Gordon G. Shepherd

Satellite observations of thermospheric tides: Results from the Wind Imaging Interferometer on UARS

Thermospheric winds measured by the Wind Imaging Interferometer (WINDII) on the upper atmosphere research satellite are analyzed for migrating solar tides. The data cover a 2-year period commencing February 1992 and are obtained from the atomic oxygen O(1S) 557.7-nm emission, which provides observations of the 90- to 200-km altitude range during daytime and the 90- to 110-km range at night. The subtropical lower thermosphere is dominated by the diurnal propagating tide which exhibits a vertical wavelength of approximately 22 km, grows in amplitude up to 95 km, and decays rapidly above where molecular diffusion greatly reduces the vertical shears. Although the phase remains fairly uniform throughout the year, a pronounced semiannual oscillation is observed in the diurnal tide amplitude. At both 20°N and 20°S the meridional and zonal wind components attain their maximum values at equinox of approximately 70 and 40 m/s, respectively, while the solstitial minima are nearly a factor of 2 smaller. At 35°N the diurnal tide semiannual amplitude oscillation is still present in the lower thermosphere, but above 100 km it is replaced by an annual cycle with a maximum in July and August. This contrasts with 35°S where the July/August peak is absent and the semiannual oscillation extends to 110 km. At midlatitudes the zonal and meridional winds are of similar magnitude, and no significant hemispheric asymmetries in amplitudes are observed. In the lower thermosphere the semidiurnal tide amplitude exhibits an annual oscillation, with maximum values of 30 to 40 m/s occurring in June/July near 100 km at 35°N, 35°S, and the equator. A bimodal structure in the seasonal variation of the semidiurnal phase is observed. This feature is characterized by rapid equinoctial transitions and is particularly well defined at the equator. Examination of the equatorial middle thermosphere indicates that the semidiurnal tide attains its maximum amplitude at 140 km and exhibits a vertical wavelength of approximately 60 km. These findings indicate the predominance of the antisymmetric (2,3) Hough mode in the tropics.

Validation of mesosphere and lower thermosphere winds from the high resolution Doppler imager on UARS

Horizontal wind fields in the mesosphere and lower thermosphere are obtained with the high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) by observing the Doppler shifts of emission lines in the O2 atmospheric band. The validity of the derived winds depends on an accurate knowledge of the positions on the detector of the observed lines in the absence of a wind-induced Doppler shift. Relative changes in these positions are readily identified in the routine measurements of onboard calibration lines. The determination of the absolute values relies on the comparison of HRDI observations with those obtained by MF radars and rockets. In addition, the degrees of horizontal and vertical smoothing of the recovered wind profiles have been optimized by examining the effects of these parameters both on the amplitude of the HRDI-derived diurnal tidal amplitude and on the variance of the wind differences with correlative measurements. This paper describes these validation procedures and presents comparisons with correlative data. The main discrepancy appears to be in the relative magnitudes measured by HRDI and by the MF radar technique. Specifically, HRDI generally observes larger winds than the MF radars, but the size of the discrepancy varies significantly between different stations. HRDI wind magnitudes are found to be somewhat more consistent with measurements obtained by the rocket launched falling sphere technique and are in very good agreement with the wind imaging interferometer (WINDII), also flown on UARS.

Combined mesosphere/thermosphere winds using WINDII and HRDI data from the Upper Atmosphere Research Satellite

This paper examines the combined mesospheric and thermospheric (50 to 200 km) longitudinally averaged winds measured by the wind imaging interferometer (WINDII) and the high-resolution Doppler imager (HRDI) onboard the Upper Atmosphere Research Satellite. The data analyzed cover 2 years from February 1992 to February 1994 and consist of both day and nighttime WINDII winds obtained from the O(1S) green line emission and mesosphere/lower thermosphere daytime HRDI winds from the O2 atmospheric band. The combination of the WINDII and HRDI data sets is first justified by comparing all the data in the lower-thermosphere overlap region for days and orbits when both instruments were observing the same volume of atmosphere. This comparison shows good agreement between the two instruments. An analysis of the combined WINDII and HRDI winds during equinox and solstice periods is then performed. The amplification with height of the diurnal tide at equinox and its subsequent decay in the lower thermosphere is clearly demonstrated by the observations. The corresponding background (i.e., diurnal mean) zonal wind component exhibits a broad region of easterlies at lower latitudes in the upper mesosphere and lower thermosphere and westerlies at midlatitudes. Above 120 km the mean winds revert to easterlies in the zonal component and a two-celled equator to pole meridional circulation. The solstice circulation is highly asymmetric about the equator in accordance with the interhemispheric difference in solar heating. The reversal of the mesospheric jets as well as the summer to winter hemisphere meridional flow in the middle thermosphere are clearly shown. At solstice a significantly weaker and more hemispherically asymmetric propagating diurnal tide is also evident.

