TIMED/SABER observations of global cold point mesopause variability at diurnal and planetary wave scales

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2010ja015945, 2011 TIMED/SABER observations of global cold point mesopause variability at diurnal and planetary wave scales Sherine Rachel John 1 and Karanam Kishore Kumar 1 Received 16 July 2010; revised 16 March 2011; accepted 23 March 2011; published 22 June [1] Cold point mesopause is characterized by the coldest point in the temperature profile of the Earth s atmosphere. TIMED/SABER observations of cold point mesopause and its variability at diurnal and planetary wave scales are discussed in this study. For the first time, the diurnal and semidiurnal tidal modulations of mesopause are quantified on a global scale during all the four seasons, namely, winter, vernal equinox, summer, and autumnal equinox. The composite of diurnal variations of mesopause height and temperature are discussed during each season and using least squares fit, diurnal and semidiurnal tidal amplitudes and phases are obtained. Most of the features exhibited by the diurnal variation of mesopause height are consistent with the present understanding of the migrating tides. The diurnal tidal modulations of mesopause show its peak over equatorial latitude and change its phase around 20 latitude. The phase of the diurnal tidal modulation is consistent during all seasons expect for a phase shift of 4 6 h observed during boreal summer. The similarities/discrepancies between the latitudinal structure of migrating tides and the diurnal variation of mesopause height are discussed. The results reveal that the diurnal tidal modulations of mesopause height show hemispherical asymmetry, which is not reflected in mesopause temperature. The diurnal and semidiurnal amplitudes in mesopause height across the globe are comparable in magnitude and it is found that over equatorial and low latitudes, the variability of mesopause is maximum at these scales as compared to seasonal scales. Quantification of mesopause height at diurnal scales is very important as it also changes the chemistry of that region. In the present study, an attempt is also made to demonstrate the modulation of the mesopause by propagating planetary waves. The results emphatically show that propagating planetary waves do modulate the mesopause height. Citation: John, S. R., and K. K. Kumar (2011), TIMED/SABER observations of global cold point mesopause variability at diurnal and planetary wave scales, J. Geophys. Res., 116,, doi: /2010ja Introduction [2] Mesopause, the coldest region of the Earth s atmosphere, is the region of demarcation between Earth s middle and upper atmosphere. Many transfer and exchange processes between the Mesosphere and Lower Thermosphere (MLT) happen at this region of minimum temperature [e.g., Ballinger et al., 2008; Kutepov et al., 2006]. The height as well as temperature of the mesopause is controlled by dynamical, chemical and radiative processes that make its variability rather complex [e.g., Holton, 1983; Chamberlain and Hunten, 1987; Taylor et al., 2001; Vineeth et al., 2007]. Variability due to tides, gravity and planetary waves are also expected to add on to this complexity [Garcia and Solomon, 1985; Kulikov, 2007; Kutepov et al., 2007]. The determination of Mesopause remains rather simple as it is characterized by the minimum temperature (often referred to as the 1 Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram, India. Copyright 2011 by the American Geophysical Union /11/2010JA Cold Point Mesopause) in the temperature profile of the Earth s atmosphere. But Mesopause region studies have suffered long enough due to inaccessibility of these heights by ordinary balloon experiments or low earth orbiting satellites. Radars or Airglow imagers fail to give continuous observations on global scales. Two commonly employed methods for these studies are Rocket measurements and Lidar observations [e.g., Fritts et al., 2004; She et al., 1993; Yu and She, 1995]. But they exist only at a few locations and this makes it difficult to have a complete picture of the mesopause. With the advent of limb viewing satellites with global coverage, this limitation is overcome to a great extent. Using space based observations, there has been numerous studies on the global structure of migrating and nonmigrating tides and their seasonal, latitudinal, longitudinal, altitudinal and inter annual variability and their role in coupling the lower and upper atmosphere using temperature and wind variations in the middle atmosphere [Oberheide et al., 2003, 2006; Huang et al., 2006; Zhang et al., 2006; Forbes et al., 2006; Liu et al., 2007; Xu et al., 2007; Zhu et al., 2008; Oberheide and Forbes, 2008; Forbes et al., 2008; Mukhtarov et al., 2009; Pancheva et al., 2009; Xu et al., 2009; 1of12

