R. H. Wiens

Springtime transition in lower thermospheric atomic oxygen

Observations from three optical ground stations and the wind imaging interferometer on the upper atmosphere research satellite have been combined to describe a “springtime transition” in atomic oxygen. At each station the transition is characterized by a rapid 2-day rise in the night-time oxygen airglow emission rate by a factor of between 2 and 3, with a subsequent decrease by a factor of about 10 in the same period of time. This latter state of extremely weak oxygen airglow indicates a depletion of atomic oxygen that persists for many days. The characteristic signature is similar at both mid-latitude and high-latitude stations and is also observed in the hydroxyl airglow, except that immediately following the enhancement, the hydroxyl emission rate does not fall below the value it had before the event. Airglow rotational temperatures behave coherently with the emission rate. WINDII data show that the airglow emission rate perturbation is a planetary scale feature associated with strong vertical motions and that the event may be associated with the winter-to-summer zonal wind reversal. Data from the northern springtimes of 1992 and 1993 are reported upon in detail, with additional data from 1995 to confirm the persistence of the phenomenon.

Photochemistry and dynamics in thermospheric intertropical arcs measured by the WIND Imaging Interferometer on board UARS: A comparison with TIE-GCM simulations

Abstract The intertropical arcs are bands of enhanced nightglow O(1D) 630 nm emission observed at 15° north or south of the magnetic dip equator. Ground-based measurements and measurements from early satellites established their main properties, including a seasonal effect that locates the more prominent of the arc structures on the winter side of the geomagnetic equator. Theoretical considerations show that asymmetry of the arc system is caused by transequatorial neutral winds pushing ionization along the magnetic field lines upward and downward into the recipient hemisphere, where recombination and therefore nightglow emission is then enhanced. Seasonal variations of arc intensity and location are a direct consequence of the seasonal variation of the projection of the meridional winds along the magnetic field lines on both large and small scales. The WIND Imaging Interferometer on UARS offers the first opportunity to measure the wind and the nightglow enhancement simultaneously across the magnetic equator, and its measurements confirm these earlier understandings, in particular the longitudinal fine structures shown by the Orbital Geophysical Observatory 4 and 6. Furthermore, such observations are also consistent with simulation of emission rate and wind generated by the thermosphere–ionosphere electrodynamics general circulation model. This verification lends support to equatorial spread-F theories that rely upon transequatorial winds for the suppression of the Rayleigh–Taylor instability.

Tidal influence on midlatitude airglow: Comparison of satellite and ground‐based observations with TIME‐GCM predictions

WINDII, the Wind imaging interferometer on the Upper Atmosphere Research Satellite measures winds and emission rates from selected excited metastable species. Measurements of the 558-nm emission from atomic oxygen provide both the wind from the Doppler shift, and the atomic oxygen concentration from the emission rate. Thus the tides and their influence on atomic oxygen are measured with the same instrument. Ground-based airglow measurements provide vertical integrals of the same quantities and coordinated observations were obtained between WINDII and ground-based instruments at Bear Lake (42.5°N, 212°E) for O(1S) 558 nm winds and O2(b1Σ) (0,1) band emission rate and temperature. The TIME-GCM model has recently incorporated airglow photochemistry, so that direct comparisons may be made with airglow observations, without inverting those observations to atomic oxygen distributions. In this study, the influence of tides on airglow emission at midlatitude is studied through the comparison of the above data sets with the TIME-GCM model, extending earlier studies conducted for the equatorial region. At the vernal equinox the upward propagating diurnal tide is found to be the dominant influence on airglow diurnal variation. At solstice the diurnal tide does not penetrate to as high an altitude, so that the dominant influence is then the in situ semidiurnal tide. This conclusion is consistent with both WINDII observations and TIME-GCM predictions, whose data sets agree extremely well with one another. The ground-based results agree well in the local time variation pattern, but the amplitudes observed are larger than for WINDII or the TIME-GCM by roughly a factor of 2. This difference illustrates very clearly the differences between a tidal pattern observed at a single site for a few nights, and a global pattern that is first zonally averaged, and then combined in local time over about 1 month, as is done with the WINDII data. The agreement of these averaged data with the TIME-GCM model strongly suggests that they accurately represent the behavior of the zonally averaged atmosphere.

Observation of polar mesospheric clouds in summer, 1993 by the WINDII Instrument on UARS

Satellite images of polar mesospheric clouds (PMCs) were taken in support of the 1993 ANLC campaign. In July, 1993, they were common at latitudes north of 65°N but increased in abundance poleward to the limit of our viewing horizon of 72°N. The southern boundary edge of the PMCs was observed to move northwards as the season progressed in 1993. Compared with the climatology of other years, the ANLC campaign observations were taken towards the end of a season in a year of modest PMC activity. WINDII, the wind imaging interferometer on the UARS satellite, is a new effective tool in the study of polar mesospheric clouds. Images of the limb give vertical distributions of the light scattered by the molecular and the particle atmosphere. Detection of the PMCs against the Rayleigh background showed that the peak altitude was around 83 km in 1993.

Gravity waves from O2 nightglow during the AIDA '89 campaign II: numerical modeling of the emission rate/temperature ratio, η

Abstract The dependence of η, the ratio of emission rate fluctuation to temperature fluctuation, on the gravity wave parameters and chemical parameters is developed using a model for the O2 airglows response to a gravity wave. The model is in the Eulerian frame of reference using the traditional linearization and perturbation method. Two photochemical mechanisms, the three-body recombination mechanism and the two-step transfer mechanism and two kinds of waves, evanescent and internal, are separately discussed in detail. Results of the model are compared with other existing models of η(O2) and found to be in agreement. The predictions of this model agree fairly well with the available gravity wave measurements in the magnitude of η, but the predicted phases are somewhat larger than those normally observed.

