Hydroxyl Nightglow Research Articles

The nocturnal behavior of the hydroxyl layer as observed by ISAMS/UARS

Abstract The nocturnal variations of the mesospheric nightglow from the hydroxyl (OH) have been characterised using measurements taken by the ISAMS instrument on the UARS satellite. In this paper, the evolution with local time of the vertically integrated emission of the OH layer is presented. The measurements used in this study have close to full latitudinal coverage for both solstice and equinox conditions, and constitute an important database for future studies of the hydroxyl nightglow behavior using dynamical models.

The nocturnal behavior of the hydroxyl airglow at the equatorial and low latitudes as observed by WINDII: Comparison with ground‐based measurements

Measurements of the hydroxyl nightglow from the Wind Imaging Interferometer (WINDII) satellite‐borne experiment and from ground‐based photometers are compared and combined in order to characterize the hydroxyl airglow nocturnal behavior at 23°S and the equatorial region. Ground‐based observations at Cachoeira Paulista (23°S, 45°W) and at Fortaleza (4°S, 38°W) from 1987 to 1994 and WINDII data from March 1992 to January 1994 are selected, and two periods are studied: the March/April equinox and the December/January solstice. The WINDII results for the equinox/solstice difference and for the nocturnal behavior of the vertically integrated emission rate of the hydroxyl nightglow are found to be in very good agreement with the ground‐based observations. The WINDII measurements revealed two general characteristics of the hydroxyl nightglow altitude variation at the latitudes above. The hydroxyl nocturnal variations are predominantly occurring below 90 km, and the vertically integrated emission rate and the peak height of the volume emission rate show an inverse relationship with seasonal, latitudinal, and inter annual effects. The interpretation of these results in terms of atmospheric tides is then discussed.

Longitudinal variations in the hydroxyl nightglow

Experimental investigations by ground-based and rocket methods during the last four decades have allowed the inference of many kinds of variations of the hydroxyl emission. However, the previous data base is insufficient to derive longitudinal variations of the emission characteristics. The WINDII instrument on board the UARS satellite provides global observations of the OH(8-3) band. In this paper we present such observations at mid-latitudes for 1993 to characterize the longitudinal variations of the zenith brightness and peak altitude of the hydroxyl emission.

The detection of the hydroxyl nightglow layer in the mesosphere by ISAMS/UARS

Measurements of night‐time radiance near 4.6 µm by the ISAMS instrument on board the UARS satellite exhibit an enhancement in the upper mesosphere. We show in this work that this increase around 85 km tangent height is due to the OH(9–8) and OH(8–7) Meinel bands. The ISAMS database therefore contains valuable information on the variability over a seasonal cycle of the OH high vibrational populations from which the chemical heating of the upper mesosphere can be derived.

The intertropical convergence zone as a source of short-period mesospheric gravity waves near the equator

Abstract On 26 October 1993, as part of the ALOHA campaign, observations of the mesospheric hydroxyl nightglow rotational temperature and band radiance were made from the NCAR Electra aircraft during a trans-equatorial flight. The power spectra of both the temperature and radiance data show significant periodic components with effective wavelengths near 41, 65 and ~250 km. The dynamic spectra produced from these data show that the increasing wavelengths attain their peak power progressively farther southward from the equator. Drawing upon previous results and theoretical predictions, it is put forward that this behaviour is suggestive of, and consistent with, the large-scale convective instability associated with the intertropical convergence zone (ITCZ), as a likely candidate for the source of these short-period mesospheric waves. If this is the case, then the ITCZ, as a source of short-period gravity waves transporting energy and momentum, could play a significant role in the equatorial circulation of the upper atmosphere.

