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1 SCIENCE CHINA Technological Sciences RESEARCH PAPER May 2012 Vol.55 No.5: doi: /s Longitudinal distribution of O 2 nightglow brightness observed by TIEMD/SABER satellite GAO Hong 1, 2*, NEE JanBai 2 & CHEN GuangMing 1 1 State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing , China; 2 Department of Physics, National Central University, Jhongli 32054, Taiwan, China Received November 26, 2011; accepted January 28, 2012; published online March 26, 2012 The global distribution of 1.27 m O 2 nightglow brightness observed by the TIMED/SABER satellite has been investigated to find the longitudinal structures for different seasons and latitudes. The results show that the O 2 airglow is dominated by wave 4 structure at latitudes between equator and 20ºS/N in both hemispheres during most seasons. At mid-latitudes around 40ºS/N, the wave 1 structure is observed for most seasons with a small contribution of wave 2 during the June solstice. A comparison of the O 2 and OH nightglows shows similarity in their global distributions which can be attributed to their similar photochemical mechanisms. O2 nightglow, longitude distribution Citation: Gao H, Nee J B, Chen G M. Longitudinal distribution of O 2 nightglow brightness observed. Sci China Tech Sci, 2012, 55: , doi: /s Introduction Airglow emissions are good sources for understanding the atmospheric dynamics. Gravity waves can be extracted from the airglow distributions observed by all-sky airglow imager (e.g. ref. [1]), winds can be derived from the airglow emission observations from Fabry-Perot interferometer (e.g. refs. [2, 3]). In addition, we can calculate the rotational temperature from airglow emission spectra (e.g. ref. [4]). In early studies by using ground-based experiments, people showed temporal variations of airglow emissions in several different time scales such as the diurnal, seasonal and secular variations. Later with rocket observations, the vertical profiles of airglow emission rates were derived by many researchers. Both the ground-based and rocket experiments were restricted to a fixed site which are lack of global structures. *Corresponding author ( hgao@spaceweather.ac.cn) The global observations of airglow became available in the last few decades with satellites and instruments such as the International Satellites for Ionospheric Studies (ISIS 2), the Upper Atmosphere Research Satellite (UARS), Odin satellite, the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite and the ROCSAT-2 (Republic of China Satellite 2) satellite. Although satellite observations can provide 3D information of airglow in principle, the complete information including latitudinal, longitudinal, and altitudinal variations is very limited. Most of satellite observations have been mainly used to study the seasonal variations of the vertical and latitudinal distributions of airglow emissions during a long satellite orbiting period. In recent years, several researchers have paid attentions to the longitudinal distributions of some atmospheric parameters. For example, by using UARS/WINDII (the wind imaging interferometer) data, Shepherd et al. [5] reported the zonal wave structure in the longitudinal distribution of the green line emission in the mesopause region and related Science China Press and Springer-Verlag Berlin Heidelberg 2012 tech.scichina.com

2 Gao H, et al. Sci China Tech Sci May (2012) Vol.55 No it to the planetary wave activities. Zaragoza et al. [6] first reported that OH nightglow had wavelike longitudinal structures varying on a timescale of days, based on the observations from the UARS/ISMS (improved stratospheric and mesospheric sounder). Wang et al. [7] analyzed the WINDII data in the mesopause region and showed that the green line nightglow exhibited zonal wave numbers 1 and 2 patterns around 35ºS and 35ºN, respectively. By analyzing the post-sunset OI nm emission from TIMED/GUVI (the Global Ultraviolet Imager) data, England et al. [8] found a zonal wave number 4 pattern in the tropics in the vernal equinox. In addition, they also used SAMI2 model to simulate this longitudinal pattern. By using the TIMED/ SABER (the Sounding of the Atmosphere using Broadband Emission Radiometry) data, Xu et al. [9] recently reported zonal wave 4 patterns in the longitudinal distributions of 2.0 µm OH nightglow brightness and the atmospheric temperature in the mesopause region and attributed this oscillation to the eastward-propagating diurnal tide with zonal wave number 3 (diurnal east propagating wave DE3). The TIMED/SABER instrument also observes the O a g X (0 0) band airglow emission at 1.27 µm g (referred to as O 2 airglow emission hereinafter). The purpose of this work is to study the longitudinal distribution of the O 2 nightglow brightness and the effects of non- migrating tides on the nightglow. This paper is organized as follows. The data processing is introduced in Section 2; the global distribution, especially the longitudinal variations of O 2 nightglow brightness is shown in Section 3; finally, a short summary is given in Section 4. 