Imaging observations of upper mesospheric nightglow emissions from Tirunelveli (8.7 o N)

Indian Journal of Radio & Space Physics Vol. 38, June 2009, pp. 150-158 Imaging observations of upper mesospheric nightglow emissions from Tirunelveli (8.7 o N) V Lakshmi Narayanan, S Gurubaran * & K Emperumal Equatorial Geophysical Research Laboratory, Indian Institute of Geomagnetism, Tirunelveli 627 011, India Received 31 December 2007; revised 18 August 2008; re-revised received and accepted 16 December 2008 An all-sky airglow imager was installed at the equatorial site Tirunelveli (8.7 o N, 77.8 o E geographic; 1.34 o N magnetic dip) during January 2007 to study the small-scale dynamics occurring in the mesosphere-lower thermosphere (MLT) region. The instrument details are presented in the paper along with methods of information retrieval from the recorded data. The presence of a variety of small-scale mesospheric dynamical events, observed with the instrument, is demonstrated. Preliminary results on the observed features, viz. the quasi-monochromatic gravity waves and instability features, such as ripples and mesospheric bores, are presented in the paper. Keywords: Airglow imagers, Mesosphere-lower thermosphere region, Gravity waves, Nightglow emissions PACS No.: 92.60.hw; 92.60.hh; 92.60.hc 1 Introduction Gathering wind and temperature information in the vicinity of the mesopause region (80 to 100 km) is still challenging. Even though MF, MST and meteor radars are used to obtain valuable wind estimates from this region, data from these instruments is not suitable for deriving parameters of small-scale waves with time periods less than one hour due to insufficient data or data gaps that often appear in the data sets. This has been posing a major constraint for a vivid understanding of small-scale gravity waves that are believed to play an important role in the transportation and deposition of energy and momentum in this region 1. Several ground-based studies have utilized airglow emissions in determination of temperature and wind in the mesopause region. A number of satellite missions like TIMED have also been carried out and are in progress to probe this region. In 1970s a novel method of probing the MLT region was evolved in which certain selected airglow emissions were monitored through wide angle cameras 2,3. This technique has undergone significant development during past two decades. All-sky airglow imagers were deployed at several new sites during this period 4-8. The advantage of these instruments over the others is that they can resolve individual small-scale gravity wave events, thereby giving a detailed insight into the complex wave-wave interactions, wave breaking processes and wave-mean flow interactions. Apart from contributing to mesospheric gravity waves, airglow imagers also find applications in the study of ionosphere by monitoring airglow depletions associated with large-scale plasma bubble events 9 which are interlinked to the equatorial spread F and nighttime ionospheric scintillations. A multi-wavelength all-sky airglow imager (ASAI) was deployed at Tirunelveli (8.7 o N, 77.8 o E geographic; 1.34 o N magnetic dip), India, during January 2007 with an objective of monitoring mesospheric gravity wave motions from this low latitude site. This is a potential geographical site near the Indian Ocean. The gravity wave sources could arise in the nearby maritime continent and due to the prominent Himalayan orography. Moreover, this site is at a distance of ~30 km to the north south aligned Western Ghats, whose orographic contribution to the gravity wave generation is poorly understood. Even though imaging studies of F-region ionosphere have been carried out from the Indian sector 10-12, observations of the mesospheric region are extremely sparse 13. The deployment of the airglow imager is, therefore, expected to increase the knowledge of the characteristics of gravity waves in the vicinity of the mesopause over the low latitude Indian sector. 2 Details of ASAI The ASAI was procured from Keo Scientific Limited, Canada and is currently equipped with four interference filters. Figure 1 depicts a schematic view

