William A. Gault

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.

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.

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.

ERWIN: an E-region wind interferometer.

A field-widened Michelson interferometer designed to measure upper atmospheric winds at three altitudes near the mesopause by using airglow emissions from O(1)S, OH, and O(2) is described. A very large path difference (11 cm) is used to suppress the fringes from the hot F-region emission of O(1)S and to facilitate accurate measurements. Field widening and thermal compensation are achieved over the large spectral range (557.7-866.0 nm) by the use of three types of glass in the interferometers arms. The instrument was installed at Resolute Bay, Canada (74.3 N, 94.5 W), in November 1992 and has been operated remotely from Toronto for four winter seasons. Some examples of data are shown to illustrate ERWINs performance.

The stratospheric wind interferometer for transport studies (swift)

Abstract The Stratospheric Wind Interferometer For Transport studies (SWIFT) is an instrument intended to measure winds to an accuracy of 5 m s −1 or better in the stratosphere, during both day and night, as well as ozone concentrations. It is based on WINDII, the WIND Imaging Interferometer on the UARS satellite, but there are a number of important differences. WINDII operated in the visible region, with widely-spaced airglow emission lines, a field-widened Michelson interferometer that uses glass combinations to provide thermal stability, and a CCD detector. SWIFT uses the thermal emission from an ozone line near 8.9 μm, a region in which the choice of refractive materials is very limited. Through a careful search for a suitable line several were found of appropriate strength that were adequately isolated, but only with a combination of etalon filters. Fortunately, HgCdTe array detectors are available so the detector is not a problem. By measuring both winds and ozone concentration it is possible to measure ozone fluxes. SWIFT will study ozone transport, transport across the sub-tropical mixing barrier, equatorial dynamics and data assimilation. The latter is an important tool for the execution of the scientific objectives.

Meridional wind in the auroral thermosphere : Results from EISCAT and WINDII-O(1D) coordinated measurements

Neutral thermospheric winds calculated from European incoherent scatter (EISCAT) radar data have been compared with winds measured by wind imaging interferometer (WINDII) in O(1D) emission during 11 passes of the WINDII fields of view near the radar facility. For the eight occasions when geomagnetic activity was low the average difference in the meridional winds measured by the two methods is less than 10 m/s. The EISCAT calculations were done with and without a “Burnside factor” of 1.7, and agreement with WINDII is somewhat better when the Burnside factor is not included. The three passes corresponding to disturbed conditions show poor agreement. In addition, agreement between EISCAT and WINDII is better when unfiltered EISCAT winds are used, rather than the 2-hour running mean used in earlier work. This finding suggests that the short-term oscillations seen by EISCAT are real oscillations of the neutral atmosphere.

Satellite Measurement of Stratospheric Winds and Ozone Using Doppler Michelson Interferometry. Part I: Instrument Model and Measurement Simulation

Abstract This paper presents an instrument model and observation simulations for the measurement of stratospheric winds and ozone concentration using a satellite instrument employing imaging and the Doppler Michelson interferometery technique. The measurement technique and instrument concept are described. The instrument model and simulations are based on initial design characteristics of the Canadian Stratospheric Wind Interferometer for Transport Studies (SWIFT) satellite instrument. SWIFT employs an imaging array and a field-widened Michelson interferometer. It will measure stratospheric winds and ozone densities using the wind-induced phase shifts of interferograms from atmospheric limb radiance spectra in the vicinity of the vibration–rotation ozone line at 1133.4335 cm−1. The measurement simulation and analysis tools have been developed to assess the SWIFT instrument performance and to evaluate the impact of instrument and measurement characteristics on expected wind and ozone errors. Sample results...

Waves Michelson Interferometer: a visible/near-IR interferometer for observing middle atmosphere dynamics and constituents

The Waves Michelson Interferometer (WAMI) is designed to provide simultaneous measurements of dynamical and constituent signatures in the upper stratosphere, mesosphere and lower thermosphere. It is being included as part of the Waves Explorer mission (G. Swenson, P.I. being proposed for NASAs MIDEX program. It is a field-widened Michelson interferometer based on the same design principle as the successful Wind Imaging Interferometer (WINDII). WAMI includes visible and near-IR channels, a segmented interferometer mirror for simultaneous fringe sampling at different optical paths and views the atmosphere in six distinct directions. Use of the segmented mirrors minimizes the aliasing of atmospheric intensity variations into the fringe parameter determinations. This technique also allows two emissions to be viewed simultaneously through the same optical channel. The emissions chosen include lines in the molecular oxygen IR-atmospheric band, a doublet in the hydroxyl Meinel bands and the oxygen green line. The daytime coverage includes winds from 45 to 180 km, and rotational temperature and ozone density from 45 to 95 km. The nighttime coverage is restricted to the airglow layer centered near 90 km where atomic oxygen, horizontal wind and rotational temperature measurements are provided. These measurements provide a rich data set from which dynamics, energetics and constituent budgets can be determined.

Optimization of a field-widened Michelson interferometer

This paper considers the optical design of a wide-angle fixed-path Michelson interferometer consisting of two arm glasses and an air gap. It is shown that this configuration can be optimized to give (a) extra large fringes (over 50 degrees in diameter) over a range of wavelength, (b) a path difference nearly independent of wavelength, or (c) a path difference specified differently at two different wavelengths for observing a pair of doublets. Specific examples refer to the airglow wavelengths of 557.7, 630.0, 732.0 nm and others, and to a path difference of 4.5 cm. The properties of different glass combinations are discussed.