2 Data processing and methodology The TIMED satellite was launched in December 2001 to an orbit of 625 km high with an inclination angle of The satellite orbits around the Earth about 15 times per day and precesses slowly such that the satellite observation takes more than 60 days to complete a full 24-h local time coverage. With a ten-channel radiometer, SABER can directly measure the atmospheric emissions covering a broad spectral range. The observations can infer various background atmospheric parameters, such as atmospheric temperature and density, ozone density, atomic oxygen density, and others. The airglow emissions measured by SABER include the OH airglow emissions at 2.0 µm (OH-A band) and 1.6 µm (OH-B band), O 2 airglow emission at 1.27 m, and NO airglow emission at 5.3 m. In this work, the O 2 and OH-A band airglow emissions from January 2002 to October 2010 are analyzed. The latitudinal coverage of SABER observations is from 53 in one hemisphere to 83 in the other hemisphere and the latitude coverage flips to the opposite hemisphere with an about 60 days interval. In order to avoid observation gaps at the high latitudes, our analysis focuses on the latitude range of 50 S 50 N. Recently, Xu et al. [9] used the SABER data between 2:00 LT and 2:00 LT local time to study the longitudinal variations of OH nightglow brightness. Following Xu et al. [9], we study in this paper the O 2 airglow brightness distribution by using the data of the same local time. Because of the seasonal variations, we set four 61-day windows centered on day 80, day 170, day 260 and day 350, respectively, corresponding to the March equinox, June solstice, September equinox and December solstice in this work. All profiles observed between 2:00 LT and 2:00 LT during the equinoxes and solstices for 9 years ( ) are averaged to produce a map of 21 latitudes from 50 S to 50 N and 18 longitudes. This is done by choosing windows of 20 longitude 10 latitude on a 5 latitude cadence so that neighboring data points overlap by 5 latitude. Finally, the profiles are interpolated to 10 longitudinal bands by using a spline interpolation. In this study, the brightness is defined as the vertical integral of the emission rate from 75 to 100 km just following the definition given by Xu et al. [9, 10]. 3 Result and discussion 3.1 Longitudinal oscillations in O 2 nightglow brightness Figure 1 shows the O 2 nightglow brightness maps during the equinoxes and solstices. Near the equator, the O 2 nightglow brightness is stronger than those at other latitudes during both equinoxes and is stronger than those during both solstices. The brightness of O 2 nightglow during the March equinox is stronger than that during the September equinox. The characteristics of O 2 nightglow emissions in the mesopause region can be attributed to the effects of migrating diurnal tides as discussed by Shepherd et al. [11], Ward [12], and Marsh et al. [13]. Satellite observations in the mesopause region over the equator (e.g. ref. [14]) have indicated that the diurnal tides near the equinoxes are stronger than those near the solstices. For both equinoxes, the intensity of diurnal tides near the vernal equinox is stronger than those near the autumn equinox [14]. During the solstices, the O 2 nightglow brightness in the summer hemisphere is in general stronger than that in the winter hemisphere. The strong brightness near the 50ºN in the June solstice and 50 S in the December solstice should be attributed to the contamination of the dayglow emission due to long daytime condition. Despite the fact that our observation time is only within 2 hours around the midnight in this work, the remanant O 2 dayglow emission from the long life O 2 ( 1 g ) molecules is so strong after sunset that it in short summer night can not be ignored and it can significantly contribute to the O 2 nightglow brightness observed in the early evening at high latitudes in the summer hemisphere.

