William E. Ward

A simple model of diurnal variations in the mesospheric oxygen nightglow

Local time variations in the OH, and O2(b¹Σ g+) and O(¹S) nightglow have recently been reported in observations by the Wind Imaging Interferometer (WINDII) and the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) and attributed to dynamical effects associated with the migrating diurnal tide. A simple model involving vertical advection, a background atmosphere from MSIS, and a latitudinal variation in amplitude given by the (1,1) Hough mode is developed and found to duplicate many of the features of the satellite observations. The model demonstrates that vertical advection is the prime process causing these airglow variations. The observed variability in the relative phases and intensities between these nightglow signatures are most likely due to variations in the phase, amplitude and/or vertical wavelength of the tide, and/or the background oxygen mixing ratio. Given the ubiquitous nature of the tidal influence in the mesopause region any attempt to determine background atmospheric parameters from nightglow signatures must take the dynamical effects of the diurnal tide into account.

Mesospheric temperature inversions with overlying nearly adiabatic lapse rate: An Indication of a well‐mixed turbulent layer

The Rayleigh lidar technique was applied to study the thermal structure of the middle atmosphere. Observations were carried out on a routine basis for one year (130 clear nights) at the main campus of York University near Toronto (44°N,80°W). Mesospheric temperature inversions were generally found to occur below a height of 70 km during winter and above during summer. The most interesting aspect of our observations was that the inversions were often associated with an overlying nearly adiabatic lapse rate which extended for several kilometres. We interpret this as being an indication (or signature) of a well-mixed turbulent layer. A one-dimensional numerical model was applied to demonstrate that a well-defined turbulent layer within the mesosphere can bring about a thermal structure quite similar to that which was commonly observed—an inversion with overlying nearly adiabatic lapse.

The Extended Canadian Middle Atmosphere Model

First results from the extended Canadian middle atmosphere model are presented. The model extends from the surface to the middle thermosphere and includes relevant physical parameterizations. To simulate unresolved gravity waves a non-orographic gravity wave drag parameterization has also been implemented. Results from a two-year simulation are discussed and are shown to be in good qualitative agreement with upper atmosphere research satellite wind observations. The realistic features which are reproduced include the reversal of the mesospheric jets and the semi-annual variation of the migrating diurnal tide in the low latitude mesosphere.

Two day wave induced variations in the oxygen green line volume emission rate: WINDII observations

Variations in the oxygen green line emission rate (at 5577 A) which correspond to a two day wave signature were observed from January 20 to January 30, 1993 by the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS). The latitude and height extent of these emission variations are summarized. From the volume emission rate data, fields of oxygen mixing ratio using Mass Spectrometer and Incoherent Scatter (MSIS) model data as the background atmosphere were calculated. Analyses of the mixing ratio fields show the variations of emission rate to be primarily the result (to first order) of vertical motions associated with the quasi two day wave (QTDW), not mixing events associated with wave breaking. The mean peak-to-peak amplitude of the vertical displacement is of the order of 1 km although at times this amplitude is as much as 4 km. The phase of the mixing ratio variations approaches the two day wave zonal wind phase above 100 km as is expected from simple considerations of the phase relationships associated with planetary waves and the lifetime of atomic oxygen.

Tidal mechanisms of dynamical influence on oxygen recombination airglow in the mesosphere and lower thermosphere

Abstract The specific mechanism causing local time variations in the atomic oxygen recombination airglow emissions is investigated by examining the Lagrangian histories of air parcels in the wind fields of the diurnal tide generated by the Global Scale Wind Model of Hagan et al. (1995). The emission rate variations are found to be due to the vertical displacements of air parcels associated with the diurnal tide. The density and temperature induced airglow perturbations associated with the tide are out of phase with that associated with the vertical displacement and hence act to supress the intensity variations. The modelled emission rate variations with local time correspond closely to those deduced from recent observations in the mesosphere and lower thermosphere by the Wind Imaging Interferometer and the High Resolution Doppler Imager on the Upper Atmosphere Research Satellite. The observed latitudinal variations in the local time behaviour of these airglow emissions are due to latitudinal variations in the vertical velocity associated with the tide.

