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\lefthead{MENDE ET AL.}
\righthead{GRAVITY WAVE MODULATED AIRGLOW}
\received{July~23,~1997}
\revised{October~27,~1997}
\accepted{October~30,~1997}
\paperid{}
\cpright{AGU}{1998}
\ccc{ }

\authoraddr{H.~U. Frey, S.~P. Geller, S.~B. Mende, Space Sciences Laboratory,
University of California, Berkeley, CA 94720.  (e-mail: hfrey@ssl.berkeley.edu, 
geller@ssl.berkeley.edu, mende@ssl.berkeley.edu).}

\authoraddr{G. R. Swenson, Electro-Optic System Laboratory, University of 
Illinois, 313 Computer and Systems Research Laboratory, 1308 W.
Main, Urbana, IL 61801.  (e-mail: swenson1@uxl.cso.uiuc.edu)}

\setcounter{secnumdepth}{4}

\begin{document}
\title{Gravity wave modulated airglow observation from spacecraft}

\author{S.~B. Mende, H.~U. Frey, S.~P. Geller}
\affil{Space Sciences Laboratory, University of California, Berkeley}

\author{G. R. Swenson}
\affil{University of Illinois, Urbana}

\begin{abstract} During the STS-75 shuttle mission a special purpose electronic
imager viewed the atmosphere below the shuttle orbiter in the wake direction
with the objective of imaging gravity wave induced airglow modulation.  A
continuous sequence of images was taken with a picture repetition rate of 4 per
second.  A narrow band (3 nm) filter was used to select the O$_2$ atmospheric
0,0 band at 762 nm.  The effectiveness of the filter was demonstrated by
comparing images of tropospheric clouds with and without filter under full moon
conditions.  The test showed that the atmospheric O$_2$ absorption combined with
the filter was effective in suppressing intensity variations of the bright full
moon illuminated clouds and any other earth albedo.  Therefore under conditions
of low moonlight the filtered images truly represented the modulation of the
airglow intensity uncontaminated by earthshine.  Longer sequences of airglow
data were processed to compensate for spacecraft motion and to dynamically
integrate the signal into a latitude and longitude map of airglow modulations.
Because of the intrinsic low pass filtering of the slant view observation
geometry preferential enhancement of the higher frequencies was needed to bring
out waves in the wavelength range of $<$ 200 km.  Although the shuttle based
limited operation of the imager did not permit to make large area global
surveys, the results demonstrated that spacecraft based imaging of the airglow
on a long duration flight would produce a global data set much needed for the
understanding of Mesosphere Lower Thermosphere dynamics.  \end{abstract}

\begin{article}
\section{Introduction}

Atmospheric Gravity Waves are a fundamental factor in the characterization of
the energetics and dynamics of the Mesosphere Lower Thermosphere (MLT) region.
Waves have a large influence on the MLT region because of their ability to
transport significant energy and momentum and to influence larger scale motions.
The mean momentum fluxes imply a strong residual circulation near the
mesosphere, with upwelling at high summer latitudes and descent in the winter
hemisphere which result in temperatures ~90 K above or below radiative
equilibrium values and a reversal of the latitudinal gradient of temperature
relative to lower altitudes [{\it McIntyre,} 1989; {\it Garcia,} 1989; {\it
L\"ubken and von Zahn}, 1991; {\it Luo et al.,} 1995].  Although our
understanding of gravity waves in the mesosphere has improved considerably in
the past decade, quantitative characterization of their role is still in its
infancy.  Significant advances could be made by a systematic survey of gravity
waves which can only be accomplished by spacecraft based imaging.

Gravity waves can be observed from space by imaging the intensity modulations of
the the 80-100 km region.  Downward (nadir) viewing from spacecraft would permit
the observation of the short wavelength (down to $\lambda_x \approx$ 10 km)
gravity waves which are most significant in controlling the MLT energy and
momentum balance.  Slant, not directly downward viewing observations of shorter
wavelength gravity waves are limited by line of sight cancellation of closely
spaced adjacent intensity maxima of the waves.

There have been numerous investigations centered on ground and space-based
optical observations of gravity waves via imaging techniques [see {\it Taylor,}
1997 and references therein].

The primary candidate for spacecraft measurement of gravity wave modulated
intense airglow emissions is the O$_2$ (0,0) band of the $b^1 \Sigma_g^+ - X^3
\Sigma_g^-$ at 761.9 nm [{\it Tarasick and Shepherd,} 1992; {\it Zhang et al.,}
1993; {\it Hickey et al.,} 1993].  This band is not visible from the ground
because of the deep absorption by the lower atmospheric O$_2$.  Another
advantage of the O$_2$ band is that the high concentration of O$_2$ in the lower
atmosphere absorbs the re-scattered emission by the earth.  This is a critical
advantage in downward viewing observations because the large non-uniformities in
the earth albedo could dominate the image modulations.  The O$_2$ absorption of
continuum radiation, such as moonlight or man made light sources had not been
measured previously in this band.