WAMDII: wide-angle Michelson Doppler imaging interferometer for Spacelab

A wide-angle Michelson Doppler imaging interferometer (WAMDII) is described that is intended to measure upper atmospheric winds and temperatures from naturally occurring visible region emissions, using Spacelab as a platform. It is an achromatic field-widened instrument, with good thermal stability, that employs four quarterwave phase-stepped images to generate full images of velocity, temperature, and emission rate. For an apparent emission rate of 5 kR and binning into 85 × 105 pixels, the required exposure time is 1 sec. The concept and underlying principles are described, along with some fabrication details for the prototype instrument. The results of laboratory tests and field measurements using auroral emissions are described and discussed.

Tidal influence on O(¹S) Airglow emission rate distributions at the geographic equator as observed by WINDII

WINDII, the Wind Imaging Interferometer on the Upper Atmosphere Research Satellite, observes winds, temperatures and emission rates in the upper mesosphere and thermosphere. In this paper we report on nighttime observations of the vertical distribution of the O(¹S) 557.7 nm emission near the geographic equator for March/April, 1993. The airglow volume emission rate distribution is found to be strongly dependent on local time. Beginning at dusk, an intense airglow emission layer descends from a mean altitude of 95 km, reaching 89 km by midnight after which the emission rapidly decays. Shortly after midnight it reappears weakly at a higher altitude and remains at this level as the emission rate gradually increases towards dawn. This strong local time dependence leads us to conclude that the effect is tidally driven. Comparison with the Forbes [1982a,b] model suggests that total density perturbations and changes in the atomic oxygen mixing ratio may the cause of the changes in emission rate distribution between dusk and midnight. The reappearance of the emission after midnight may be caused by downward winds bringing oxygen-rich air from above.

Validation of O(1S) wind measurements by WINDII: the WIND Imaging Interferometer on UARS

This paper describes the current state of the validation of wind measurements by the wind imaging interferometer (WINDII) in the O(1S) emission. Most data refer to the 90-to-110-km region. Measurements from orbit are compared with winds derived from ground-based observations using optical interferometers, MF radars and the European Incoherent-Scatter radar (EISCAT) during overpasses of the WINDII fields of view. Although the data from individual passes do not always agree well, the averages indicate good agreement for the zero reference between the winds measured on the ground and those obtained from orbit. A comparison with winds measured by the high resolution Doppler imager (HRDI) instrument on UARS has also been made, with excellent results. With one exception the WINDII zero wind reference agrees with all external measurement methods to within 10 m s−1 at the present time. The exception is the MF radar winds, which show large station-to-station differences. The subject of WINDII comparisons with MF radar winds requires further study. The thermospheric O(1S) emission region is less amenable to validation, but comparisons with EISCAT radar data give excellent agreement at 170 km. A zero wind calibration has been obtained for the O(1D) emission by comparing its averaged phase with that for O(1S) on several days when alternating 1D/1S measurements were made. Several other aspects of the WINDII performance have been studied using data from on-orbit measurements. These concern the instruments phase stability, its pointing, its responsivity, the phase distribution in the fields of view, and the behavior of two of the interference filters. In some cases, small adjustments have been made to the characterization database used to analyze the atmospheric data. In general, the WINDII characteristics have remained very stable during the mission to date. A discussion of measurement errors is included in the paper. Further study of the instrument performance may bring improvement, but the utimate limitation for wind validation appears to be atmospheric variability and this needs to be better understood.

Longitudinal structure in atomic oxygen concentrations observed with WINDII on UARS

WINDII, the Wind Imaging Interferometer on the Upper Atmosphere Research Satellite, began atmospheric observations on September 28, 1991 and since then has been collecting data on winds, temperatures and emissions rates from atomic, molecular and ionized oxygen species, as well as hydroxyl. The validation of winds and temperatures is not yet complete, and scientific interpretation has barely begun, but the dominant characteristic of these data so far is the remarkable structure in the emission rate from the excited species produced by the recombination of atomic oxygen. The latitudinal and temporal variability has been noted before by many others. In this preliminary report on WINDII results we draw attention to the dramatic longitudinal variations of planetary wave character in atomic oxygen concentration, as reflected in the OI 557.7 nm emission, and to similar variations seen in the Meinel hydroxyl band emission.

