Science Officer Don Pettitt works inside the Zvezda Service
Space Chronicles #3
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ISS Science Officer Don Pettit
Cloud Observations in the Southern Hemisphere
I have been
observing and photographing large-scale noctilucent clouds in the
southern hemisphere. Observing noctilucent clouds are almost like
chasing after a techno-version of Willo-the-Wisp. These clouds are
located on the fringes of space and shrouded in mystery. Because
they are so thin, they are difficult to see and require special
viewing conditions where the clouds are bathed in sunlight but seen
with a dark-sky background. Otherwise, the contrast is insufficient
to define them from the sky. Sunset and sunrise in polar-regions
are one of the few viewing places on the surface of Earth that satisfies
these conditions. Are these clouds only in polar regions, or do
they have global extent but can only be seen in these locations
due to the required viewing geometry? Nobody knows for sure. They
are believed to be water-ice crystals in the 1-micrometer size range
forming layers a few kilometers thick at altitudes of 80 to 82 kilometers.
From where this water comes from to form these crystals is a mystery.
There is no known transport mechanism that forces water from the
lower meteorological layers of our atmosphere into these altitudes.
At these altitudes, called the mesosphere, you are wedged between
the ionosphere and the stratosphere and are at the point where molecular
mean-free path arguments begin to collapse into continuum physics.
This is the region where meteors burn up in a fiery flash. This
is where the space shuttle upon its entry, begins to glow bright.
And what are these clouds doing there, and where did they come from?
They have only been observed in the northern hemisphere for about
a hundred years and usually during the summer months. Before that
there is no record. This leads one to speculate that they may have
something to do with industrialization and some global man-derived
effect on our environment. Rarely have they been seen in the southern
hemisphere, and perhaps only for the last twenty years. Even rarer
is photographic evidence. This is why I am so excited about our
southern hemisphere space station observations.
images are downlinked and will be posted on the NASA
Earth Observation Web site. The film will have to wait until
we return this spring. We noticed these displays when we changed
our station attitude to XPOP, a fancy way of saying that the same
side always points towards the sun as we orbit the Earth. This means
our Earth-looking window sometimes points directly nadir at Earth,
but other times it points outward towards the stars. In-between
these extremes it points to Earth's horizons which present a most
marvelous view of sunrise and sunsets across the day-night terminator.
This is perfect geometry for observing noctilucent clouds.
clouds appear as a thin but distinct cloud layer well above the
visible part of the atmosphere. They appear to be about twice the
distance from the horizon than the visible thickness of the atmosphere.
To the eye, they look like a thin shell of glowing wisps suspended
above our atmosphere in the blackness of space. On a digital image
though, the effect is less dramatic due to the sensitivity of the
CCD array that shows a gradual transition between the upper atmosphere
and these clouds and then an abrupt change into blackness.
Here is a simple
first order estimate to determine their altitude. The tops of thunderheads
are visible on edge and have tops that are typically at 10 km (35,000
feet). Using this as a ruler, the "blue" part of the atmosphere
above the cloud tops is about 3 to 4 times this thickness or 30
to 40 km. Therefore, the visible thickness of the atmosphere is
40 to 50 km. The noctilucent clouds, as seen with your eye, are
twice the visible atmosphere thickness. This places them in the
range of 80 to 100 km.
Dec. 18, 2002:
Looking south over Australia during transition over
the night-day terminator, a thin and well lighted noctilucent
cloud layer about two visible atmospheric thickness
above the horizon. Some layering is visible. The cloud
layer extends for about 15 degrees along the horizon.
It is visible for about 3 minutes.
No noctilucent clouds visible.|
Dec. 19, 2002:
Near the tip of South America, no noctilucent clouds visible.|
over tip of South American with station at S35 W75, good
display noctilucent clouds visible for about 4 minutes
with an extent of about 30 degrees on horizon at 2 atmospheric
thickness above horizon; some layering is visible within
the clouds. Two distinct layers seen.|
near South American at S38 W79; noctilucent cloud layer
2 atmospheric thicknesses above horizon; 24 degree arc
(just filled the horizontal field of view of the 85 mm
camera lens), lasted about 3 minutes.|
same observations of noctilucent clouds, a most wonderful
No noctilucent clouds visible.|
No noctilucent clouds visible.|
display of noctilucent clouds, covered nearly 90 degrees
on the horizon up to the very edge of the terminator.
At the terminator, the viewing geometry is not square
edge-on but slightly skewed so three dimensional structure
is visible. The cloud edge at the terminator showed at
least 4 layers of structure with similar delicate webbing
that is often seen from the ground. The display lasted
only 1½ minutes. I took digital and film images.
weak display noctilucent clouds. Station at S25 W120.|
weak noctilucent cloud display, only 5 degrees on horizon
and very faint. Over Australia.|
station attitude to LVLH +YVV for thermal constraints
on the truss segments. No good views of horizon in this
attitude so noctilucent cloud observations put on hold.
went back into XPOP however, noctilucent clouds are
not visible due to the position of our orbit with Earth's
terminator, at least that is what I surmise. Definitely
no clouds seen. I determined from our moving map program
the extent of our visible horizon from orbit when at
high latitudes. When at S50, the horizon extends from
S25 to S70 or 45 degrees in latitude! When at S42, the
horizon extends from S22 to S64 or 42 degrees in latitude.
In longitude, the horizon is 42 degrees wide. The horizon
becomes disproportionately skewed to the high latitudes
when at high latitudes.
Jan. 3, 2003:
sighting of noctilucent clouds in some time, while moving
across the day-night terminator instead of the night-day
terminator like all early sightings. Station was at S50
E65 which is way south of India. Good display at two visible
atmosphere thicknesses above the horizon for about 3 minutes
with a 20 degree arc on the horizon. Some layering was
visible in the telephoto lens. At the same moment, there
was a brilliant display of aurora australialus forming
a bright green arc with one fat edge. There were upward
directed streamers dancing over the tops of the auroral
arc. Was most beautiful. I was able to observe the aurora
for perhaps 6 minutes, until our motion caused it to move
from the field of view. This was the first time I have
seen aurora australialus and I was not disappointed.|
noctilucent cloud sightings as moving at sunset into darkness
with station at S50 E40. Saw the most amazing structure.
At first, there were two distinct clouds, both about 30
degrees in extent on the horizon separated by a gap of
about 30 degrees. As we moved across the terminator, maybe
over 1½ minutes, the gap narrowed and the two clouds
merged, forming one cloud that was perhaps 45 degrees
in extent. Was able to follow this cloud for another 2
minutes. It is obvious that these clouds are present over
an extended area and I can only see a sectional wedge
due to the required viewing geometry. Most fascinating!
Also saw a weak display of aurora australialus.|
noctilucent clouds, weak aurora over Australia. |
noctilucent clouds, no aurora.|
Jan 8 2003:
attitude to LVLH +XVV in preparation for next week's EVA.
No more horizon views for observation of noctilucent clouds
for awhile. |