In this paper we describe observations of airglow intensity modulations observed
from the space shuttle.  The modulations are attributed to the passage of
gravity waves through the airglow layer.  In order to image the airglow and
resolve the finer scale waves it is necessary to take longer exposures to
enhance the SNR and overcome the intrinsic faintness of the airglow, while
compensating for the spacecraft motion.  We perform such image enhancements data
taken on the STS-75 mission with the objective to show feasibility of the
technique for long term systematic observation of the airglow and provide the
much needed data base for global gravity wave studies.

\section{Observations}

The Tether Optical Phenomena (TOP) imager instrument was flown on the STS-75,
TSS-1R shuttle mission to observe discharge glows caused by the interaction of
the long electrodynamic tether with the atmosphere/ionosphere.  The TOP imager
was a high sensitivity image intensified CCD camera capable of taking extended
exposures.  The data discussed in this paper were taken with an interference
filter centered at 762 nm with a bandwidth of 3 nm.  The imager was mounted
inside the orbiter cabin to look through one of the windows to observe the
atmosphere below and was shielded from interior light.

The observing scenario is illustrated in Figure 1.  The shuttle orbiter was in a
circular orbit of 300 km altitude and the camera was looking back along the
orbit track at the airglow emission layer located $\approx$ 92 km.  The broken
lines represent the shuttle position radius vector and the radius vector to the
airglow layer tangent point from the center of the earth.  At the view direction
towards the tangent point the observer is looking parallel to the airglow layer
and small scale intensity modulations of the airglow would be averaged due to
line of sight cancellation.  When the view direction is aimed below the limb
tangent (e.g.  $2^\circ$ or below) the structure in the airglow can be resolved
with a resolution element defined by the airglow layer thickness ($\approx$ 8
km) divided by the tangent of the angle subtended by the view direction and the
airglow layer (Van Rhijn angle).

Figure 2 illustrates the effectiveness of the filter and the lower atmosphere to
suppress backscattered full moon moonlight from the earth and clouds.  The
figure shows the comparison between the unfiltered and filtered images.  The
strong features of moonlit earth limb and clouds (shown in the unfiltered image)
are completely suppressed in the filtered image and their intensity is
considerably lower than the features of the O$_2$ airglow.  Resonance scattering
in the high density lower atmosphere below absorbs all emissions of the same
wavelength as the O$_2$ band components.  However for continuum sources such as
moonlight considerable leakage would be expected between the absorbing lines of
the O$_2$ band.  A code named "MODTRAN" is generally used for the prediction of
atmospheric extinction as a function of wavelength and we have run it for the
case presented on Figure 2 assuming that the reflecting layers (clouds) are
either at 0 km or 8 km altitude.  To simulate the geometry we assumed that the
instrument was looking $2^\circ$ and $6^\circ$ below the direction of the
airglow tangent height.  By including the elevation of the moon ($15^\circ$) we
obtained a total in band transmission of 7.2 \% for 0 km cloud elevation at
$2^\circ$ view angle and 37 \% for the 8 km at $6^\circ$ case.  Since the
observations were taken near full moon, the intensity of the moonlit clouds were
about 3 to 5 kR per nm.  In the filter band of 3 nm the MODTRAN code predicted a
luminosity level ranging from 1 kR to 5 kR with strong intensity dependence on
the slant angle.  Our test showed that there was no significant (structured)
background and no apparent angular dependence of intensity, consequently we
believe that the suppression must have been more effective than the model
prediction showing that the broadband filter combined with the atmospheric O$_2$
removes most of the moonlight even in full moon conditions.  From this we are
reasonably certain that during low moon conditions white light backscatter is
not a significant contaminant of our observations.  The wave data presented in
this paper were taken during low or moon-less conditions.

A typical image of the airglow taken by the TOP camera is shown on Figure 3
essentially upside down with the limb enhanced airglow spanning the image
diagonally.  Above the bright airglow limb the faint airglow layer is seen.  As
we gradually look away downward from the tangential view the apparent intensity
diminishes towards the top left corner of the image.  Superimposed on this
apparent gradual intensity variation there are discernible intensity structures
showing a few wave-like airglow features almost but not exactly parallel to the
enhanced limb.