Longitudinal variations of green line emission rates observed by WINDII at altitudes 90–120 km during 1991–1996

Abstract Longitudinal variations in green line airglow are investigated. The volume emission rate data used for the analysis are taken by Wind-Imaging Interferometer (WINDII) at latitudes 70°S–70°N and altitudes 90– 120 km in December/January and March/April of 1991–1996. Relatively stable or stationary patterns, which persist from year to year, are found to exist at fixed local times during solstice and equinox. In general, the nightglow exhibits zonal wavenumber one and two variations in the southern and northern latitudes around 35°, respectively. The dayglow displays a wavenumber two variation near the equator, with a tendency towards 10°S–20°S for the solstitial period. All structures are seen to extend over 10° or more of latitude. The emission rates vary significantly, changing by a factor 2 or more at the minima and maxima of the wave structures. Primary maxima are around ∼180°E–240°E for the nightglow, and ∼300°E–360°E for the dayglow, while secondary maxima are near ∼90°–120°E for both airglows. Their magnitudes also show systematic decrease by a factor of 3 from year 1991 to 1996, perhaps due to decreases in solar activity and tidal amplitude. The existence of a stationary planetary wave structure suggests the importance of coupling processes between the mesopause and the earth (or lower atmosphere) in understanding the airglow variations, in addition to the well-known solar-driven photochemistry.

Multiple reflections in a wide-angle Michelson interferometer

The effect of multiple reflections from a semireflecting surface such as an interference filter located in front of or behind a wide-angle Michelson interferometer (WAMI) is examined. By considering the instrument as a complex operator on the incident electric field, theoretical results are obtained which describe a large variety of configurations. Experimental results are presented which are consistent with these results. It is concluded that since the presence of these reflections changes the form of the observed fringes and affects measurements of the phase and visibility of the fringes, care must be taken to avoid such reflections in designing WAMIs.

Correlations between the mesospheric O(1S) emission peak intensity and height, and temperature at 98 km using WINDII data

Data from O(1S) emissions measured by the WIND Imaging Interferometer (WINDII) on the UARS spacecraft for two days in the January/February 1992 time period are used to analyse the behaviour of the oxygen green-line peak in the mesosphere. Correlations between the peak emission and height are observed for the night-time measurements but not for the day-time measurements. Extremes in the peak emission rate and height are compared to the temperature field at 98 km. It is found that when the emission rate is high, the height of the peak is low and the temperature is high and vice versa. This is suggestive of what might occur if the motions associated with these extrema were quasi-adiabiatic and the oxygen mixing ratio conserved.

On the role of atomic oxygen in the dynamics and energy budget of the mesosphere and lower thermosphere

Atomic oxygen affects both the heating and cooling of the mesosphere and lower thermosphere through its recombination and collisional deactivation of CO2. Recent increases in the accepted value of the deactivation constant, ko in CO2-O collisions enhance the role of the latter process in the dynamics and energy budget of this region. There are strong increases in the radiative cooling due to CO2 in the 90–120 km region. The sensitivity of the cooling rate in the 15 μm band to variations in atomic oxygen concentration, [O], is enhanced so that the atmosphere in this region is unstable to these variations in concentration. Convection appears possible thereby providing a vertical mixing mechanism which is ineffective in the diffusion of heat but is effective in mixing constituents, and some justification for the use of different eddy diffusivities for heat and constituents. Heating rate calculations are intrinsically more complicated because both the heating and the cooling processes involving atomic oxygen are nonlinear functions of its concentration. The damping of atmospheric waves will be affected because radiative damping will be stronger up to 100 km and the cooling rate dependence on the atomic oxygen mixing ratio affords a new mechanism for photochemical damping. It appears that the overall dynamics and energy budget of this region must be reconsidered.

The O(1D) dayglow emission as observed by the WIND Imaging Interferometer on UARS

Volume emission profiles of the O(1D) dayglow measured by the WIND Imaging Interferometer on the Upper Atmosphere Research Satellite are analyzed to examine the O(1D) excitation mechanisms in the sunlit thermosphere. The observed emission profiles are compared with theoretical profiles calculated using a model which takes into account all of the known daytime sources of O(1D). We find that a yield of 0.4 for O(1D) production from the reaction of N(2D) with O2 is required to explain the measured profiles. This yield is about a factor of 2 smaller than the value used in earlier studies of the O(1D) dayglow. Near the peak of the emission profile the contributions from dissociative recombination of O2+ and the reaction of N(2D) with O2 are comparable to each other and their relative variation shows a strong latitudinal and solar zenith angle dependence.