Seasonal dependence of mesospheric gravity waves ( <100 Km) at Peach Mountain Observatory, Michigan

We present results from a 14‐month study of all‐sky camera observations of the Hydroxyl (OH) nightglow made at the Peach Mountain Observatory, Michigan (42.3°N; 83.7°W) Spatial variations in the observed OH airglow images have been used to assess gravity‐wave (GW) occurrence frequency at ∼85 km altitude as a function of season. A Strong seasonal dependence of mesospheric GW activity is observed, with peak activity in the summer months and much reduced activity during the winter months. Gravity waves (as defined by observed coherent variations in relative OH brightnesses of >∼7.5) were found to be present on about 70% of the clear‐sky nights during the summer months. During the spring, fall, and winter months, however, the observed GW occurrence frequency was very low (<10%). Most of the GWs were observed to propagate towards the eastward hemisphere. We suggest that the tropospherically‐generated GWs are anisotropic (eastward) thus passing through to the mesosphere only in the summer and being filtered out by the intervening neutral winds during other seasons. It is also possible that the GWs are able to reach higher altitudes without breaking because of their smaller amplitudes at lower altitudes during the summer season relative to the winter season.

Atmospheric transmission coefficients for hydroxyl rotational lines used in rotational temperature determinations

In an effort to retrieve accurate rotational temperatures from hydroxyl (OH) nightglow spectra, a computer model of atmospheric absorption has been combined with a Doppler model of the Λ-doubled OH rotational lines in order to derive transmission coefficients for the individual lines. Atmospheric transmission coefficients have been tabulated for the first three to four major P, Q and R-branch rotational lines of the OH Meinel (4,2), (3,1), (7,4), (6,2) and (8,3) bands. These coefficients have been calculated for OH Doppler temperatures over the typical mesospheric range from 150 to 250 K and for mid- and high-latitude, winter and summer conditions. Finally, the resulting atmospheric transmission coefficients have been applied to a low-noise, near-infrared data set obtained during the high-latitude summer under quiet geophysical conditions. It is shown that these coefficients correct the selective absorption of individual rotational lines in different OH vibrational bands, and thus, resolve the differences in the rotational temperatures inferred from the different vibrational levels of the uncorrected spectra.

Hydroxyl temperature and intensity measurements during noctilucent cloud displays

Abstract. Two Fourier transform spectrometers have been used to investigate the properties of the near-infrared hydroxyl (OH) nightglow emission under high-latitude summertime conditions and any association with noctilucent clouds (NLCs). The measurements were made from Poker Flat Research Range, Alaska (65.1°N, 147.5°W), during August 1986. Simultaneous photographic observations of the northern twilight sky were made from Gulkana, Alaska (62.2°N, 145.5°W), approximately 340 km to the south to establish the presence of NLCs over the spectrometer site. Data exhibiting significant short-term variations in the relative intensity (as much as 50–100%) and rotational temperature (typically 5–15 K) were recorded on six occasions when NLCs were observed. Joint measurements were also obtained on several "cloud-free" nights. No obvious relationship was found linking the mean OH intensity or its variation with the occurrence of NLCs. However, a clear tendency was found for the mean OH temperature to be lower on NLC nights than on cloud-free nights. In particular, a significant fraction of the OH(3–1) band spectra recorded by each instrument (16–57%) exhibited temperatures below ~154 K on NLC nights compared with <3% on cloud-free nights. This result is qualitatively consistent with current models for ice particle nucleation and growth, but the mean OH temperature on NLC nights (~156 K) was significantly higher than would be expected for long-term particle growth in this region. These observations raise questions concerning the expected proximity of the high-latitude, summertime OH layer and the NLC growth region.

Hydroxyl temperature and intensity measurements during noctilucent cloud displays

Two Fourier transform spectrometers have been used to investigate the properties of the near-infrared hydroxyl (OH) nightglow emission under high-latitude summertime conditions and any association with noctilucent clouds (NLCs). The measurements were made from Poker Flat Research Range, Alaska (65.1°N, 147.5°W), during August 1986. Simultaneous photographic observations of the northern twilight sky were made from Gulkana, Alaska (62.2°N, 145.5°W), approximately 340 km to the south to establish the presence of NLCs over the spectrometer site. Data exhibiting significant short-term variations in the relative intensity (as much as 50–100%) and rotational temperature (typically 5–15 K) were recorded on six occasions when NLCs were observed. Joint measurements were also obtained on several "cloud-free" nights. No obvious relationship was found linking the mean OH intensity or its variation with the occurrence of NLCs. However, a clear tendency was found for the mean OH temperature to be lower on NLC nights than on cloud-free nights. In particular, a significant fraction of the OH(3–1) band spectra recorded by each instrument (16–57%) exhibited temperatures below ~154 K on NLC nights compared with <3% on cloud-free nights. This result is qualitatively consistent with current models for ice particle nucleation and growth, but the mean OH temperature on NLC nights (~156 K) was significantly higher than would be expected for long-term particle growth in this region. These observations raise questions concerning the expected proximity of the high-latitude, summertime OH layer and the NLC growth region.

