Introduction

The distribution of most chemical species in the stratosphere is affected by both dynamical and chemical processes. Conversely, the distribution of certain photochemical species, such as ozone, can influence the radiative budget of the stratosphere, affecting temperatures and motions. Until recently, satellite remote observations of the stratosphere have provided only temperature and constituent measurements. Global horizontal winds have been deduced from temperature fields by using the thermal-wind relationships, which relate the vertical shear of the geostrophic wind components to horizontal temperature gradients. These equations, however, are only a good approximation for large-scale, low-frequency, extratropical flows (D.J. Andrews, Academic Press, 1987). The High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) is providing the first direct measurement of the global horizontal wind field in the stratosphere, mesosphere, and thermosphere. UARS was launched September 12, 1991, into a 585 km circular orbit inclined 57 deg to the equator (C.A. Reber, EOS 71, 1867, 1990). The spacecraft carries ten instruments designed to study the chemistry and dynamics of the stratosphere and above. Knowledge of the stratospheric wind field will help to determine how transport and mixing influence the ozone budget in the lower stratosphere, and to quantify the mean and eddy circulations associated with the observed semiannual oscillations. In the mesosphere, the wind measurements will contribute to the understanding of the relative role of turbulent diffusion and bulk advection in accounting for mesospheric tracer budgets.

The goal of HRDI is to measure the vector winds in the stratosphere (10-40 km), mesosphere and lower thermosphere (~50-120 km) during the day, and the lower thermosphere at night (~95 km) to an accuracy of 5 m/s. The horizontal wind vector is measured by observing the Doppler shift of rotational lines of molecular oxygen along two lines of sight (P.B. Hays, Appl. Opt. 21, 1136, 1982; V.J. Abreu et al., Appl. Opt. 28, 2128, 1989). In addition to winds, temperatures and volume emission rates are obtained in the mesosphere and lower thermosphere, and cloud top heights, effective albedos, aerosol phase functions, and scattering coefficients are determined in the stratosphere. Figure 1 schematically depicts the physical processes involved in the measurement. Depending on the band and atmospheric region, molecular oxygen lines can be observed in either emission or absorption, the latter dominating below ~50 km. Solar radiation incident on the atmosphere is absorbed, exciting molecular oxygen into the state. This can occur directly through resonance or photochemical reactions (L. Wallace and D.M. Hunten, J. Geophys. Res. 73, 4813, 1968; A. Bucholtz et al., Planet. Space Sci. 34, 1031, 1986). When this process takes place in the mesosphere or above, where quenching effects are small, the O2 emission lines in the A band of the () transition are observable. The emission in the B (1,0) and gamma (2,0) bands can be ignored because the excited vibrational states are collisionally quenched into the nu'=0 state before the electronic transition takes place (L.C. Lee and T.G. Slanger, J. Chem. Phys. 69, 4053, 1978; M.J. Gauthier and D.R. Snelling, Can. J. of Chem. 52, 4007, 1974; U. Schurath, J. Photochem. 4, 215, 1975). Below ~50 km electronic quenching () rather than emission dominates the loss process. In this case, the absorbed photons are not reemitted, and absorption lines develop in the spectrum. Absorption features in the A (~762 nm, 13,120 cm-1), B (~688 nm, 14,525 cm-1), and gamma (~629 nm, 15,902 cm-1) bands can be observed when solar radiation is elastically scattered by molecules and aerosols into the field of view of the observer. This scattering may occur before or after light is reflected from the surface. The line features generated by the Atmospheric bands (see P.B. Hays, Appl. Opt. 21, 1136, 1982) are generally quite narrow, with a width of ~0.001 nm (2 x 10-2 cm-1), and are therefore suitable for the Doppler shift measurement technique. The HRDI instrument and early results have been discussed elsewhere (P.B. Hays et al., in Digest of the Topical Meeting on Optical Remote Sensing of the Atmosphere, Vol. 4, DC, p. 7; V.J. Abreu et al., Optics & Photon. News 2, 28, 1991; P.B. Hays et al., J. Geophys. Res. 98, 10,713, 1993; P.B. Hays et al., Planet. Space Sci. 40, 1599, 1993; W.R. Skinner et al., Proc. NASA Symp. on Global Wind Measurements, Deepak Press, VA, 1986, p. 129; W.R. Skinner et al., IGARSS '87, 1987; Y.T. Morton et al., Geophys. Res. Lett. 20, 1263, 1993). This paper will review the instrument, discuss some of the important in-flight calibrations, and give examples of the type of data recovery possible. Several atmospheric parameters can be obtained with HRDI, but this paper will deal exclusively with the recovery of winds.

Instrument Requirements

The requirement to measure a Doppler shift on the order of 10-4 cm-1 demands an instrument that has high throughput and high spectral resolution. The Fabry-Perot interferometer that is used in HRDI is such a device (see P. Jacquinot, J. Opt. Soc. of Amer. 44, 761, 1954). High throughput and high resolution are not independent parameters, and optimization must be performed to define the best system. The optimization procedure must consider practical limitations. For example, the resolution (FWHH) of a Fabry-Perot is given by:

(1)

(2)

(3)

The quantity of light collected by a Fabry-Perot is proportional to the effective area of the etalon and the solid angle passing through it. The solid angle through the etalon is

(4)

Determination of Wind Shift

In order to determine winds, which is the primary product of the HRDI instrument, it is necessary to determine the Doppler shift of an absorption or emission line. Figure 5(a) shows the signal observed by an emission line on the IPD with and without a Doppler shift. The unshifted line is centered on channel 16 and the shifted line is moved by 20 m/s. Figure 5(b) clearly demonstrates that the shift has a very small effect at line center and in the wings, and is most notable on the sides of the line. The Doppler shift causes the signal in some channels to increase and to decrease in others. The proper interpretation of these signal changes allows the Doppler shift to be determined. The algorithm used to recover the Doppler shift is complex, and only a conceptual demonstration of the technique is presented here. The signal in the presence of a shifted emission or absorption line is given by

(5)

(6)

(7)

Fig. 5. Illustration of the effect Doppler shifts have on the spectrum observed by HRDI. a) Spectra with and without a small Doppler shift. b) The difference between the two spectra. The center and wings are unaffected, but the sides show a dramatic change.