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.
Figure 1. Schematic illustrating the physical processes involved in the HRDI
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
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.
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:
and N is a combination of the finesses due to reflectivity, instrument defects
and detector broadening. To a good approximation, this is given by
The amount of light collected in the central order (assuming no true absorption)
is close to
There are three important features to note from these equations. First, the
amount of light collected decreases monotonically as the reflectivity increases.
Second, the light in the central order depends only on the free spectral range
(which in turn depends on the gap spacing) and the etalon reflectivity, while
the resolution depends on the defects as well. Defects increase the interferometer
halfwidth and decrease the peak transmission. Third, if the finesses are significantly
different, then the resolution is determined by the smallest finesse. A practical
rule is to try to make the finesses approximately equal. The HRDI detector is
a multi-channel device which has 32 channels equally spaced in wavelength. The
number of channels was limited by telemetry and manufacturing capabilities.
The aperture finesse is approximately the number of channels per order. This
was set to be 30 at 762 nm (i.e., ~1 order on the detector) and it decreased
linearly with wavelength to 25 at 630 nm. The Fabry-Perot plates were manufactured
to a matched flatness of ~lambda/200, and in order to ensure that the etalons
would survive launch stresses, they were firmly attached to their mounts. This
had the unfortunate effect of significantly stressing and distorting the plates
so that the defect finesses were about 20. The reflectivity of the etalons was
about 0.90 at all wavelengths of interest, corresponding to a reflectivity finesse
of about 30. These combine to give an overall finesse of about 12. The large
degree of distortion provided by the etalon mountings was unanticipated and
the etalons were coated to a higher than optimal value.
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
HRDI examines a spectral range of 0.5 cm-1 at ~13,100 cm-1. The solid angle
is then approximately 2.4 x 10-4 sr and, with an effective etalon diameter of
8.9 cm, the area-solid angle product is 1.49 x 10-2 cm2 sr. This number must
be matched by all other optical elements in the system. This requirement is
readily met by the rest of the interferometer optics; the only real concern
is the telescope that observes the atmosphere. The diameter of the telescope
is limited in size by mass constraints, cost of the primary mirror, and the
length of the baffle. A large-diameter telescope requires a long baffle adding
its own mass and moment of inertia, which is a consideration since the telescope
must move rather rapidly and with a minimum of wobble. Since HRDI is a limb
viewer, the vertical field of view is limited by the vertical scale of the features
in the atmosphere which the instrument is designed to study. The scale height
of the atmosphere in the stratosphere, mesosphere and lower thermosphere ranges
from 5 to 8 km. A vertical field of view of 0.12 degrees corresponds to a vertical
range on the limb of 5 km at a satellite altitude of 600 km. The horizontal
field of view can be much wider, since typical horizontal scales in the atmosphere
are much larger than vertical features. Since HRDI measures the winds by determining
the Doppler shift, care must be taken to properly account for the Doppler shift
provided by the spacecraft motion. If the field of view is too large there will
be a significant Doppler shift variation across the field of view. A consideration
of these factors leads to a telescope design which incorporates a primary 17.78
cm in diameter and field of view of 0.12 x 1.3 degrees (a secondary field of
view of 0.12 x 0.12 degrees is available for star observations). The telescope
is an off-axis Gregorian which minimizes small-angle scattering, and the area-solid
angle product is then 1.1 x 10-2 cm2 sr. This demonstrates that the etalon and
telescope are not optimally matched. The entire etalon (area and solid angle)
is filled by the use of a fiber optic bundle which illuminates it with a lower
intrinsic brightness than if the etalon and telescope were matched.
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
Equation 5 can be inverted in a least square fashion using all channels and
converted by the Doppler equation to give
The statistical uncertainty of velocity can be estimated by assuming the only
source of error is photon statistics in Ci(U). This is the largest source of
error and can adequately describe the statistical error. Using Poisson statistics
which describe photon noise, the uncertainty in U is given by
Equation (7) demonstrates that to minimize wind errors the signal should be
as bright as possible and the slope of the signal should be large. The instrumental
width (~0.001 nm, 0.02 cm-1) is significantly greater than the shift due to
atmospheric winds. It is important to recognize that HRDI measures Doppler shifts
on the order of 10-5 nm, and is not attempting to resolve spectral features
of this magnitude. Since the width of the spectral line depends on convolution
of the instrument width and the atmospheric line shape, there is no reason to
make the instrument resolution much narrower than the line as the resultant
signal will not become sharper and the signal-to-noise ratio will decrease.
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.
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