OH Nightglow Emission Research Articles

Dual‐site imaging observations of small‐scale wave structures through OH and OI nightglow emissions

Four all‐sky CCD imagers have been simultaneously operated at Shigaraki (34.9°N, 136.1°E) and Murou (34.6°N, 136.0°E), Japan from January to March 1998 to obtain OH and OI (557.7 nm) nightglow images. We determined height of observed small‐scale wave structures (horizontal wavelengths of ∼7–9 km) by a triangulation technique. For an event of January 26, the structures seen in both nightglow images are estimated to have existed at around 91 km, suggesting that the structures occur at altitudes where these emission layers overlap. For another event of January 30, the structures with a height of 82 km were observed only in the OH images. These results indicate that the height extent of the small‐scale wave structures is fairly small, suggesting that such structures are instability features rather than gravity waves. Even if we assume that the structures are gravity waves, their vertical wavelengths estimated by using the dispersion relation are very small.

Long base‐line measurements of short‐period mesospheric gravity waves during the SEEK Campaign

Simultaneous observations of short‐period (<1 hour) mesospheric gravity waves were made over an extended base‐line (∼660 km) from two sites in Japan (at Shigaraki and Yamagawa) during the SEEK (Sporadic‐E Experiment over Kyushu) campaign, 9–23 August, 1996. Coincident image data obtained on four nights provided a rare opportunity to investigate the horizontal spatial extent and propagation characteristics of the waves in detail. Surprisingly, out of a total of 12 events observed from Shigaraki and 10 from Yamagawa at most only three exhibited similar propagation characteristics at each site. The majority of the wave displays (∼70–75%) had quite dissimilar characteristics suggesting a preponderance for localized wave motions of limited lifetimes and geographical extent. A marked preference for wave progression towards the N‐NE indicates that the gravity waves imaged in the near infrared OH and OI (557.7 nm) nightglow emissions were probably not directly associated with the observed E‐region irregularities.

A two‐dimensional spectral analysis of short period gravity waves imaged in the OI(557.7 nm) and near infra red OH nightglow emissions over Arecibo, Puerto Rico

In January 1993 an extensive set of radar and optical data was gathered from various key sites around the world during a coordinated “10 Day Run” designed to investigate the coupled dynamic behavior of the upper atmosphere on a large, medium and small scale. As part of this campaign an all‐sky CCD imaging system was operated at Arecibo Observatory, Puerto Rico, to help quantify the response of the low latitude mesosphere‐thermosphere system to short period (<1 hour) gravity waves. Measurements of the OI(557.7 nm) and near infrared OH nightglow emissions were made in conjunction with photometric and ISR radar soundings and revealed an abundance of small‐scale structure in the 80–100 km range. In this letter we apply two‐dimensional spectral analysis techniques to aid in the interpretation of a complex set of image data that consisted of two intersecting quasi‐monochromatic gravity wave patterns progressing on approximately orthogonal headings. An investigation of the spectral content and temporal evolution of these wave motions at each emission altitude is presented.

Seasonal and latitudinal variations of gravity wave‐driven fluctuations in OH nightglow

The seasonal and latitudinal variations of the gravity wave‐driven fluctuations in the OH nightglow are investigated theoretically using a model that accounts for emission from an extended OH layer and includes the effects of eddy diffusivities in the gravity wave dynamics. The mean (unperturbed) state is obtained from a two‐dimensional, nighttime model so that mean‐state number densities, temperatures and eddy diffusivities are all self‐consistent. Seasonal and latitudinal variations in the background OH nightglow emission and in the propagation and dissipation characteristics of the gravity waves influence how the OH nightglow modulations due to gravity waves depend on season and latitude. At intermediate gravity wave periods (i.e., periods between 0.5 and 3 hours for λx = 100 km; between 3 and 10 hours for λx = 500 km; and greater than about 4 hours for λx = 1000 km) possible trends in the behavior of <η> (equal to , where I is the airglow intensity, T is intensity‐weighted temperature, an overbar denotes the mean state, a prime denotes a perturbation about that mean state and the brackets indicate vertical integration over all emission levels) with latitude and season could be masked by interference effects. At long periods (i.e., periods greater than 3 hours and 10 hours for λx = 100 and 500 km, respectively) trends are complicated by the competing effects of eddy diffusivities, which directly modify both local values of and the altitudes of maximum wave amplitude, and the vertical distributions of the minor species. Only at periods of a few hours or less (depending on horizontal wavelength) are any seasonal trends seen in <η>, and these are more noticeable in the magnitude of <η> than in its phase. These seasonal variations in <η> are primarily due to seasonal variations in the undisturbed temperature which affect the temperature‐dependent chemical rate constants involved in the OH nightglow. A specific case showing latitudinal trends is presented, but the result is not representative of all of our results obtained involving latitudinal variations.