NARAYANAN et al.: UPPER MESOSPHERIC NIGHTGLOW EMISSIONS FROM TIRUNELVELI 151 Fig. 1 Schematic diagram of ASAI of the instrument. The front end optical system consists of a circular fisheye lens (f/4 Mamiya medium-format achromatic all-sky lens) with a focal length of 24 mm and 180 o field of view (FOV) followed by a plano-convex lens pair making up the telecentric arrangement. Between the fish eye lens and telecentric lens pair, an external shutter is provided to prevent the filters and CCD from excess light during non-operational hours. As a precautionary measure, a light sensor is provided, which closes the shutter in case the ambient light level is above the fixed threshold. A six position filter wheel with temperature controller follows the front end optics in which currently four filters are in use. The interference filters are designed for operation at room temperature around 27 o C. The characteristics of interference filters are given in Table 1 along with the most probable centroid height of the corresponding airglow emission. A software is used for interactive control over the filter position, filter wheel temperature and shutter status. Emission wavelength (nm) Table 1 Filter characteristics Assumed centroid height (km) Filter peak transmission wavelength (nm) Filter bandwidth (nm) Peak transmission (%) OI (557.7) 97 557.8 1.79 53.5 Na (589 and 90 589.2 1.98 71 589.6) OH meinel 87 705.3-928.2 93.6 band (notch ~865) Background filter 572.3 1.95 84 The image that follows the interference filter is captured on the CCD detector through a pair of achromats comprising of re-imaging optics. The purpose of re-imaging is to preserve the gathered light flux while storing the data in smaller sized CCD array. The reimaged light is allowed to fall on CCD detector s active area through an ultrafast Canon camera lens (f/0.95). The CCD detector is back-illuminated and air-cooled with an array of 512 512 pixels with a

152 INDIAN J RADIO & SPACE PHYS, JUNE 2009 pixel depth of 16 bits, in which every pixel covers an area of 24 24 µm. The physical dimension of the CCD array is 12.3 12.3 mm. The camera system is thermoelectrically cooled to temperature below -70 o C (dark charge is 0.0023 electrons/pixel/sec at -70 o C) during data acquisition. The CCD response is almost flat and differential gain of pixels is merely statistical in nature. Data from the CCD camera is read and stored on a windows-based computer via USB 2.0 interface. 3 Image processing Several methods of retrieval of information from all-sky airglow images are available in the literature 3,14-16. The standard method for airglow image processing is adopted in the present study for processing the acquired images. The fisheye lens makes it possible to capture almost the entire night sky and projects the image onto the CCD in such a way that each pixel subtends equal angle of view. The raw image is said to be wrapped, as the image is compressed and curved at low elevation angles owing to the projection by fisheye lens. In addition, while viewing at low elevation angles, a thicker emission layer results in the increase of intensity and is known as Van Rhijn effect. The aim of image processing is to compensate for such intensity effects and noise factors to a considerable extent and then unwrap the image. 3.1 Determination of field-of-view (FOV) The FOV for each campaign is determined with the help of elevations of star positions on raw images. Elevation angles are noted for few stars around the edges of an all-sky image and the FOV for a given operating condition is computed from these angles. 3.2 Image calibration Gravity wave parameters like apparent phase velocity and horizontal wavelength are more accurate only after projecting the image into an equidistant grid. Moreover, it is impossible to subject the image to two dimensional spectral analyses without removing the noise, intensity effects and the curvature brought about by fisheye lens projection. The unwrapping (projection of raw image in an equidistant grid) process requires transformation of the image through a set of coordinate systems 10. The azimuth (A) and elevation (E) values for the bright star positions in the image are noted from astronomical ephemeris and the pixel coordinates (i,j) are transformed to distance (x,y) or geographic coordinates. A standard coordinate system (f,g) is defined 3 such that the horizon (elevation angle 0 o ) is of unit radius from zenith (elevation angle 90 o ) and a linear transformation of pixel coordinates (i,j) is prescribed by the following set of equations: f = a i + a j + a 1 2 3 g = b i + b j + b 1 2 3 (1) where, (f,g), are the standard coordinates for corresponding (i,j) of pixel coordinates and the coefficients a and b are called calibration constants. To get (f,g) values of the noted set of stars, their azimuth and elevation values have to be used in the following equations: f = G( E)sin( A) g = G( E)cos( A) (2) where, G(E), is a function that relates the elevation angle and distance covered by a fisheye lens from its centre. Here, G(E) is assumed to be a linear function which is a good first order approximation for fisheye lenses in what is known as the equidistance model 14,17. Once the calibration constants a and b are found, one can get (A,E) from Eqs (1) and (2). In the images, with the help of the azimuth values, the top of the image has been made to point northward and the right to point eastward. In order to get the latitude longitude (λ,φ) values or distance values (x,y) for all (i,j) positions, one needs to know an angle, called earth-centered angle (ECA), as is evident from the viewing geometry (Fig. 2). The ECA (ψ) is given by: 1 r ψ = 90 E sin cos E (3) r + z where, r, is the distance of the observation station from the Earth s centre (usually, radius of the earth for stations near mean sea level); and z, is the altitude of the airglow emission layer from the observing station. With the aid of ECA, the real distance, l, for any point on the image from image center can be found, which gives the distance coordinates as below: x = l sin( A) y = l cos( A) (4)

