The
ripple-like
structures
in these photographs are not
clouds but large
gravity waves travelling through the night-time sky at a height
of 80-100 km
(50-60 miles). The waves extended across the entire sky and were
also visible to the naked eye.
Gravity waves are a
common feature in every region of the Earth's atmosphere and oceans.
They are oscillations
in density, pressure, temperature, and wind speed and they contribute
significantly to the energy and momentum budgets of both regions.
In particular, gravity waves are particularly important in governing
the dynamics and photochemsitry of the upper regions of the Earth's
atmosphere.
1.
Night-time Airglow
The nighttime
upper mesosphere at 80-100 km altitude contains several narrow
(~10 km) layers of glowing gas.
1. Neutral oxygen (O(¹S)) emission at 557.7 nm which occurs near 96 km.
2. Molecular oxygen (O2) emission from bands in the near-infrared at 862.0-869.0 nm which occurs near 94 km.
3. Neutral atomic sodium emission from the doublet at 589.0 and 589.6 nm which occurs near 92 km.
4. The Meinel bands due to hydroxyl (OH), which occur in the visible through to the infrared, originate from near 87 km.
2. All-sky Optical Imaging Measurements
Mesospheric gravity-wave activity is recorded using an extremely sensitive camera equipped with a fish-eye lens which images the entire sky every 1-2 minutes (see picture below). The camera is computer-controlled and observations are made on clear, dark nights during the two-week period centered on the new Moon.
Available Nights of Observations
We
have several all-sky imagers located at
several places around the
world:
(i)
Millstone
Hill, Massachusetts
(ii)
Arecibo
Observatory, Puerto Rico
(iii)
McDonald
Observatory, Texas
(iv)
El
Leoncito, Argentina
(v) Mount John, New Zealand
(vi) Mercedes, Argentina
All-sky images from Arecibo Observatory showing two simultaneous gravity wave events in the atomic oxygen layer (O) at 96 km,
and the hydroxyl layer (OH) at 87 km. The horizon runs around the edge of the images and directly overhead is at the centre.
One wave is moving towards the south-west and covers the whole sky as a series of bright bands.
The other wave is moving due south and appears as dark bands in the O image and bright bands in the OH image. This indicates
that the wave has changed phase ove the height interval (about 9-10 km) between the two emission layers.
3.
Sharp
frontal wave events - Ducted Waves and Bores
Bores
are horizontally-propagating jumps in the density, pressure,
temperature, etc. They
are seen commonly in rivers and tidal estuaries, in the ocean
and lower atmosphere. They have only recently been identified in
the upper mesosphere (Smith et al., 2003; 2005, 2006a).
Bores come in two types:
(i) Undular bores
- the
leading
front is followed by a series of trailing waves which are phase-locked
to the front Image (see picture below).
(ii) Turbulent bores
- the
leading front is followed by a region of turbulence.
Here
is an all-sky
image of a bore event that was visible to the naked eye over Texas
(comapre to the
colour photos above). The waves are travelling due south towards bottom of the image.
The tower on the North-East horizon (black spike)
is also visible in the colour photos above.
Click here for a movie of the event.
4. Mesopause Temperature Measurements
We have recently developed a spectrographic system to measure the rotational temperature of the night-time OH layer at 87 km.
The method uses the brightness ratio of two spectral lines (P1(2) at 840.0 nm and P1(4) at 846.5 nm in the OH(6-2) Meinel
spectral band in the near infra-red. Below are two plots of the OH temperature during two consecutive nights at Millstone Hill.
Both plots show large changes in temperature (~25 K) due to wave and tidal activity. A recent comparison of the spectrograph
temperatures with the temperatures obtained from the TIMED SABER instrument during overpasses of Millstone Hill
yielded excellent agreement (Smith et al., 2006b).
5. Miscellaneous Irrelevent Stuff