3 1260 Gao H, et al. Sci China Tech Sci May (2012) Vol.55 No.5 Figure 1 O 2 nightglow brightness maps in four seasons. The center point of each map is (0º latitude, 0º longitude). The unit of brightness is kr. From Figure 1, we can see that the longitudinal structure of O 2 nightglow brightness changes with latitude and season. During both equinoxes, the longitudinal distribution at the equator has four peaks; the distributions around 20 S during the March equinox and around 20 N during the September equinox have four valleys. During the June solstice, the distributions around 20 N and 20 S show four peaks and valleys, respectively. During the December solstice, the longitudinal structure is complicated except that there are 4 peaks around 50 S. Figure 2 gives the longitudinal variations of O 2 nightglow brightness at five selected latitudes: 40 S, 20 S, equator, 20 N and 40 N. During the March and September equinoxes as shown in Figure 2, the oscillations in O 2 nightglow brightness are generally in phase, while during the June and December solstices, the oscillations are out of phase at most longitudes. Table 1 gives the wave numbers which are dominant in the longitudinal variations of O 2 nightglow brightness at each latitude and season. In Table 1, symbol ~ means close to. From this table, we can see that wave numbers 1 and 4 components are, in general, dominant for most of latitudes and seasons. At 20 N during both equinoxes, and 20 S during both solstices, wavelike number 4 variation is the strongest feature. At 40 S/N, wavelike number 1 is the most dominant variation except for the June solstice, when the wavelike number 2 structure is also important. In addition, zonal wavelike number 3 oscillation is strong at 20 N during the June solstice. Xu et al. [9] have given maps of OH nightglow brightness and atmospheric temperature at 87 km in 6 months: January, March, May, July, September and November. Comparing nightglow brightness map of O 2 shown in Figure 1 with that of the OH given by Xu et al. [9], we find that the longitudinal structures of O 2 nightglow are very similar to those of OH nightglow in March, July, September and November. Although Xu et al. [9] have shown the OH nightglow brightness maps for 6 months, we will show here the OH nightglow brightness maps during both equinoxes and solstices for easy comparison. The OH nightglow brightness maps and the longitudinal distributions at five latitudes during the equinoxes and solstices are given in Figures 3 and 4, respectively. Comparing the nightglow brightness maps of OH in Figure 3 with that of O 2 in Figure 1, we can see that the global structures of both airglows are similar except for 50 latitude in the summer hemisphere. As for the intensity of brightness, unlike the O 2 nightglow, the high latitude OH nightglow brightness around 50 in both summer hemi-

4 Gao H, et al. Sci China Tech Sci May (2012) Vol.55 No spheres is not stronger than those at other latitudes. This should be due to that the OH dayglow is weaker than the OH nightglow and it does not contribute to the OH nightglow observation. As discussed before, the O 2 dayglow, however, can significantly affect the O 2 nightglow observation. During both equinoxes, the strongest OH and O 2 nightglow brightness appear at the equator. It is worth noting that during both solstices, the peaks of OH and O 2 nightglow brightness around 20 latitude in the summer hemisphere are larger than those around 20 latitude in the winter hemisphere. The zonal band with the lowest OH or O 2 nightglow brightness is situated around 20 latitude in the winter hemisphere. The longitudinal variations of OH nightglow brightness at each latitude shown in Figure 4 are respectively similar to those of O 2 nightglow brightness shown in Figure 2. As for the intensities of brightness, the OH nightglow at 40 S during the June solstice is higher than that during the September equinox. However, the O 2 nightglow brightness at 40 S during the June solstice is slightly lower than that during the September equinox. Similarly, at 40 N, the O 2 nightglow brightness during the December solstice is close to that during the March equinox; while the OH nightglow brightness during the December solstice is stronger than that during the March equinox. 