NARAYANAN et al.: UPPER MESOSPHERIC NIGHTGLOW EMISSIONS FROM TIRUNELVELI 153 intensity values, it is better to carry out regridding as the last stage of image processing. After regridding, one may median filter the image for better clarity. It was found that the median filtering with 3 3 or 5 5 neighborhood is optimum. 3.4 Star field removal After the calibration of images, the stars are of no use and act as noise. They can be removed by replacing interpolated values from the rows and columns that contain them or by replacing a mean (or median) value from the surrounding undisturbed pixels. In the present case, the pixels affected by starlight and their neighboring pixels have been replaced with the mean of surrounding pixel values. Care has been taken in retaining the undisturbed pixel values while replacing star light pixels. Fig. 2 Airglow viewing geometry (curvatures of earth surface and emission layers are exaggerated): r - distance of observing station from earth s center (usually radius of earth); z - distance of airglow layer from observing station; E - elevation of point P; ψ - earth centered angle (ECA); l - distance between the point P and center of the image. The center of image represents the zenith of observing site This set of equations results from discarding the curvature of the emission region. This is a valid assumption since the curvature is much smaller than our analyzing area. To obtain the latitude and longitude values for all pixel positions the following equations can be utilized: p 1 λ = sin (sin λ cosψ + cosλ sinψ cos A) 1 sinψ sin A φp = sin cosλ p (5) where, (λ p, φ p ), are the latitude and longitude of the points; and λ, is the latitude of the observing site. 3.3 Regridding The image has to be re-sampled into a uniformly spaced distance grid from the obtained distance coordinates and this process is called regridding. It is accomplished by means of 2D interpolation of the image. The nearest neighbor interpolation method has been utilized to re-sample the image into an equidistant grid. This process is necessary before subjecting the image to spectral analysis like Fourier analysis as they require uniformly spaced data. Since regridding is the process of re-interpolating the 3.5 Background and flat-field corrections Usually noise removal contains two steps, viz. background subtraction and flat-field correction. To carry out background subtraction, images with the background filter should be taken periodically with the same exposure time of other wavelengths. The background images are subtracted from the acquired images. This process ensures that the sky noise and constant instrumental noise are removed from acquired emission intensities. But this process could not work effectively for broadband OH images since their bandwidth is very large and estimation of sky noise for such a wide bandwidth is not straightforward. Hence, the sky noise was assumed as more or less a constant for a limited time interval and a set of images acquired within the interval were averaged to use as a background image. Subtraction of this image has the advantage in that it accounts for constant instrumental noise and intensity effects like Van Rhijn effect. It also enhances the contrast of structures by removing the mean airglow signal. Moreover, it can be adapted to OH images also. The flat-field correction accounts for differences in gain between the pixels. If the gain is almost uniform throughout the CCD array, flat-fielding does not improve the quality much. Flat-fielding is a must for situations that need precision photometry. Nevertheless, flat-fielding brings about uniformity in the images by accounting for differential pixel gains. Garcia et al. 14 have suggested the use of an average image for flat-fielding in order to compensate Van Rhijn effect. In the present case, star-removed average of images acquired with the background filter