2 Huang et al., 2010; Zhang et al., 2010]. Earlier studies also reported the presence of longer period waves that interact with tides in the mesosphere and thermosphere regions [Garcia et al., 2005; Riggin et al., 2006]. Though all these aspects have been studied in detail, there has been no attempt to study the variability of the Mesopause height due to diurnal and semidiurnal tides and to quantify it. [3] It was thought for a long time that the mesopause is situated at around km and this is seen in most of the standard text book atmospheric temperature profiles. But recent studies have shown that this is not the case. von Zahn and Neuber [1987] used Lidar measurements at 69 N during winter months and found that the Mesopause was consistently near 100 km. In summer, at the same observational site, the mesopause height was found to be close to 88 km using meteorological rocket observations by von Zahn and Meyer [1989]. Lübken and von Zahn [1991], using Lidar and sounding rockets at the arctic (69 N, 16 E) established beyond doubt that the polar mesopause indeed showed a bimodal character with the winter mesopause being higher by more than 10 km than the summer mesopause. This was in addition to the mesopause temperature that was already known to be cooler in summer than in winter by about 70 K at the poles [Stroud et al., 1959]. Further studies in mid latitudes (41 N) by She et al. [1993] and Yu and She [1995] reported prevalence of two distinct mesopause heights with the lower height ( 86 km) persisting all through summer but not very pronounced in winter. von Zahn et al. [1996] and She and von Zahn [1998] had observations carried out at more mid latitude and polar sites supporting the same findings. One limitation of the data used was that Lidar observations were carried out during nighttime that might not account for the diurnal variation in the mesopause structure. In the tropical region, nocturnal structure of the mesopause was studied by Friedman and Chu [2007] at Arecibo Observatory (18.35 N, W) using Lidar observations and they reported a three level structure, attributing it to seasonal variation. Annual and seasonal variability and climatology of Mesopause temperature as well as winds were derived at the Colorado State University Na Lidar Facility in Fort Collins, Colorado (41 N, 105 W) [Yuan et al., 2008a, 2008b]. They obtained strong seasonal variation in height and temperature, consistent with the findings of others. Satellite observations provide new insights into the MLT structure and processes as 24 h local coverage is possible over a period of time. Mertens et al. [2004] used SABER temperature data processed using nonlocal thermal equilibrium (non LTE) algorithm and have reported similar double mesopause structure. Global mesopause structure was studied using SABER data and the two level global mesopause structure was established by Xu et al. [2007]. They concluded that the mesopause of middle and high latitude regions is at lower altitudes in the summer hemisphere around summer solstice and is at higher altitudes during other seasons. At the equator the mesopause is at the higher altitude for all seasons. Inter annual variability is not profound but tides as well as gravity waves modulate the structure globally. The higher equatorial mesopause was also reported by Venkat Ratnam et al. [2010]. They conclude that the equatorial mesopause is always at 100 km with no seasonal or long term variations. A secondary minimum in temperature is seen between 75 and 80 km which have some variations related to summer and winter solstices. Most of the variability reported so far are attributed to the effect of tides, gravity and planetary waves [e.g., Li et al., 2007; Yuan et al., 2008b; Morris et al., 2009]. But there has not yet been any quantification to the extent of variability induced by each of these sources. The aspect of whether the diurnal variability is averaged out in estimating the Mesopause height and temperature is also a question to ponder over. Most of the earlier studies showed seasonal/monthly averages of mesopause height, which averages out the tidal and planetary wave modulations. On the other hand, tidal modulations of mesopause height are very significant across the globe [Xu et al., 2007]. For example, if we consider only the seasonal or monthly mean of equatorial mesopause, it always remains at 100 km altitude [Venkat Ratnam et al., 2010]. However, at diurnal scales equatorial mesopause height shows considerable variability and thus one has to consider the total spectrum of mesopause variability to arrive at a solid conclusion. Even though there have been studies in the past on diurnal variation of mesopause, there are no attempts to quantify the tidal modulations (in terms of amplitude and phase). One more important scale at which mesopause variability is expected is planetary wave scales. As par the authors knowledge, there is no attempt in the past to study planetary wave modulations of mesopause height though this has been studied in temperature [e.g., von Savigny et al., 2007; Morris et al., 2009]. In the present study, an effort is made to bridge this gap in our understanding of the spectrum of mesopause variability at shorter time scales. We focus on two specific aspects of Mesopause variability: (1) the quantification of diurnal variation of Mesopause height and temperature in terms of amplitude and phase on global scale and (2) variation of mesopause structure at planetary wave scales over selected latitudes. [4] The data used for this global study is from Sounding of the Atmosphere by Broadband Emission Radiometry (SABER) onboard Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. In the present communication, we initially present the seasonal variations of global mesopause structure (Height and Temperature) using four years of SABER data ( ) and then the analysis and discussion focuses on the tidal and planetary wave modulations of the Mesopause. 