorter than 1 hour in winter, when we have largest numbers of selected IGWs. Gavrilov et al. [1996, 2000] used such separation for the analysis of the MU radar data. Bottom plots in Figure 9 gives histograms of propagation azimuths similar to Figure 5. One can see that the shorter-period IGWs have generally smaller amplitudes than the longer-period ones. The maxima of NBS propagation azimuths in the sectors deg and deg. Figure 9 reveals also substantial dependence of NBS magnitudes on the azimuth. Both longer- and shorter-period NBS have maximum magnitudes in the sector deg, although largest amplitudes of longer-period NBS are observed in the beginning and shorter-period NBS: at the end of this sector in Figure 9. At the same sector of azimuths we have the maximum of numbers of selected NBS (see above). Horizontal wavelengths and phase speeds have also azimuthal dependence in Figure 9. For NBS with periods larger than 1 hour maximum c h and l h are observed between 90 and 220 deg. Waves with periods less than 1 hour have largest c h and l h at azimuths deg and deg. Minimum c h and l h for all NBS periods exists between 0 90 and deg. [29] Figure 10 represents azimuthal dependencies similar to Figure 9, but for the O 2 emission in winter season. Here dependencies of NBS parameters on azimuth are more complicated, but one can see some correlation between largest magnitudes, values c h and l h on one hand and the maxima of azimuth histograms on the other hand. In general, Figures 9 and 10 show that the main difference between longer and shorter-period NBS is in their amplitudes. Azimuthal distributions of the other parameters in Figures 9 and 10 are quite similar Discussion [30] It is interesting to compare the present results with the previous studies of IGW characteristics in the MLT region. Nakamura et al. [2001] have analyzed the seasonal variations of IGW intensity and horizontal propagation directions in the MLT region from radar and airglow imager observations. They showed that the wave propagation directions depend very much on the frequency and wavenumber spectral ranges of analyzed IGWs. [31] Recently, all-sky airglow imagers make possible visual observations of short wave structures having periods 5 30 min, horizontal wavelengths km and phase velocities m/s [e.g., Taylor et al., 1995; Nakamura et al., 1999; Ejiri et al., 2001a]. Hodograph analysis of vertical wind profiles obtained with the MU radar at Shigaraki from turbulent induced radio echoes at altitudes km [Nakamura et al., 1993] and from meteor track echoes [Nakamura et al., 1993; Namboothiri et al., 1996] provides information about IGWs with periods 5 15 hours and horizontal wavelengths km. Both methods select IGWs with relatively small horizontal phase speeds of a few tens m/s. All of mentioned studies show the domination of eastward propagating IGWs in winter and westward IGWs in summer. From all-sky OH and O557.7 nm airglow images Ejiri et al. [2001b] obtained two main directions, i.e., northwest and southwest in winter and northeast direction in summer. Our Figure 5 shows additional southeast IGW direction in winter for the OH emission. For O 2 emission in winter southwest direction becomes less populated in Figure 5 and summer directions shift to the north. [32] The question about IGW propagation directions is important in connection with the directions of the vertical component of horizontal momentum flux. IGW theory gives the same or opposite directions of wave propagation and momentum flux for IGWs having upward or downward energy fluxes, respectively [Gavrilov et al., 1996]. A statistical study of NBS attributed to IGWs with periods hours and horizontal wavelengths km from the data of the MU radar observations at altitudes km reveals the broad distributions of NBS momentum and propagation directions with smaller anisotropy in different seasons [Gavrilov et al., 2000]. Our Figure 5 also shows that the numbers of NBS propagating eastward and westward in winter and in summer in the O 2 emission layer are close to each other. The reason for that may be the differences in the filtering of IGW spectrum propagating from the lower atmosphere wave sources by the mean wind in the middle atmosphere. The IGWs observed with airglow