SATI: A SPECTRAL AIRGLOW TEMPERATURE IMAGER

Abstract An empirical understanding of planetary scale perturbations in the upper atmosphere is most effectively gained by the complementary use of both satellite and ground-based instrumentation. Limb observations from space provide vertical information and broad global coverage, while ground-based observations provide detailed system development in both solar and universal time. Ground-based instruments must be stable, accessible to but not dependent upon operator interaction, and inexpensive. The technique of interference filter spectral scanning has shown itself to satisfy these requirements when embodied in the instrument MORTI, a mesopause oxygen rotational temperature imager. MORTI has been especially useful in studies of fluctuations of the temperature and emission rate of the O2 Atmospheric airglow layer. SATI represents a complete redesign of the MORTI concept in order to make it an even better instrument for ground-based networks. The major new features are the replacement of cryogenic by thermo-electric cooling, real time temperature and emission rate readout, remote operation, and the addition of an OH channel. Examples of spectral airglow images taken with SATI are presented and its potential use in the COSTEP PSMOS project (planetary scale mesopause observing system) is discussed.

Tides in emission rate and temperature from the O2 nightglow over Bear Lake Observatory

The mesopause oxygen rotational temperature imager, MORTI, was operated at Bear Lake Observatory (41.9°N, 111.4°W) during the period November, 1991 to May, 1993. Fluctuations of long period in both emission rate and temperature of the O2 Atmospheric (0–1) nightglow layer are evident that seem related to the diurnal and semidiurnal tides. The data show a dominant semidiurnal tidal mode in January, but a dominant diurnal component at the spring equinox. Other long-period fluctuations also appear for which a linear tidal explanation does not seem applicable.

Nightglow zenith emission rate variations in O(1 S) at low latitudes from wind imaging interferometer (WINDII) observations

Volume emission rate data of the O( 1 S) nightglow layer measured by the wind imaging interferometer (WINDII) aboard the Upper Atmosphere Research Satellite (UARS) are vertically integrated to give equivalent zenith column emission rates. The data are restricted to tropical latitudes and categorized according to season. The migrating tidal signature is extracted by plotting zonally averaged data as a function of local time. The main features are a premidnight maximum at the equator during equinox intervals and a midnight maximum at 20°N and S. The two equinoxes are not equivalent, nor is the behavior identical in the two hemispheres. Winter periods show little or no well-defined tidal signature. Longitudinal variations at fixed local time were also studied. These variations are larger than the tidal signature and are geographically random, showing no consistent identification with land masses. Some of these variations could be of planetary scale, while others are probably long-wavelength gravity waves, but in any case they were seen to extend over 10° of latitude at least. Zenithal emission rate measurements exceeding 500 R occurred at the coincidence of these longitudinal wave structures and the tidal maxima. The suggestion is put forward that much of the perceived tidal variability in the tropical lower thermosphere may be explained by such wave-tide superpositions.

Airglow intensity variations induced by gravity waves. Part 2: comparisons with observations

Abstract The generalized gravity wave–airglow interaction theory presented in the first paper of this two part series by Wang et al. (2001) is further developed for comparisons with observations. As a counterpart of Krassovskys airglow intensity-to-temperature ratio η , we propose the ratio of the line-of-sight (LOS) perturbation wind to the intensity, η w . The behaviors of these ratios are determined by the intrinsic wave frequency, rather than the observed frequency. The Doppler-shifting effects are investigated and found to be quite important. For a given observed frequency, an increasing (decreasing) magnitude and phase angle of η is generally expected, in response to the downward (upward) Doppler-shifting of the intrinsic frequencies for the waves propagating along (against) a mean flow. In a wind-temperature-stratified atmosphere, the magnitude of η may vary by a factor of ∼2 or more, with larger values corresponding to small-scale waves, and the phase angles may change by ∼40–90°, with larger values occurring for either large-scale or very small-scale wave modes. For η w , which is proportional to 1/ η , its Doppler response generally exhibits a tendency opposite to that of η . These ratios are also dependent on zenith angle θ , at which observations are made. In particular, θ -dependence of η w is primarily controlled by different contributions of horizontal and vertical motions to the LOS wind. For limb scans, small-scale gravity waves with relatively large vertical perturbation velocities are shown to play an important role in producing the observed large airglow intensity variations. In order to explore the importance, we simulate the green line airglow images observed by WINDII, the WIND Imaging Interferometer on the Upper Atmosphere Research Satellite (UARS), by numerically integrating the Michelson equation of the instrument. The model-calculated ratios and simulated images are compared with realistic observations from the ground and space in good agreements.

Airglow measurements of possible changes in the ionosphere and middle atmosphere

Abstract A brief review of mechanisms for oxygen airglow excitation is presented in the light of predicted mesosphere and lower thermosphere changes for increased carbon dioxide and methane. From this it is concluded that airglow is extremely sensitive to global change. Possible airglow measurements are reviewed, and an assessment given of the potential for the detection of change. The primary problem in the determination of trends is shown to be the natural variability of airglow, for which the two main factors are solar variability, and dynamics. Examples of solar and dynamical influences are shown. Independent monitoring of the solar flux can in principle allow the separation of the solar influence, but the dynamical one is more difficult. One solution proposed is to make ground-based measurements around a zonal ring of fixed latitude, with longitude separations that are as equally spaced as possible. Such an observing system has now been initiated as a SCOSTEP project, with the name PSMOS, the Planetary Scale Mesopause Observing System. It is expected that PSMOS will make a significant contribution towards the determination of such trends.