Simultaneous intensity, temperature and imaging measurements of short period wave structure in the OH nightglow emission

A low light TV camera co-aligned with a narrow field Michelson interferometer have been used to investigate how variations in the intensity and temperature of the near infrared hydroxyl nightglow emission are related to the passage of short period mesospheric gravity waves. The observations were made from Sacramento Peak, NM (32.8°N, 105.8°W) on the night of 14/15 June 1983, during a spectacular nightglow display that also exhibited wave structure visible to the dark adapted eye. A coherent wave-like pattern of horizontal wavelength 23 ± 1 km moving uniformly with a velocity of 28 ± 2 m s −1 (apparent period 13.7 ± 1.2 min), was measured at several azimuths over an interval of 2.5 h. The induced intensity and temperature perturbations were wave-like in nature and exhibited a high degree of correlation. The intensity variations were large, up to 30%, yet the brightest wave forms were ⩽ 10 K hotter than the adjacent dark forms. The amplitude of the intensity fluctuation was found to be ∼ 8 times larger than the temperature variation. In all cases the intensity wave led the thermal wave, but only by a small amount. High contrast wave patterns of this type are uncommon and are a principal feature of “bright night” displays.

On the origin of ripple-type wave structure in the OH nightglow emission

The origin of small-scale mesospheric wave structure with horizontal wavelengths of only 5–15 km and lifetimes of less than 45 min, termed “ripples”, has been investigated using video observations of the near infra-red OH nightglow emission taken over a period of several years. Previous studies in the literature indicate that ripples are associated with lower lunar transit, lunar high tide and with geomagnetic activity (as measured by K p). Examination of our data indicated a peak in the occurrence of ripples around lower lunar transit. However, the distribution of observing time was also strongly peaked towards lower lunar transit since this occurred at night during New Moon periods, when most of the observations were made. Statistical analysis of these data revealed no significant correlation with lower lunar transit and no evidence was found to support the theory that ripples arise from the breakdown of lunar tides in the upper atmosphere. The comparison of ripple occurrence with K p phase showed a strong correlation, with ripples tending to occur when K p was falling. However, in an analysis using the AE magnetic index, no evidence was found to support the hypothesis that ripples are associated with the breakdown at mid-latitudes of travelling ionospheric disturbances (TID) generated at polar latitudes by auroral processes. It appears more likely that ripples are the result of short-lived velocity shears generated in situ by the chance combination of wind and wave motions.

Observations of wave‐driven fluctuations of OH nightglow emission from Sondre Stromfjord, Greenland

OH nightglow emissions from the (6, 2) band were observed during February 12–14, 1986, at Sondre Stromfjord, Greenland. The data were analyzed using time series analysis techniques to determine , where ΔI and ΔT represent correlated fluctuations from the mean OH intensity (Ī) and temperature ( ) in various frequency bands. For correlated fluctuations due to measurement error, |η| < 1, while for correlated fluctuations caused by atmospheric gravity waves (AGWs) that pass through the OH emission layer the measured |η| should be generally greater than 1.5. While some of the observed OH intensity‐temperature fluctuations were correlated and had an |η| < 1, two correlated intensity‐temperature waves with periods of 4 and 2 hours were seen with an |η| > 1.5. The 4‐hour (2‐hour) period wave had an |η| equal to 3.1 ± 1.1 (1.6 ± 0.5) and a phase angle, between the intensity and temperature components, of 5° ± 23° (40° ± 20°). These results are consistent with a recent theoretical model for the passage of AGWs through an OH emission layer. Agreement is closest for the 4‐hour (2‐hour) period wave when the layer is peaked at 87 km (83 km) and the O scale height is −2.0 (−4.0km).