3.2 Mechanisms for the longitudinal oscillations of O 2 nightglow The analogies between the O 2 nightglow brightness and OH-A band nightglow brightness are expected because of similar photochemical mechanisms for both airglow emissions. The OH-A band nightglow emission observed by SABER, which mainly includes OH(9,7) band and OH(8,6) band, is produced by the following reactions: ko+o 2 +M 2 3 O+O +M O +M H + O 3 3 k v 2 H+ O OH( 9) O Figure 2 Longitudinal distributions of O 2 nightglow brightness at 40 S, 20 S, equator, 20 N and 40 N during the equinoxes and solstices. Table 1 The dominant wave numbers in the longitudinal variation of O 2 nightglow (1 means wave number 1, etc.) 40 S 20 S equator 20 N 40 N March , ~4 June ~2 4 2, ~4 3, ~4 2, ~4 September 1 ~2 1, ~4 4 1, 4 December 1, 3, , ~4 1 Therefore, OH-A band emission is in direct proportion to [O 3 ] ([] means number density). [O 3 ] and [O] satisfy the relationship [ O3] ko+o 2+M[ O][ O2 ][M]/( kh+o [H]) under 3 photochemical equilibrium of O 3. Therefore, the OH-A band nightglow emission is approximately positively correlated to [O]. In addition, we must keep in mind that k H+O 3 is a function of temperature. As for O 2 nightglow emission, there are three possible photochemical mechanisms [10]: O+O+M O ( ) M (R1) ko O M 1 2 g koh ( v ) O g OH( v)+o OH+O ( ) (R2) koh ( v ) O 1 2 g OH( v)+o O ( ) H (R3) Reaction (R1) implies that O 2 nightglow emission is approximately correlated with [O] 2. Reactions (R2) and (R3) indicate that the O 2 nightglow emission is proportional to OH() v directly and to [O] indirectly. Thus, the O 2 nightglow emission is also positively correlated with [O] by either mechanism. In addition, the reaction rates producing O 2 ( 1 g ) molecule are temperature dependent. These indicate that O 2 nightglow emission is related with the OH-A band nightglow, and both emissions are influenced by [O] and temperature. A lot of studies have indicated that the atmospheric temperature in the mesopause region is strongly modulated by tides [14, 15]. The long-lived O atoms in the mesopause region can be also modulated by dynamical processes including tides. Therefore, both OH and O 2 nightglow emis-

5 1262 Gao H, et al. Sci China Tech Sci May (2012) Vol.55 No.5 Figure 3 The same as Figure 1 but for OH nightglow brightness. sions can be modulated by the same tidal processes simultaneously. Xu et al. [9] have studied the effects of non-migrating tides on the longitudinal distribution of atmospheric temperature, and analyzed the roles played by non-migrating tides in the longitudinal structure of OH nightglow by comparison with atmospheric temperature. Their results indicated that strong wavelike oscillations in temperature and OH nightglow brightness are caused by non-migrating tides. As discussed above, the longitudinal structures of O 2 nightglow brightness and OH nightglow brightness are similar. Therefore, some dominant wavelike oscillations in the longitudinal distributions of O 2 nightglow brightness should be attributed to non-migrating tides. According to the result given by Xu et al. [9], the zonal wave number 1 structure near the equator at a fixed local time results from the combined modulation of the westward-propagating diurnal tide with wave number 2 (DW2) and the standing or zonally-symmetric (wave number 0) diurnal oscillation (D0). The wave number 4 structure is mainly caused by DE3. Hagan and Forbes [16] have shown that DW2, D0, and the eastward-propagating diurnal tide with zonal wave number 3 (DE3) are strong near the equator. In addition, the wave number 3 structure around 20ºN in the June solstice should be produced under the action of the eastward-propagating diurnal tide with zonal wave number 2 (DE2) as explained by Xu et al. [9]. 4 Summary By using the 9 year data of the 1.27 µm O 2 airglow emission from the TIMED/SABER for , we analyzed the global seasonal distributions of O 2 nightglow. The global distributions of O 2 and OH nightglow brightness were compared to study the effects of tides on O 2 nightglow. The results indicated the similarity in the longitudinal variations of both airglows. This can be understood in terms of their similar photochemical mechanisms. There are clear longitudinal oscillations in the O 2 nightglow brightness. During both equinoxes, the longitudinal distribution at the equator has four peaks; the distribution around 20 in the autumn hemisphere has four valleys. During the June solstice, the distributions around 20 N and 20 S show four obvious peaks and valleys, respectively. Zonal wavelike numbers 1 and 4 variations are dominant at most latitudes. Zonal wavelike number 4 is the most dominant oscillation at 20 N during both equinoxes and 20 S

