R. L. Walterscheid

Small‐scale gravity waves in the upper mesosphere and lower thermosphere generated by deep tropical convection

A time-dependent, nonlinear, fully compressible, axisymmetric, ƒ-plane, numerical model is used to simulate the generation of small-scale gravity waves in the upper mesosphere and lower thermosphere by intense deep convection in the troposphere. The simulations show that major convective storms in the tropics excite a broad spectrum of upper mesosphere-lower thermosphere gravity waves above the storm centers. The wave field includes a component that is guided in a thermal duct in the lower thermosphere and propagates horizontally outward from above the storm. Storms excite oscillations over the source which are initially confined to a stratospheric duct but leak into the thermospheric duct over time generating a long train of small-scale-ducted waves. This ringing phenomenon persists for several hours after the storm has ended. The ducted disturbances may propagate large distances from the source and explain observations of a strong summertime anisotropy favoring southward propagation of small-scale waves observed in the airglow over Adelaide more than 2000 km to the south of the storm events.

Large-amplitude semidiurnal temperature variations in the polar mesopause: evidence of a pseudotide

Current atmospheric tidal models1,2 predict an amplitude of ≲2 K for the solar semidiurnal temperature variations in the polar mesopause (∼85 km altitude) during the winter solstice period. In contrast, here we report an amplitude of about 13 K for the 12-h variations in the mesopause temperature at Longyearbyen, Spitsbergen (78.2° N) on UT day 359 in 1984. The kinetic temperature of the mesopause was deduced from the thermalized rotational distribution of the mesopause airglow OH(8,3) vibrational–rotational band emission in the near-infrared spectral region. A semidiurnal ‘pseudotide’ is proposed as the source of the observed large-amplitude 12-h temperature variation. This pseudotide is generated by gravity-wave momentum fluxes modulated by the true semidiurnal tide3. The semidiurnal pseudotide is very efficiently excited at very high latitudes. Based on conservative values of gravity-wave amplitudes, we estimate a temperature amplitude of ∼10 K.

First measurements of the two‐dimensional horizontal wave number spectrum from CCD images of the nightglow

A narrow wavelength band CCD camera, built at Aerospace, has been used to obtain images of the OH Meinel (6,2) band and the O2 atmospheric (0,1) band nightglow. The field of view of the camera is approximately 100 by 100 km at an altitude of 90 km, the nominal height of the nightglow. It is shown how Fourier techniques can be applied to these data to optimally smooth the images, to identify the presence of monochromatic waves, and to obtain both the one-dimensional and, for the first time, the two-dimensional horizontal wave number spectrum of gravity waves passing through the emission layers. Both measures of the spectrum depend, to a certain extent, on a technique which makes use of Krassovskys η ratio. These techniques are applied to sample data taken from May 9, 1989, during the Arecibo Initiative in Dynamics of the Atmosphere (AIDA) campaign in Puerto Rico, and from the recent Collaborative Observations Regarding the Nightglow (CORN) campaign in Illinois. While future papers will describe these data in more detail, a brief comparison is made with recent models of the two-dimensional horizontal wave number spectra presented by Gardner et al. (1993) and Gardner (1994).

Acoustic wave heating of the thermosphere

A numerical model is used to study the dissipation in the thermosphere of upward propagating acoustic waves. Whereas dissipating gravity waves can cool the upper atmosphere through the effects of sensible heat flux divergence, it is found that acoustic waves mainly heat the thermosphere by viscous dissipation. Though the amplitudes of acoustic waves in the atmosphere are poorly constrained, the calculations suggest that dissipating acoustic waves can locally heat the thermosphere at rates of tens of kelvins per day and thereby contribute to the thermospheric energy balance. It is shown that viscous heating cannot be calculated from the divergence of the wave mechanical energy flux. Acoustic waves that are barely detectable at mesopause heights can become significant heaters of the atmosphere high in the thermosphere. We suggest that acoustic waves might be responsible for heating the equatorial F region to produce the hot spot observed in the O I 630 nm airglow over the Andes Mountains.

Observations of the OH Meinel (6,2) and O2 atmospheric (0,1) Nightglow emissions from Maui during the ALOHA-90 Campaign

Observations are reported from a CCD nightglow camera deployed on Maui during ALOHA-90. These data show that there were periods of both strong and weak gravity wave activity, that background winds in the direction of wave propogation can be derived using airglow data alone, and that there may be a connection between sporadic Na and large-scale wave activity. These data are in fair to good agreement with models of wave induced airglow fluctuations.