Evidence of preferential directions for gravity wave propagation due to wind filtering in the middle atmosphere

All‐sky TV images of wave structure in the near‐infrared hydroxyl (OH) nightglow emission were recorded over a 3‐month period during May, June, and July 1988 from a high‐altitude site at the Mountain Research Station (40.0° N, 105.6° W, 3050 m), near Nederland, Colorado. Well‐defined, coherent wave patterns associated with the passage of short period (&lt;1 hour) gravity waves were observed on a total of 22 occasions. The wave motions exhibited similar spatial and temporal properties during each month but a distinct tendency for northward propagation (68% of the wave azimuths within ± 40.0° N), with some eastward motion in May and June, was observed throughout the campaign. Although it is theoretically well known that upward propagating gravity waves can be blocked at a critical layer produced by the interaction of the waves with the horizontal background wind, observational evidence of this phenomenon is rare. To investigate the possibility that the asymmetry in the wave propagation directions was caused by the critical layer, a model based on mean climatological background winds and numerical tidal wave modes valid for any mid‐latitude site and time of the year was constructed to show the regions forbidden to upward gravity wave propagation from critical layer theory. These “blocking diagrams” which vary with height and time were constructed for the OH altitude (∼87 km) for the present paper. Comparison of the predicted (i.e., least restricted) and the observed directions of the wave motion show almost complete agreement. This suggests that middle atmospheric winds can play an important role in determining the flux and the azimuthal distribution of short‐period waves reaching the upper atmosphere.

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.

Temporal variations in the hydroxyl nightglow observed during ALOHA‐90

Ground‐based observations of the hydroxyl nightglow were made with a Fourier transform spectrometer from Haleakala, Maui during the ALOHA‐90 campaign. Vibrational level populations and rotational temperatures were obtained with a temporal resolution of 1 minute. Temperature precision was ±1 K. Highly correlated in‐phase variations in temperature and population were seen on all nights. However, a long term trend in the vibrational population was not reflected in the temperature.

Observations of short period mesospheric wave patterns: In situ or tropospheric wave generation?

Near infrared images showing wave structure in the hydroxyl (OH) nightglow emission have been obtained from Maui, Hawaii during the ALOHA‐90 campaign. Analysis of two nights during this campaign (25 and 31 March) indicate extensive, highly coherent, linear wave patterns of very short apparent period (∼5 and 10 min respectively). Both displays exhibited several features characteristic of the in situ breakdown of a large scale, long period, upper atmospheric wave disturbance. Data in support of this mechanism was found by other ALOHA instruments which detected concurrent long period (1–2 hour) mesospheric wave disturbances on both occasions. However, a tropospheric source for these waves cannot be ruled out. At least on 25 March a weather front occurred at ∼1400 km range with a favourable orientation and location. Although its range was relatively large, background winds may have substantially increased the path length of the waves through the intervening atmosphere.

Near infrared imaging of hydroxyl wave structure over an ocean site at low latitudes

Coordinated observations of wave structure in the near infrared hydroxyl (OH) nightglow emission have been made from Maui, Hawaii using a suite of narrow angle and all‐sky TV cameras. Two sets of data were obtained, the first in conjunction with the ALOHA‐90 campaign and the second during the subsequent new moon period. Well formed, short period (&lt;20 min) wave patterns of comparable morphology, dynamics and abundance to those regularly imaged from mid‐latitude mountain sites were detected on several occasions. Although the Hawaiian islands comprise several high volcanic peaks, the patterns were not consistent with gravity waves generated by the interaction of strong winds with the local island topography. This suggests that other mid‐latitude wave patterns may also not be of mountain origin. The wave patterns imaged during ALOHA‐90 were of significantly lower contrast than those detected later. This effect may be related to changes in the characteristics of the middle atmosphere that occur shortly after the spring equinox.