6 Gao H, et al. Sci China Tech Sci May (2012) Vol.55 No This work was supported by the National Natural Science Foundation of China (Grant Nos , , , , ), the National Basic Research Program of China ( 973 Project) (Grant No. 2006CB806306), China Postdoctoral Science Foundation Funded Project (Grant No ), the Specialized Research Fund for State Key Laboratories, and the National Science Council for postdoctor position at NCU (Grant No. NSC M ). We are greatly grateful for the airglow data provided by the SABER team. Figure 4 The same as Figure 2 but for OH nightglow brightness. during both solstices. At 40 S/N, wavelike number 1 is the most dominant variation except for the June solstice; in this season the wavelike number 2 structure is also important. The wavelike number 4 variation should be mainly caused by DE3, and the wavelike number 1 variation is due to the combination of DW2 and D0. In addition, zonal wavelike number 3 oscillation is strong at 20 N during the June solstice. This should be a signature of the non-migrating DE2 tide. 1 Li Q, Xu J Y, Yue J, et al. Statistical characteristics of gravity wave activities observed by an OH airglow imager at Xinglong in northern China. Ann Geophys, 2011, 29: Yuan W, Xu J Y, Ma R P, et al. First observation of mesospheric and thermospheric winds by a Fabry-Perot interferometer in China. Chin Sci Bull, 2010, 55: Jiang G Y, Yuan W, Ning B Q, et al. A comparison of mesospheric winds measured by FPI and meteor radar located at 40N. Sci China Tech Sci, this special issue, Zhu Y J, Xu J Y, Yuan W, et al. First experiment of spectrometric observation of the Hydroxyl Emission and Rotational Temperature in the mesopause in China. Sci China Tech Sci, this special issue, Shepherd G G, Thuillier G, Solheim B H, et al. Longitudinal structure in atomic oxygen concentrations observed with WINDII on UARS. Geophys Res Lett, 1993, 20: Zaragoza G, Taylor F W, López-Puertas M. Latitudinal and longitudinal behavior of the mesospheric OH nightglow layer as observed by the Improved Stratospheric and Mesospheric Sounder on UARS. J Geophys Res, 2001, 106: Wang D Y, Ward W E, Solheim B H, et al. Longitudinal variations of green line emission rates observed by WINDII at altitudes km during J Atmos Solar-Terr Phys, 2004, 64: England S L, Immel T J, Sagawa E, et al. Effect of atmospheric tides on the morphology of the quiet time, postsunset equatorial ionospheric anomaly. J Geophys Res, 2006, 111: A10S19 9 Xu J, Smith A K, Jiang G Y, et al. Strong longitudinal variations in the OH nightglow. Geophys Res Lett, 2010, 37: L Gao H, Xu J Y, Chen G M, et al. Global distributions of OH and O 2 (1.27 μm) nightglow emissions observed by TIMED satellite. Sci China Tech Sci, 2011, 54: Shepherd G G, Roble R G, Zhang S P, et al. Tidal influence on midlatitude airglow: Comparison of satellite and ground-based observations with TIME-GCM predictions. J Geophys Res, 1998, 103: Ward W E. A Simple Model of Diurnal Variations in the Mesospheric Oxygen Nightglow. Geophys Res Lett, 1999, 26: Marsh D R, Smith A K, Mlynczak M G, et al. SABER observations of the OH Meinel airglow variability near the mesopause. J Geophys Res, 2006, 111: A10S05 14 Xu J Y, Smith A K, Liu H L, et al. Seasonal and QBO variations in the migrating diurnal tide observed by TIMED. J Geophys Res, 2009, 114: D Yuan T, She C Y, Krueger D, et al. A collaborative study on temperature diurnal tide in the midlatitude mesopause region (41 N, 105 W) with Na lidar and TIMED/SABER observations. J Atmos Solar-Terr Phys, 2010, 72: Hagan M E, Forbes J M. Migrating and nonmigrating diurnal tides in the middle and upper atmosphere excited by tropospheric latent heat release. J Geophys Res, 2002, 107: