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IMAGE: ISS Science Officer Don Pettitt
ISS Science Officer Don Pettitt works inside the Zvezda Service Module.
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Don Pettit Space Chronicles

Expedition Six
Space Chronicles #3

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By: ISS Science Officer Don Pettit

Noctilucent 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.

Our digital 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.

noctilucent 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.

Notebook logs:

GMT day 351,
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.

21:30 No noctilucent clouds visible.
GMT day 352,
Dec. 19, 2002:
06:00 Near the tip of South America, no noctilucent clouds visible.
09:05 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.
10:40 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.
21:40 same observations of noctilucent clouds, a most wonderful sight.
GMT 354,
Dec 20:
12:00 No noctilucent clouds visible.
19:40 No noctilucent clouds visible.
GMT 355,
Dec 21:
7:58Spectacular 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. Spectacular!
14:15Very weak display noctilucent clouds. Station at S25 W120.
11:02Very weak noctilucent cloud display, only 5 degrees on horizon and very faint. Over Australia.
GMT 356,
Dec 22:
 Changed 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.
GMT 360: 

We 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.

GMT 003,
Jan. 3, 2003:
16:04First 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.
17:34More 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.
GMT 004,
Jan.4, 2003:
13:40No noctilucent clouds, weak aurora over Australia.
15:06No noctilucent clouds, no aurora.
GMT 008,
Jan 8 2003:
 Changed attitude to LVLH +XVV in preparation for next week's EVA. No more horizon views for observation of noctilucent clouds for awhile.

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/20/2004
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