Role of ozone in the sodium and hydroxyl nightglow

One-dimensional photochemical diffusion model which includes oxygen-hydrogen-sodium atmosphere has been used to examine the relation between sodium and hydroxyl nightglow and the role of ozone in it. It is found that both emissions can be obtained on the basis of photochemistry. The following reactions Na + O3→ NaO + O2 and H + O3→ OH +O2 play key role in sodium and hydroxyl emission respectively. Further it is found that variations in both emissions are controlled by the variation in the concentration of ozone.

Wave‐driven fluctuations in OH nightglow from an extended source region

Hydroxyl nightglow emissions arise from a region of the atmosphere several to 10 km thick centered at about the mesopause. The intensity of the nightglow 〈I〉 is therefore a superposition of contributions from individual thin emitting layers. The inferred rotational temperature 〈TI〉 is an intensity weighted average of the temperatures throughout the emitting region provided that the overall temperature variation of the region is small compared with the average temperature, and that the Boltzmann exponent in the rotational line intensity distribution is not much larger than unity. Acoustic‐gravity waves propagating through the emitting region produce fluctuations in the OH nightglow; intensity and temperature oscillations, 〈δI〉 and 〈δT〉, are conveniently related by the ratio , where the overbar refers to time‐averaged quantities. When the vertical wavelength λυ of a disturbance is large compared to the vertical thickness of the emitting region, 〈η〉 is accurately given by for a thin layer at the peak emission height. However, when λυ is comparable to or smaller than the emission region thickness, 〈η〉 values can vary widely from the peak emission height ratio η. When λυ is small, the magnitude of the normalized fluctuation in net intensity |〈δI〉/〈Ī〉| is considerably reduced from the large λυ value due to cancellation effects of out‐of‐phase layers in the emitting region. Therefore, there is an observational bias toward detection of nightglow fluctuations from waves with λυ greater than emission region thickness. Values of 〈η〉 versus period are reported for several horizontal wavelengths based on a dynamical‐chemical model that accounts for realistic OH chemistry and the dynamics of acoustic‐gravity waves propagating vertically through a height‐dependent basic state atmosphere.

Altitude Distribution of Hydroxyl Bands of the Δν = 2 Sequence in the Nightglow

Rocket borne measurements of the altitude distribution of the hydroxyl nightglow emission in the Δν = 2 sequence have been made. Bands originating from the upper vibrational levels (5, 6, and 7) were measured with a band-pass filter in the 1.8 μ. spectral region and bands from the lower levels (3, 4, and 5) were measured in the 1.6 μ. region. The observed emission was distributed in a layer 10 km thick with a maximum at 87 km. In the altitude region below 90 km, the distribution of O2 (1Δ) measured on the same flight was similar to the hydroxyl profiles. These observed distributions extend over a narrower range of altitude than most previous observations, but agree well with the distributions predicted by model atmosphere calculations.

THE OH NIGHTGLOW EMISSION

Abstract The several determinations of the relative vibrational transition probabilities for the OH vibration-rotation bands are examined. The discrepancies in the results may be attributed to their strong dependence on the observed intensities. More recent intensity measurements of the airglow, which are better suited for such a determination, indicate that terms of order higher than the first, in the usual dipole moment expansion, can he neglected. To represent the laboratory measurements, however, the higher order terms are necessary. The principal characteristics of the hydroxyl nightglow emission are reviewed. The emission rate shows a summer minimum and a winter maximum. Most of the spectroscopically determined gas temperatures exhibit a similar trend except that the amplitude of their seasonal variations shows a more distinct increase with the latitude. A comparison of the spectroscopic temperatures with the temperature profiles and emission altitudes obtained with rockets indicates that either the...