154 INDIAN J RADIO & SPACE PHYS, JUNE 2009 (572.3 nm) has been utilized. By taking the mean of almost all the background images acquired throughout the night, the sky noise is presumed to get averaged out. This flat-field frame is a good uniformly illuminated surface, provided stars were removed and only cloudless images were used. This also removes the intensity perturbations caused by other light noise like city lights in the horizons to an extent. 3.6 Time differencing Time differencing uses the difference of successively obtained images that are of improved image quality for visualizing the wave events. The advantage of this method is that while subtracting, the fixed background and Van Rhijn effect are minimized considerably, thus improving the contrast. This method has been used in a few cases for measuring phase velocity and a better display of wave structures. Swenson & Mende 15 were the first to propose this method to increase the contrast of OH images. But this method, though not mandatory, can be used only to observe the morphology and propagation direction of the wave, and care should be taken while measuring wavelengths as these estimates may have errors. 4 Results and discussion Figures 3(a)-(f) show a sequence of the processing steps used for an image recorded on the night of 8-9 February 2007 with the OH filter. The raw image is shown in Fig. 3(a) in which brightness in the top right corner of field of view is caused by city lights and vehicle lights. Flat-fielding is done with the starremoved average of images acquired with background filter (572.3 nm) [Fig. 3(b)]. The brightness caused by city lights is, thus, expected to be accounted for by flat-fielding. The star removal algorithm in the present study detects the pixels influenced by stars with sudden intensity enhancements and replaces them with the mean value of surrounding pixels. This algorithm leaves only very few exceedingly bright stars [Fig. 3(c)]. The average of a set of star-removed OH images is shown in Fig. 3(d) that serves as a background image. Figure 3(e) is the difference between the image under consideration and the average image in which the intensity effects and noise are accounted for quite reasonably. On comparing Fig. 3(e) with Fig. 3(b), one infers the absence of centre brightness and enhanced contrast of the wave event. The last image [Fig. 3(f)] shows the coordinate-transformed equidistant projection of the Fig. 3 Image processing steps: a) raw image; b) flat-fielded image; c) star removed image; d) average image (used as background); e) difference image; f) equidistance projection of difference image (median filtered) [x and y axis represent pixel coordinates from (a) to (e) and distance in km in (f)]. This image is an OH image obtained on 8-9 February 2007 at 21:03 hrs IST with an exposure time of 90 s

NARAYANAN et al.: UPPER MESOSPHERIC NIGHTGLOW EMISSIONS FROM TIRUNELVELI 155 image. This image is median-filtered for better clarity. A significant drawback in the current location of the imager is the presence of light disturbances caused by moving vehicles on nearby road. Even though FOV of the instrument has been reduced, the effect still persists though to a lesser degree, thereby, forbidding any rigorous analysis leading to estimates of intensity values and power spectral density of dominant wave features. Because of this constraint, only those images have been considered that are undisturbed by any vehicular passage. Nevertheless, this effect does not affect the information retrieved on upper atmospheric structures and motions recorded in the images. Based on the past studies with ASAI, most of the events observed are classified into different types. A few of them are: 4.1 Quasi-monochromatic waves The wave event shown in time-differenced images of Figs 4(a), (c) and (d) was observed on the night on 14-15 February 2007 beginning at 22:30 hrs IST in OH, Na and OI (557.7 nm) airglow emissions with a clear display in OH for more than 3.5 h. It is propagating northwards at an azimuth of ~350 o. Figure 4(b) shows the cross-sections along the white arrow indicated in Fig. 4(a) from a sequence of images. The crests and troughs in the extracted crosssections of the wave are clearly seen to propagate. The horizontal phase speed of this wave is estimated to be 61.5 ms -1 and the horizontal wavelength is ~28 km, yielding an observed periodicity of ~7.5 min. Figure 5(a) reveals another wave feature detected on the night on 20-21 January 2007. The large-scale structures noticed herein filled almost the entire field of view and had an estimated wavelength of ~41 km and phase velocity of ~45 ms -1 with an estimated periodicity of ~15 min. They too persisted for several hours propagating towards north-west at an azimuth of 312 o. The phase velocities of wave structures measured from these images are called apparent phase Fig. 4 Time difference images of wave event observed on the night on 14-15 February: a) OH image; b) Cross sections from a sequence of images taken along the arrow shown in OH image at the mentioned times. The slanting lines represent the motion of crests to the left as time progress; c) Na image; d) OI image. Exposure time for Na and OI images is 180 s (x and y axis show distance in km)

156 INDIAN J RADIO & SPACE PHYS, JUNE 2009 velocities since they are expected to be Doppler shifted by background winds. Hence, the derived wave periodicity is also apparent. These types of waves are most frequently observed and widely studied through airglow imaging. Past studies of these waves have determined phase velocities observed in the range 20-80 ms -1 and wavelengths between 15 and 50 km. Typically these waves will have a lifetime of about 1 h or more. The wave parameters of events that are described above lie within the limits reported earlier. It may be mentioned that a significant portion of the waves observed with all-sky airglow imagers are ducted 18,19 and these waves are capable of propagating large distances before they dissipate. 4.2 Ripples Ripples are very short-scale localized transient features noticed in airglow images. They are not observed to persist for several hours and are not likely to travel long distances. They are believed to arise from local instabilities, either convective or dynamical, and hence are generated in situ 20-23. Ripple lifetime will be typically from a few min to 45 min and they show a tendency to align with the background wind irrespective of their origin. The features noted by circled regions in Fig. 5(b) serve as examples for ripples observed over Tirunelveli. On the night on 20-21 January 2007, an extensive ripple activity is recorded. The observed ripples are almost perpendicular to the large-scale bands and they propagate towards north-east (whereas band propagation is towards north-west). Their wavelengths are very short, ranging 5-13 km. They are highly transient and less intense. In many occasions, the entire ripple structure is seen to drift along south-east. The smaller horizontal wavelengths and the perpendicular alignment of ripple phase fronts with the bands suggest that these ripples might result from a convective instability 22. The drift of the entire ripple structure towards the south-east direction might have resulted from a component of the neutral wind that might have persisted along that direction. 4.3 Prominent fronts The night on 22-23 March 2007 was marked by a noticeable wave event when the prominent front (Fig. 6) was observed around 00:15 hrs IST and lasted up to 01:30 hrs IST. The front has horizontal extension all through the field of view and is followed by a few phase-locked undulation crests whose numbers seem to increase with time. The leading front is the brightest of all the observed crests and its propagation direction is from north-west towards south-east at an azimuth of 140 o with an apparent phase velocity of ~68 ms -1. The wavelength calculated from the distance between the first and second crests is ~15 km. The intensity shows an increase behind the leading front and acts as a boundary between bright and dark regions of the image. Such prominent fronts Fig. 5 OH image recorded on 20-21 January 2007 at 02:16 hrs IST: a) The arrow indicates direction of propagation of NE-SW aligned bands; and b) The circles highlight the ripples (x and y axis show distance in km)

NARAYANAN et al.: UPPER MESOSPHERIC NIGHTGLOW EMISSIONS FROM TIRUNELVELI 157 Fig. 6 Mesospheric frontal system recorded with OH imagery on 22-23 March 2007 (x and y axis show distance in km) accompanied by either increase or decrease in intensities are usually classified as either mesospheric bores 24 or wall events 25. Dewan & Picard 26,27 developed a widely accepted model of mesospheric bores based on the theory of tidal bores. Considering the similarities of the observed event with Dewan & Picard model, it is believed that this might be a weak mesospheric undular bore event reported for the first time from the Indian sector. 5 Conclusions In the present paper, the operational details of the all-sky airglow imager deployed at Tirunelveli and methods of information retrieval from the recorded data are given. The ASAI is capable of monitoring wave disturbances in shorter scales (< 1 h time period and < 100 km wavelength). Imaging observations made so far yield estimates of wavelengths and apparent phase velocities of individual quasimonochromatic waves. The wave events shown in the present work are among a variety of interesting events observed from this site for over 30 clear moonless nights since early 2007. Detailed analysis of a few of those events is in progress and will be communicated in due course. Acknowledgements One of the authors (VLN) is thankful to the Director, Indian Institute of Geomagnetism, for research scholarship. The authors acknowledge the