Observations of gravity wave propagation in the OI (557.7 nm), Na (589.2 nm) and the near infrared OH nightglow emissions

Images of wave-like structure in three nightglow emissions with different height profiles [the OI (557.7 nm) and Na (589.2 nm) lines and the OH bands between 715 and 810 nm] have been obtained with a low light TV system. A bright, well defined, curved wave pattern seen in these emissions on the evening of 14 August 1980 from the Gornergrat Observatory, Switzerland (45.98°N, 7.78°E) has been identified as the signature of a short period gravity wave propagating freely through the upper atmosphere in the height interval 80–100 km from a region over South-West France. The best fit parameters of the wave disturbance which produced the structures in the three airglow layers are wavelength = 6.5 ± 1.5 km, phase velocity = 5 ± 2 ms −1 (period = 21 ± 1 min) and phase propagation angle = 75° ± 3°, and were obtained by assuming a background wind with a horizontal component of 18 ms −1.

Seasonal variations of the correlation among nightglow radiations and emission mechanism of OH nightglow emission

Seasonal variations of correlations are studied, between the intensity changes of the OH(6-2) and (8-3) bands, of the OH(6-2) band and the O 2(0-l) atmospheric band, of the O 2(0-l) band and the NaD lines, of the OH(6-2) band and the NaD lines, of the 5577 A line and the O 2(0-l) band, of the 5577 A line and the NaD lines and of the 5577 A line and the OH(6-2) band, based on the data which are taken nearly simultaneously on the same nights from March 1978 to February 1979. It is found that the strength of intensity correlations between these radiations indicates a clear seasonal variation, except for the intensity correlations between the OH(6-2) and (8-3) bands and between the OH(6-2) band and the O 2(0-l) band. The emission mechanism of OH bands is investigated, based on the frequency distribution of the experimental π value [= ( ΔI I )/( ΔT T ) ] and on the intensity correlation between the OH(6-2) band and the O 2(0-l) band. It is found that the emission mechanism of OH bands favors the ozone mechanism in place of the O 2 ∗ mechanism.

Airglow variations and dynamics in the lower thermosphere and upper mesosphere—III. Variations during stratospheric warming events

The 5577 Å, Na and OH nightglow emissions observed at Haute Provence were examined for possible changes during the stratospheric warming events. It has been found that during the midwinter major warmings the emission intensities exhibit marked increases of the order of 100%, persisting for about a week. In addition, it has been shown by an analysis of the superposed epoch type that the sudden enhancement in the emission intensities occurs almost simultaneously with the splitting of the stratospheric polar vortex. The results may confirm that the influence of the circulation changes associated with the stratospheric warming extends up to at least the lower thermosphere, causing redistributions of photochemical minor constituents by the effect of transport processes and the change of aeronomic properties.

Hydroxyl rotational temperatures and intensities in the nightglow

Spectra of several bands of the OH nightglow emission are obtained at Tromsø and Ås during the autumn 1963 and spring 1964, respectively. Rotational temperatures have been derived, giving an average of 210 ± 10°K at both stations. No seasonal dependence or latitude effect is found. OH temperatures obtained by earlier investigators are discussed and found to agree mutually and with those reported here, assuming the OH emission height near 100 km and the atmospheric temperature at this height to vary with solar activity. Absolute band intensities are computed, giving mean values in agreement with earlier investigations. At both stations the observations seem to indicate a seasonal variation of the intensity, showing a minimum during winter. Any latitude effect should be small, possibly giving less intense OH bands north of 60°N.