2. Database [5] The data used for this study is from the SABER payload onboard the TIMED satellite. SABER instrument is a limb viewing broadband multichannel infrared radiometer which makes measurements ranging from 1.27 mm to17mm. TIMED orbits at 625 km making more than 14 orbits per day with a period of 1.6 h and an orbital inclination of Thus SABER provides data with near global coverage spanning 24 h in local time over a period of 60 days. The kinetic temperature from upper troposphere to lower thermosphere is retrieved from CO 2 limb emissions at 15 mm, employing non LTE inversion technique [Mertens et al., 2001; Remsberg et al., 2003]. The errors in retrieval are found to be 1.4 K at 80 km to 22.5 K at 110 km by Mertens et al. [2001]. The inclusion of non LTE has improved the temperature values even up to 35 K in the MLT region [Mertens et al., 2004]. In the region of interest in the present 2of12

3 Figure 1. Seasonal mean of global mesopause (a) height and (b) temperature. study (say km), the uncertainties in the data would be 4 5 K. The mesosphere lower thermosphere (MLT) measurements made by SABER are unique in their vertical resolution. The daytime as well as nighttime measurements during both ascending and descending orbits are obtained. In this study, we use the kinetic temperature, version 1.06 of SABER level 2A data. Comparisons of SABER temperature data with various ground based measurements have been carried out and the reliability of the data has been ascertained [Mertens et al., 2004; Kishore Kumar et al., 2008] and therefore we do not intend to do this in the present study. 3. Results and Discussions 3.1. Seasonal Variations of Global Mesopause [6] For the present study, SABER temperature observations for four years ( ) are gridded into 2.5 latitudes and further sorted into local time (0 23 h with 1 h interval) across the globe. As mentioned earlier, SABER takes 60 days to sample at all local times over any given latitude. The Mesopause temperature is identified as the coldest point in the atmosphere and the corresponding height is taken as Mesopause height. On occasions when there were two minima in the temperature profile, the coldest was taken as the mesopause. To study the global structure of the mesopause first on seasonal scale, the mesopause height and temperature are diurnally averaged and then seasonal mean was taken during Winter (December February (DJF)), Vernal Equinox (March May (MAM)), Summer (June August (JJA)) and Autumnal Equinox (September November (SON)) with respect to Northern hemisphere. Figures 1a and 1b show the global Mesopause structure averaged for each season. It is clear that seasonal variations in the Mesopause occur prominently over the mid and higher latitudes with lesser variation over the low and equatorial latitudes. At low latitudes, the mesopause temperature doesn t show a seasonal variation, but the mesopause height does show a very slight seasonal variation. [7] By now, there are numerous studies on seasonal variation of mesopause structure and the present results are consistent with the present understanding. The tropical latitudes (generally 20 S 20 N) are nearly always within the maximum variation of the solar declination angle throughout the year. Thus, tropical latitudes experience the maximum variation in solar zenith angle [nearly overhead sun ( 0 degrees), or at minimum ( 23 degrees), to below the horizon at night (>90 degrees)] each day throughout the year. Therefore, it is expected that the variation in the tropical mesopause structure should be dominated by diurnal variation with little seasonal dependence. On the other hand, the high latitudes experience the smallest variation in solar zenith angle. Polar winter is nearly dark for the entire 24 h earth rotation period while polar summer has light for nearly the entire 24 h earth rotation period. Over the duration of a 24 h period, the polar region switches from nearly completely dark to nearly completely daylight in going from winter to summer, respectively. Thus, it is expected that the variation in the high latitude mesopause structure should be dominated by seasonal variation with little diurnal variation. Thus it is found that the winter hemisphere mesopause is warmer and at a higher altitude compared to the summer hemisphere and the equatorial mesopause is not showing any seasonal variation. These are the conclusions arrived at by many studies focused on global mesopause structure [Huaman and Balsley, 1999; Xu et al., 2007]. Now, a basic understanding exists about the mesopause and its variability. The thermal structure around the mesopause is dictated by radiative processes and solar heating above 100 km and circulation induced cooling below 90 km. The annual variation of mesopause is caused by solar heating and the seasonal variations are from nonradiative processes. Xu et al. [2007] also reported the hemispherical asymmetries in mesopause height and temperatures and showed beyond any doubt that the austral summer mesopause is higher in altitude and warmer when compared to the boreal summer. From Figure 1a, one can notice this hemispherical asymmetry in the mesopause structure. The possible mechanisms responsible for the observed hemispherical asymmetry are shown by Xu et al. [2007] which range from Earth s orbital eccentricity to gravity wave asymmetries. One more noteworthy observation from Figure 1a is the lowering of equatorial mesopause height to km during the boreal summer. Though the mean equatorial mesopause height is usually seen around 100 km, it is to be noted that during boreal summer months the height is lower at km. The mesopause temperature around the equatorial latitudinal belt 3of12

4 Figure 2. Composite diurnal pattern of the mesopause height for (a) winter, (b) vernal equinox, (c) summer, and (d) autumnal equinox. The white patches show the data gaps. is also seen to be at 170 K, irrespective of the season as shown in Figure 1b. It is envisaged that on a day to day basis the equatorial mesopause may be varying, but seasonal/ monthly averaging might not bring this fact to light as smaller variations are averaged out. Therefore it is essential to bring out the global mesopause variations at shorter time scales, which is the central objective of the present analysis Diurnal Variations of Global Mesopause [8] Using three months of observations at a stretch, composite diurnal cycles are constructed for each season for four consecutive years ( ) and are averaged to get global diurnal structure of Mesopause height and temperature. We have thus averaged the temperature along the latitude circle for each local time hour and each latitude bin (2.5 ) over the three months period. Construction of these types of composite diurnal cycles will reduce the nonmigrating tidal components and will retain the migrating tidal components. Figures 2a 2d and 3a 3d show diurnal patterns of the mesopause height and temperature for winter, vernal equinox, summer and autumnal equinox respectively. The gaps in Figures 2a 2d and 3a 3d are hours when there was no satellite coverage. On most of the seasons and over most of the latitudinal belts, local noon hours are not covered by SABER. From these diurnal patterns of global mesopause, one can find that the seasonal signatures of mesopause structure are superimposed on the diurnal variations. The first striking thing from these diurnal variations is the propagating structures, which can be readily attributed to migrating diurnal tides; however this aspect will be discussed in detail later. In polar latitudes, for summer and winter seasons, we see summer lows ( 85 km, 130 K) and winter highs ( 100 km, 200 K) in both height and temperature with no significant diurnal pattern. This pattern is not seen in the equinoxes. i.e., polar Mesopause structure is mostly subject to seasonal variability. However, one should keep in mind that Polar Regions are sampled alternatively every 60 days by the satellite. Over the equator and low latitudes, for winter (Figure 2a), the mesopause remains >100 km between 0800 and 2000 h up to 20 latitude on either side of the equator and <100 km at other times. Both equinoxes follow a similar trend in the tropics (Figures 2b and 2d). For summer (Figure 2c), the mesopause height is 85 km found between 0800 and 1600 h in contrast to what is observed during winter. A peak to peak diurnal variation of 15 km in mesopause height and 30 K in its temperature can be noticed over equatorial latitudes. Thus, the diurnal variation is strongest over equatorial latitudes and seasonal variations are strongest over polar latitudes. Further, to quantify the mesopause variability at diurnal scales for the first time across the globe, we interpolated the data gaps in mesopause height and temperature to extract the diurnal and 4of12

5 Figure 3. Diurnal pattern of the mesopause temperature for (a) winter, (b) vernal equinox, (c) summer, and (d) autumnal equinox. The white patches show the data gaps. semidiurnal amplitudes and phases. As the Polar Regions have more time gaps, we restricted our analysis to 55 to 55 latitudes. [9] The complete diurnal cycle obtained for each season using four years of data is averaged to study the diurnal and semi diurnal tidal influence on Mesopause height and temperature. Using the least square fitting method, diurnal and semidiurnal tidal amplitudes and phases are estimated. Figures 4a 4d show the latitudinal structure of reconstructed diurnal tides in mesopause altitude for the four seasons respectively. One can notice the consistent phase of diurnal tides over the equatorial latitudes during all the four seasons. However, the phase is slightly advanced in time during the boreal summer over equatorial latitudes. During all the three seasons (austral summer and equinoxes), the time of maximum magnitude is around Hrs local time whereas during boreal summer it is around Hrs. McLandress [1997, 2002] using the Canadian middle atmosphere model, emphatically showed the shifting of diurnal tidal amplitude by 4 6 h between winter and summer in Northern hemisphere. All other features of diurnal variation of mesopause height are consistent with