Dynamical-chemical model of fluctuations in the OH airglow driven by migrating tides, stationary tides, and planetary waves

The theory of tidally driven fluctuations in the OH airglow, generalized to account for emission from an extended layer, is applied to airglow fluctuations due to both migrating and zonally symmetric tides and free traveling planetary (Rossby) waves. Krassovsky`s ratio {eta}{sub E} (the ratio of normalized airglow fluctuation intensity to normalized temperature fluctuation) is evaluated for a suite of tidal modes and for the traveling planetary waves. Values of {eta}{sub E} are distinct enough to allow for identification of a variety of tidal modes and planetary waves in airglow observations. The theory predicts that airglow observations at very high latitude should contain prominent signatures of zonally symmetric tidal modes with periods of 6,8, and 12 hours. 32 refs., 4 figs., 3 tabs.

Formation characteristics of sporadic Na layers observed simultaneously by lidar and airglow instruments during ALOHA-90

Sporadic Na (Nas) layers were observed by the airborne lidar during ALOHA-90 on the 22, 25 and 27 March flight missions. Perturbations in the O2 and OH nighttime airglow emission intensities and temperatures were also observed by instruments on the aircraft and at Haleakala Crater (20.8°N, 156.2°W) during these events. The most striking correlation between the airglow and lidar measurements occurred during the northbound flight leg of the 25 March mission. When the Nas layer formed at 90.7 km, while the Electra aircraft was between 750 and 500 km south of Haleakala, the O2 temperatures near 95 km above the Electra and Haleakala increased by approximately 45 K. The data for this night suggest a connection between Nas and a large-scale wave, and suggest that the wave is tidal in nature. The data also suggest that some Nas layers can form very quickly over large geographic areas. Fast chemical processes are required to generate the large amounts of atomic Na involved in some of these events.

Depletion of oxygen in aurora : Evidence for a local mechanism

A ground-based and rocket investigation of the response of the neutral atmosphere to E region auroral heating has been carried out at Poker Flat, Alaska. The temporal evolution of the atomic oxygen to molecular nitrogen ratio (O/N2) in the lower thermosphere has been monitored using the optical emissions from the aurora as a diagnostic. Comparisons between the changes in the O/N2 ratio and the auroral Joule and particle heating have shown several examples of close similarity between the durations of the heating events and the depletions. Using the thermospheric winds measured during the rocket flights and the temporal structure of the depletions, the upper limit on the horizontal scale size of the depletions has been estimated at 200–400 km. Moreover, in situ rocket measurements of atomic oxygen showed significant differences at points separated horizontally by approximately 220 km. It is also concluded from the near coincidence between the depletion events and the Joule heating events that the dynamical mechanism(s) that drive the depletions were not far distant from the observing site, that is, local processes are sometimes dominant f during periods of moderate auroral activity. We suggest that the observation of a strong wind shear in the 100- to 120-km altitude region [Larsen et al., 1997] could be responsible for turbulence that contributes to the changes in minor constituent composition.

Comparison of theories for gravity wave induced fluctuations in airglow emissions

A comparison is undertaken of theories for the gravity wave induced fluctuations in the intensity of airglow emissions and the associated temperature of the source region. The comparison is made in terms of Krassovskys ratio ηE for a vertically extended emission region (ηE is the ratio of the vertically integrated normalized intensity perturbation to the vertically integrated normalized intensity-weighted temperature perturbation). It is shown that the formulas for ηE in the works by Tarasick and Hines (1990) and Schubert et al. (1991) are in agreement for the case of an inviscid atmosphere. The calculation of ηE using the theory of Tarasick and Hines (1990) requires determination of their function χ; we show that χ is simply related to the “single-level” Krassovskys ratio η of Schubert et al. (1991). The general relationship between χ and η is applied to a simple chemical-dynamical model of the O2 atmospheric airglow and the altitude dependence of these quantities is evaluated for nonsteady state chemistry. Though the Tarasick and Hines (1990) formula for ηE does not explicitly depend on the scale heights of the minor constituents involved in airglow chemistry, ηE implicitly depends upon these scale heights through its dependences on chemical production and loss contained in χ. We demonstrate this dependence of ηE for the OH nightglow on atomic oxygen scale height by direct numerical evaluation of ηE; in this case the dependence originates in the chemical production of perturbed ozone.

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 (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 , and these are more noticeable in the magnitude of than in its phase. These seasonal variations in 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.