158 INDIAN J RADIO & SPACE PHYS, JUNE 2009 support from the Department of Science and Technology, Government of India for carrying out the study. References 1 Fritts D C & Alexander M J, Gravity wave dynamics and effects in the middle atmosphere, Rev Geophys (USA), 41 (2003) 1003, doi:10.1029/2001rg000106. 2 Peterson A W & Kieffaber L M, Infrared photography of OH airglow structures, Nature (UK), 242 (1973) 321. 3 Hapgood M A & Taylor M J, Analysis of airglow image data, Ann Geophys (France), 38 (1982) 805. 4 Nakamura T, Higashikawa A, Tsuda T & Matsushita Y, Seasonal variations of gravity wave structures in OH airglow with a CCD imager at Shigaraki, Earth, Planet Space (Japan), 51 (1999) 897. 5 Smith S M, Mendillo, Baumgardner J & Clarke R R, Gravity wave imaging from a sub-auroral site: first results from Milestone hill, J Geophy Res (USA), 105 (2000) 27119. 6 Batista P P, Clemesha B R, Simonich D M, Taylor M J, Takahashi H, Gobbi D, Batista I S, Buriti R A & de Medeiros A F, Simultaneous lidar observation of a sporadic sodium layer, a Wall event in the OH and OI5577 airglow images and the meteor winds, J Atmos Sol-Terr Phys (UK), 64 (2002) 1327. 7 She C Y, Li T, Williams B P & Yuan T, Concurrent OH imager and sodium temperature/wind lidar observation of a mesopause region undular bore event over Fort Collins/Plateville, Colorado, J Geophys Res (USA), 109 (2004) D22107, doi:10.1029/2004jd004742. 8 Shiokawa K, Ejiri M K, Ogawa T, Yamada Y, Fukunishi H, Igarashi K & Nakamura T, A localized structure in OH airglow images near the mesopause region, J Geophys Res (USA), 108 (2003) 4048, doi:10.1029/2002jd002462. 9 Mendillo M, Baumgardner J, Colerico M & Nottingham D, Imaging science contributions to equatorial aeronomy: initial results from the MISETA program, J Atmos Sol-Terr Phys (UK), 59 (1997) 1587. 10 Sinha H S S & Raizada S, Some new features of ionospheric plasma depletions over the Indian zone using all sky optical imaging, Earth, Planet Space (Japan), 52 (2000) 549. 11 Sinha H S S, Rajesh P K, Misra R N & Dutt N, Multiwavelength imaging observations of plasma depletions over Kavalur, India, Ann Geophys (France), 19 (2001) 1119. 12 Mukherjee G K, Carlo L, Mahajan S H & Patil P T, First results of all-sky imaging from India, Earth, Planet Space (Japan), 50 (1998) 119. 13 Mukherjee G K, The signature of short-period gravity waves imaged in the OI 557.7 nm and near infrared OH nightglow emissions over Panhala, J Atmos Sol-Terr Phys (UK), 65 (2003) 1329. 14 Garcia F J, Taylor M J & Kelly M C, Two-dimensional spectral analysis of mesospheric airglow image data, Appl Opt (USA), 36 (1997) 7374. 15 Swenson G R & Mende S B, OH emission and gravity waves (including a breaking wave) in all-sky imagery from Bear Lake, Geophys Res Lett (USA), 21 (1994) 2239. 16 Taylor M J & Garcia F J, A two dimensional spectral analysis of short period gravity waves in the OI (557.7 nm) and near infra red OH nightglow emissions over Arecibo, Puerto Rico, Geophys Res Lett (USA), 22 (1995) 2473. 17 Xiong Y & Turkowski K, Creating Image-Based VR using a self-calibrating fisheye lens, Paper presented at the IEEE Computer Society Conference on Computer Vision and Pattern Recognition, San Juan, Puerto Rico, 17-19 June 1997. 18 Isler J R, Taylor M J & Fritts D C, Observational evidence of wave ducting and evanescence in the mesosphere, J Geophys Res (USA), 102 (1997) 26301. 19 Walterscheid R L, Hecht J H, Vincent R A, Reid I M, Woithe J & Hickey M P, Analysis and interpretation of airglow and radar observations of quasi-monochromatic gravity waves in the upper mesosphere and lower thermosphere over Adelaide, Australia (35 o S, 138 o E), J Atmos Sol-Terr Phys (UK), 61 (1999) 461. 20 Taylor M J, Pendleton Jr W R, Clark S, Takahashi H, Gobbi D & Goldberg R A, Image measurements of short period gravity waves at equatorial latitudes, J Geophys Res (USA), 102 (1997) 26283. 21 Hecht J H, Walterscheid R L, Vincent R A, Airglow observations of dynamical (wind-shear induced) instabilities over Adelaide, Australia, associated with atmospheric gravity waves, J Geophys Res (USA), 106 (2001) 28189. 22 Hecht J H, Instability layers and airglow imaging, Rev Geophys (USA), 42 (2004) RG1001, doi:10.1029/ 2003RG000131. 23 Li T, She C Y, Williams B P, Yuan T & Collins R L, Concurrent OH imager and sodium temperature/wind lidar observation of localized ripples over northern Colorado, J Geophys Res (USA), 110 (2005) D13110, doi:10.1029/ 2004JD004885. 24 Taylor M J, Turnbull D N & Lowe R P, Spectrometric and imaging measurements of a spectacular gravity wave event observed during the ALOHA-93 campaign, Geophys Res Lett (USA), 22 (1995) 2849. 25 Li F, Swenson G R, Liu A Z, Taylor M J & Zhao Y, Investigation of a wall wave event, J Geophys Res (USA), 112 (2007) D04104, doi:10.1029/2006jd007213. 26 Dewan E M & Picard R H, Mesospheric bores, J Geophys Res (USA), 103 (1998) 6295. 27 Dewan E M & Picard R H, On the origin of mesospheric bores, J Geophys Res (USA), 106 (2001) 2921.