the present understanding of migrating tides. One of the striking features observed in Figures 4a 4d, which is consistent with migrating tides, is the phase reversal at around 20 latitude. The tidal modulations of mesopause are strongest during the vernal equinox as compared to autumnal equinox, again consistent with migrating diurnal tidal theory. Apart from the equatorial maxima in diurnal tidal amplitudes in the cold point mesopause height, there is a secondary maximum which is not symmetric around the equator, changing with season. For example, during austral summer as shown in Figure 4a, the secondary maximum appears at 50 latitude in southern hemisphere and during boreal summer a broad secondary peak appears between 20 and 40 in northern hemisphere as shown in Figure 4c. Thus, the secondary peak in diurnal tidal amplitudes in mesopause height appears in the summer hemisphere. Similarly, during vernal equinox the secondary peak appears between 25 and 50 latitude in the northern hemisphere as shown in Figure 4b and between 25 and 40 latitude in southern hemisphere during autumnal equinox as shown in Figure 4d. Thus the secondary peak in diurnal tide amplitudes in mesopause height during equinoxes appears in that hemisphere which is transiting in to summer. Even though most of the features shown in Figures 4a 4d coincide with that of well known migrating tides, the hemispherical asymmetry in the secondary peak could not be explained. To further investigate the tidal modulation of mesopause height, similar analysis was carried out on mesopause temperatures and the results are depicted in Figures 5a 5d. Figures 5a 5d reveal an interesting aspect of the tidal modulations of the mesopause. The tidal modula- 5of12

6 Figure 4. Latitudinal structure of reconstructed diurnal tides in mesopause altitude for (a) winter, (b) vernal equinox, (c) summer, and (d) autumnal equinox. tion of mesopause temperature is again consistent with migrating tidal theory and moreover shows hemispherical symmetry in the secondary peak. The cold point mesopause height does not show this hemispherical symmetry. Thus this is a new observation on the tidal modulation of mesopause height and temperature and needs further analysis to investigate why there is a hemispheric asymmetry in tidal modulations of mesopause height but not in its temperature. [10] After establishing the diurnal tidal modulations of mesopause and its temperature, similar procedure is used to study the semidiurnal modulations of the same. Figures 6a 6d show the semidiurnal tidal amplitudes of the mesopause height for winter, vernal equinox, summer and autumnal equinox respectively with respect to the northern hemisphere. The amplitudes of semidiurnal modulations of mesopause are comparable with that of diurnal tides. Again, the latitudinal variation of semi diurnal tidal amplitudes in mesopause height is consistent with the tidal theory showing relatively large amplitudes in mid latitudes as compared to equatorial latitudes. It is known that the mesospheric semidiurnal tide dominates over the diurnal tide at latitudes greater than 40 on either side of the equator [Manson et al., 1999; Yi, 2001; Mitchell et al., 2002]. This dominance of the semidiurnal tide over the diurnal tide is attributed to trapping of diurnal tide as its period is longer than the inertial period in the mid latitudes. In the present study also, semidiurnal modulations of mesopause height is peaking in mid latitudes in all seasons except for boreal summer, during which the peak is observed around 20 latitude in the southern hemisphere. Figures 7a 7d show the semidiurnal tidal amplitudes in mesopause temperatures, which are consistent with our present understanding of latitudinal structure of mesospheric semi diurnal tides. Thus we quantify the modulation of the mesopause height and its temperature in terms of diurnal and semidiurnal tides and it is found that diurnal and semi diurnal tides cause variation of mesopause height as high as 5 km and temperature as high as 8 K. The present results show that the largest variation of mesopause over tropics is at diurnal scales and seasonal variation is negligible where as seasonal variation is profound at higher latitudes Planetary Wave Modulations [11] Variety of wave motions ranging from gravity waves to planetary waves pervade the mesopause region. There is every reason to believe that these waves, especially planetary waves, can modulate the mesopause height and its temperature. Until now, there is no attempt to study the 6of12