Climatology of middle- and low-latitude daytime F region disturbance neutral winds measured by Wind Imaging Interferometer (WINDII)

We have modeled the global climatology of middle- and low-latitude F region daytime disturbance neutral winds using extensive measurements by the Wind Imaging Interferometer (WINDII) instrument on board the UARS. The perturbation winds were obtained by subtracting the quiet time values from the disturbed winds along the satellite orbit, which effectively removes average measurement bias. The zonal disturbance winds are mostly westward (except in the early morning sector), increase with latitude, and have largest values in the late afternoon sector. In general, the meridional perturbation winds are equatorward, increase linearly with latitude, and decrease from early morning to afternoon hours. The zonal and meridional perturbations increase roughly linearly with Kp and expand to lower latitudes with increasing magnetic activity. The meridional disturbance winds are largest for low solar flux conditions. We present empirical analytical models for longitudinally averaged disturbance winds from 60° to the equator. Our model winds are in poor agreement with results from the empirical wind model Horizontal Wind Model-93 during the entire daytime period. There are also important discrepancies between the average perturbations winds from WINDII and the National Center for Atmospheric Research thermosphere-ionosphere electrodynamic general circulation model, particularly at midlatitudes. These differences could be explained in part by the storm time dependence of the disturbance winds and by the variability of the high-latitude electric fields.

The Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite: A 20 year perspective

The Wind Imaging Interferometer (WINDII) was launched on the NASAs Upper Atmosphere Research Satellite on 12 September 1991 and operated until 2003. Its role in the mission was to measure vector winds in the Earths atmosphere from 80 to 110 km, but its measurements extended to nearly 300 km. The approach employed was to measure Doppler shifts from a suite of visible region airglow lines emitted over this altitude range. These included atomic oxygen O(1S) and O(1D) lines, as well as lines in the OH Meinel (8,3) and O2 Atmospheric (0,0) bands. The instrument employed was a Doppler Michelson Interferometer (DMI) that measured the Doppler shift as a phase shift of the cosinusoidal interferogram generated by single airglow lines. An extensive validation program was conducted after launch to confirm the accuracy of the measurements. The dominant wind field, the first one observed by WINDII, was that of the migrating diurnal tide at the equator. The overall most notable WINDII contribution followed from this; determining the influence of dynamics on the transport of atmospheric species. Currently, non-migrating tides are being studied in the thermosphere at both equatorial and high latitudes. Other aspects investigated included solar and geomagnetic influences, temperatures from atmospheric scale heights, nitric oxide concentrations and the occurrence of polar mesospheric clouds. The results of these observations are reviewed from a perspective of twenty years. A future perspective is then projected, involving more recently developed concepts. It is intended that this description will be helpful for those planning future missions.

Tidal influence on the oxygen and hydroxyl nightglows: Wind Imaging Interferometer observations and thermosphere/ionosphere/mesosphere electrodynamics general circulation model

Longitudinal zonally averaged Wind Imaging Interferometer (WINDII) (on UARS) night-time oxygen (O(1S)) and hydroxyl (P(3) line in the OH(8,3) Meinel band) volume emission rates exhibit dramatic spatial and temporal variations. The recently improved thermosphere/ionosphere/mesosphere electrodynamics general circulation model (TIME-GCM) produces simulations for the two airglows through the input of (1,1) upward propagating diurnal tides. The model simulations show excellent agreement with WINDII observations in both the local tune domain and the latitudinal domain between 40°S and 40°N. The influence of diurnal tides on the two airglows in strongest in the tropical region. In the local solar time domain the emission rate and peak altitude at the equator show large tidal perturbations, but they are fairly stable at midlatitude. In the latitudinal domain there is an equatorial trough in the oxygen emission rate which exists regardless of local time and season. The hydroxyl emission rate is more dependent on local time and season. At equinox it has a prominent equatorial maximum which disappears at dawn, whereas at solstice it has a very weak equatorial maximum at dusk, changing soon after midnight to an equatorial minimum. These features of emission rates are also compared to TIME-GCM simulations for meridional wind, temperature, and atomic oxygen density, [O], with and without upward propagating diurnal tides. The results are as follows: (1) The large oscillations of the two nightglows as well the atomic oxygen density in the tropical region are driven by the diurnal propagating tides. In altitude the mesosphere and lower thermosphere is divided into two type of cells, one with meridional winds converging at the equator, higher temperature, and enhanced [O] and airglow emission rates, and the other with meridional winds diverging from the equator, lower temperature, and depleted [O] and airglow emission rates; all these are essentially related to the wavelength and phase of the diurnal tides. (2) The relatively stable airglow emission rates at around 20° are related to the minimum fluctuation of temperature and stable meridional wind directions in spite of the maximized wind speed in this region. (3) The year-around equatorial depletion in the oxygen airglow and the double-peaked profile in the hydroxyl airglow are most likely produced by the diurnal tides. Further investigation is needed particularly for the absolute value of the oxygen emission rate, the interhemispheric comparison of the oxygen emission rate, why WINDII observes maximum summer emissions whereas the TIME-GCM gives maximum winter emission, and the effects of waves in addition to the diurnal tides.