During the STS-75 mission several sequences of such images were taken over
various geographic regions.  The repetition rate was 4 images per second (1/4
sec exposure each).  The signal to noise ratio of the images, which was limited
by the relatively short exposure, could be greatly improved by co-adding the
signal from each volume element while it is in the field of view.  As shown in
Figure 1, the TOP camera views a $2^\circ$ wide region starting $2^\circ$ below
the limb tangent.  To perform the intensification, each CCD pixel within this
region was projected on a map of resolution elements which were referenced to a
geographical grid located on a surface $\approx$ 90 km above the solid earth.
Then as each consecutive exposure was taken the CCD pixels from each exposure
were mapped on the same grid taking into account the motion of the shuttle.  The
signal from each CCD pixel was added to the signal already present at the
corresponding resolution element.  As the field of view of the CCD moved across
a geographic region, the signal was integrated at each geographic resolution
element and the signal to noise ratio was thus enhanced.  We term this process
"dynamical integration".

\section{Discussion}

Examination of Figure 4 shows that some of the long wavelength wide area regions
have a very high contrast.  To enhance the smaller, shorter wavelength, waves
selective contrast enhancement was performed by Fourier filtering.  The image
(Figure 4) was transformed so that the X and Y axes were perpendicular and
parallel respectively to the orbit trajectory.  The two dimensional Fourier
transform of this image was obtained and the very high contrast, low frequency
components of the center region of the Fourier transform were blacked out.  The
resultant transform was inverted and transformed back in its appropriate
geographic orientation.  This filtering method was tried on several ground based
images and it was found that as long as the images do not contain high frequency
components from sharp edges or stars the technique does not generate image
artifacts.  The images described here lack such sharp features and thus they
well represent the airglow structure in the selected frequency range.  From the
transformation of the resultant image into to geographic coordinates (Figure 5)
we can see two interfering wave patterns.  We have also superimposed the sketch
of two wave systems which are consistent with our observations.  Recent
observations of CO$_2$ band modulations in the stratosphere show that gravity
waves closer to the lower atmospheric source exhibit circular waves pattern
propagating from a central origin [{\it Dewan et al.,} 1997].  Our data is
consistent with such a wave propagating mainly in the zonal direction seen in
our small observing field and interfering with a weaker wave propagating in the
meridional direction.

In ground based zenith observations [e.g {\it Swenson and Mende,} 1994] the
amplitude of short wave induced airglow intensity modulations is usually of the
same order of magnitude as long wavelengths and in fact {\it Hecht et al.}
[1997] show that the slope of the O$_2$ A band wave number spectrum is - 2.42.
However, in the STS-75 data, the amplitudes of waves of wavelength 100 km or
shorter are so small that they had to be selectively amplified to present them.
The need for the enhancement can be explained purely by the geometry of the
slant angle, near limb observations.  Referring to Figure 1, it can be shown
that observing from $2^\circ$ to $4^\circ$ below the limb tangent results in
line of sight integration spanning $\approx$ 100 km resolution elements
accounting for the need to preferentially amplify the shorter wavelengths.

A shortcoming of the STS-75 data set is the relatively sporadic nature of the
airglow data taken during the mission.  Nevertheless the STS-75 TOP mission
provided several example data sets (one of which was presented in this paper)
which demonstrate what can be obtained by an unoptimized slant viewing camera.
We have also shown that the O$_2$ (0,0) band is ideal for topside airglow
imaging because of the absorbing qualities of the lower atmosphere in this band.
This work can be considered a precursor for performing a future investigation in
which long duration continuous synoptic observation of the waves will be made
from spacecraft.


\acknowledgments The authors are greatly indebted to all persons at Lockheed
Palo Alto Research Laboratories and at NASA Marshall and Johnson Spaceflight who
were involved in the STS-75 flight.  The authors acknowledge the contribution of
J.  L.  Mergenthaler and J.  B.  Kumer of Lockheed-Martin for their contribution
in running the MODTRAN code.  The work was supported under subcontract No V08971
at Berkeley by the University of Michigan under the SETS NASA contract number
NAS8-39381.

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\end{thebibliography}{}
\end{article}

\begin{figure}
\caption{Observation scenario (not to scale).  View directions towards the limb 
tangent and $2^\circ$ and $4^\circ$ below that are illustrated.}
\end{figure}

\begin{figure}
\caption{The effectiveness of the filter (combined with O$_2$ atmosphere) in 
suppressing white moon-lit earth.  Scene with filter, only the airglow is 
visible (top) filterless scene taken 3 sec later illuminated by the near full 
moon (bottom).}
\end{figure}

\begin{figure}
\caption{Video frame of the O$_2$ band airglow layer and wave induced airglow 
modulations.}
\end{figure}

\begin{figure}
\caption{Geographically corrected dynamically integrated data taken between 1507 
and 1510 on the 26th of February 1996.} \end{figure}

\begin{figure}
\caption{A projection of the Fourier filtered data into geographic coordinates.  
Broken lines were drawn on the image to illustrate wave patterns which could be 
consistent with the observed waves.} \end{figure}

\end{document}