7 Figure 5. Latitudinal structure of reconstructed diurnal tides in mesopause temperature for (a) winter, (b) vernal equinox, (c) summer, and (d) autumnal equinox. aspect of planetary wave modulation of mesopause height. However, there are studies which show the equatorial wave modulations of tropopause [Tsuda et al., 1994; Satheesan and Krishna Murthy, 2005]. Scott and Cammas [2002] reported the breaking of Rossby waves on an isentropic surface that modulates the tropopause structure. In this regard, to examine the mesopause variability at planetary wave scales, we adopted a gridding method discussed by Pancheva et al. [2009]. At each 5 latitude, we constructed a time series as the satellite passes over that belt regardless of the longitude over which it passes. By doing so, we will be obtaining the temperature profiles over that latitudinal belt with 1.5 h time resolution. These temperature profiles are then used to determine the mesopause height and temperature. Using this technique, the time series is constructed and the same is used to study the signature of planetary waves in mesopause height and temperature. [12] To determine the predominant periods of the planetary waves present in the mesopause height, the time series is subjected to wavelet analysis. The planetary waves with periods between 2 and 30 days are considered here. As it will be an intensive computational task to quantify the modulation of mesopause by planetary waves across the globe, we demonstrate this aspect by considering a specific example over low latitude as this region is known for planetary wave activity during winter season. Figures 8a and 8b show the wavelet spectra of mesopause height and its temperature respectively during the winter of 2004 over northern hemisphere low latitude (10 15 N). From these spectra it is evident that the mesopause height and temperature are modulated by quasi 16 day wave (periodicity of quasi 16 day wave ranges from 12 to 20 days). One can find more details on quasi 16 day wave shown by Mitchell et al. [1999]. The modulation of the mesopause height by quasi 16 day wave is about 2 km and temperature is about 4 K. The amplitude of quasi 16 day wave in mesopause height and temperature are significant at 95% level. One can notice the signature of quasi 5 day wave also, but its amplitude is not significant. Similar analysis was carried out at other latitudes, which have shown mesopause modulations at periods ranging from 2 to 30 days with significant amplitudes (figures not shown). Thus for the first time it is demonstrated that propagating planetary waves do modulate the mesopause height. This result has important implications on the chemistry of the mesopause region. For example, it is known that the peak height of OH radicals in the MLT region closely follow the mesopause. Thus the modulation of mesopause height will also affect the altitudinal distri- 7of12

8 Figure 6. Latitudinal structure of reconstructed semidiurnal tides in mesopause altitude for (a) winter, (b) vernal equinox, (c) summer, and (d) autumnal equinox. 8of12

9 Figure 7. Latitudinal structure of reconstructed semi diurnal tides in mesopause temperature for (a) winter, (b) vernal equinox, (c) summer, and (d) autumnal equinox. 9of12

10 mesopause height as high as 5 km and temperature as high as 8 K. One interesting aspect observed in the tidal modulations of mesopause height is the hemispherical asymmetry in the secondary maximum in diurnal tide amplitudes. This asymmetry is observed only in the mesopause height but not in its temperature. Except for this asymmetry, other features of diurnal variation of mesopause are consistent with present understanding of atmospheric tides. These results concluded that the largest variation of mesopause over tropics is at diurnal scales and seasonal variation is not significant. Present results for the first time demonstrated that the planetary waves of various periodicities do modulate the mesopause structure which also significantly contributes to the short term mesopause variability. [14] Acknowledgments. Sherine Rachel John expresses her gratitude to ISRO for providing Research Fellowship. The authors acknowledge the efforts of the TIMED/SABER team in making the data available and freely downloadable. [15] Robert Lysak thanks the reviewers for their assistance in evaluating this paper. Figure 8. Wavelet spectra of mesopause (a) height and (b) temperature for the winter of 2004 over 10 N 15 N latitude. bution of chemical species around that region. Thus, the present study quantifies the variability of mesopause height and its temperature at diurnal scale across 55 to 55 latitudes and demonstrates the modulation of mesopause by planetary scale waves. 4. Summary [13] TIMED/SABER observations are used to study the mesopause modulations by diurnal tides and planetary waves. The satellite observations of temperature profiles are gridded in local time and latitudes and the same are used to study the seasonal and then diurnal variation of mesopause. High latitudes show strong seasonal dependence with colder and lower Mesopause ( 150 K, 85 km) in summer hemisphere and hotter and higher mesopause ( 200 K, 100 km) in winter hemisphere with equinoxes being transition periods. The tropics do not follow this seasonal dependence but have stronger diurnal scale variations. Recent studies on mesopause concluded that tropical mesopause is always at 100 km using the monthly mean observations of mesopause. In the present study, tidal modulation of the mesopause height and temperature are quantified in terms of diurnal and semidiurnal tides. Diurnal and semi diurnal tides cause variation of References Ballinger, A. P., P. B. Chilson, R. D. Palmer, and N. J. Mitchell (2008), On the validity of the ambipolar diffusion assumption in the polar mesopause region, Ann. Geophys., 26, , doi: /angeo Chamberlain, J. W., and D. M. 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