8 ACL 7-8 GAVRILOV ET AL.: GRAVITY WAVES FROM SATI OBSERVATIONS imagers have smaller horizontal phase speeds and are the subjects for stronger filtering than the faster IGW considered in this paper [see Nakamura et al., 2001]. [33] Results of Gavrilov et al. [2000] also show some differences in the histograms of the directions of wave propagation and wave momentum flux. This may be caused by IGWs, energy of which propagates downwards. The main part of IGW energy is recently supposed to propagate from sources located in the lower atmosphere. But a portion of IGW may come from sources located above the MLT region (i.e., disturbances in auroral zone). A partial reflection of upward IGW energy flux may occur due to inhomogeneities of the mean wind and temperature fields. The wave momentum flux of downward propagating IGW has direction opposite to the direction of wave propagation. Therefore, histograms of the wave propagation and momentum flux directions may be different for an IGW harmonic ensemble containing a portion of downward propagating waves. [34] Radar studies of seasonal variations of IGW intensity in the middle latitude mesosphere frequently show semiannual variation with maxima in winter and summer and minima in equinoxes [e.g., Meek et al., 1985; Vincent and Fritts, 1987; Tsuda et al., 1990; Nakamura et al., 1996; Gavrilov et al., 2000]. NBS magnitudes for the OH and O 2 emissions shown in Figures 7 and 8 have maxima in winter. It is difficult to get complete information about summer IGW amplitudes, because cloudy weather in Shigaraki in summer prevents almost entirely airglow observations between middle of June and middle of August (see Figures 7 and 8). [35] Consideration of the mean and median values of IGW parameters in Table 2 show that their seasonal variations are relatively small and not very stable for both the OH and O 2 emissions. Figures 7 and 8 show an increase in NBS amplitudes in November and December for both Figure 9. NBS parameters versus propagation azimuth for the OH emission in winter season for NBS with periods larger than 1 hour (left) and smaller than 1 hour (right). Vertical bars show individual measured NBS parameter values. Figure 10. Same as Figure 9 but for the O 2 emission.

9 GAVRILOV ET AL.: GRAVITY WAVES FROM SATI OBSERVATIONS ACL 7-9 OH and O 2 emissions. It is accompanied with the increases in NBS horizontal phase velocities and wavelengths. This might reflect seasonal variations of IGW sources and conditions of wave propagation in the middle atmosphere over Japan. [36] Interpreting the seasonal variations of IGW intensity one should take into account their possible height changes. Measurements of ionospheric drift velocity variations at Collm (52 N, 15 E) and with medium frequency radar over Hawaii (22 N, 160 W) Gavrilov et al. [2001a, 2001b] showed a height changes of semiannual variations of wind variances with periods hours having solstice maxima and equinox minima at altitudes near and below 80 km to the maxima of IGW intensity in spring and autumn and minima in solstices at larger altitudes. This was confirmed by numerical simulation of the IGW spectrum propagation in nonhomogeneous dissipative atmosphere [Gavrilov and Fukao, 1999; Gavrilov et al., 2001a, 2001b], which showed that such altitude changes in the seasonal variations of IGW intensity may be connected with seasonal changes in the background fields of wind and temperature and related to the changes in the strength of wave sources at different heights in the lower and middle atmosphere. This may partly explain the instability of seasonal changes in NBS magnitudes in Figures 7 and 8 and in Table 2 for OH and O 2 emissions having maxima at altitudes 87 and 94 km, respectively. [37] One should keep also in mind that the relatively large number of registered NBS attributed to IGWs we have only in winter season. In other seasons the numbers of observed IGWs are substantially smaller (see Table 2) due to weather conditions at Shigaraki Conclusion [38] In this paper we use a method of statistical analysis to study parameters of relatively large-scale IGWs from the data of SATI observations of the OH and O 2 nightglows at Shigaraki, Japan (35 N, 136 E) during The method uses high- and low-frequency filtering, spectral analysis of the OH and O 2 emission rates and rotational temperatures, calculation of phase correlated and random components, and estimating horizontal wavelengths, phase speeds and directions of propagation of spectral components attributed to IGWs from simultaneous measurements at 12 sectors located at the sky circle with a zenith angle of 30. [39] Average relative magnitudes of spectral components are % for the emission rates and 0.5 1% for the rotational temperature at different seasons. Distributions of periods, horizontal wavelengths and phase speeds have the main maxima at hours, km and m/ s, respectively. In winter, the distributions of propagation azimuths are broad with a dominance in the northwest sector and in the southern half of the sky for OH and in the northwest and southeast sectors for O 2. In spring and summer, IGWs propagate mainly to the northern half of the sky with large concentration in the northeast sector for the OH emission. In autumn, IGW azimuths have broader distributions mainly to the northern half of the sky. Seasonal variations of the average and median NBS magnitudes are not exceeded 20 30%. In winter, IGWs have maximum occurrence with larger magnitudes, horizontal wavelengths, and phase speeds, particularly, at azimuths deg. [40] Maximum IGW occurrence is in winter. 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