[79] The atmosphere is an extremely complex and variable structure. It has been studied from the ground and from various vehicles. Skylab's position above the atmosphere and the long duration of the missions enabled the crews to gather information about the atmosphere that could hardly have been obtained in any other way.
Figure 6-1 shows a sunrise photographed from Skylab. Orbiting the Earth once every 93 min, the astronauts of Skylab saw 15 sunrises during every 24-hr period.
The atmosphere is almost homogeneous up to some 95 km. The composition is fairly constant and much the same as at sea level. The principal constituents are nitrogen (78%), oxygen (21%), argon (1%), and-minor constituents (-0.04%). Water vapor, aerosol particles, and an ozone layer are also present. Above the homogeneous zone the gases separate according to their masses....
....under the influence of gravity. Molecular oxygen and nitrogen occupy the lowest levels, whereas helium and hydrogen dominate at the highest levels. Between them lies a level of predominately atomic oxygen, which has an intermediate mass.
In the thin upper reaches of the atmosphere, above 50 to 55 km, is the ionosphere where sunlight ionizes the air, producing free electrons and ions of the gases. The density of electrons fluctuates between night and day, from season to season, and from place to place above the Earth's surface. Some of these variations are predictable, and the general structure of the ionosphere is fairly well understood.
The ionosphere is subject to many disturbances. One of these results in the emission of visible light called the aurora, a condition that occurs when high-energy particles interact with air at high altitudes.
Figure 6-2 shows diagrammatically the composition, temperature, characteristic phenomena, and electron-density profile of the atmosphere.
Many Skylab experiments concerned the Sun and observed it directly without interference from the atmosphere. In other experiments, the Sun's light was used as a probe to study the atmosphere itself.
The geometry of the experiments is seen in figure 6-3. As Skylab moves toward the Earth's shadow, the sunlight reaching Skylab begins to pass through the uppermost reaches of the atmosphere. As the orbit continues, the distance traveled by the light through the atmosphere becomes greater, and the light passes through lower and lower layers of the atmosphere. The reverse sequence is observed at dawn. Moonrise and moonset offer the same opportunity for study of the various layers of the atmosphere. Since Skylab made about 15 revolutions in 24 hr, there were many sunrises and sunsets each "day," occurring over widely different.....
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....locations. Because of the speed at which Skylab was traveling, the sunset or sunrise occurred so rapidly that atmospheric conditions did not change much.
Several methods have been developed for calculating altitude profiles, that is, changes in atmospheric properties throughout its height. Measurements of absorption of sunlight were made at several wavelengths from Skylab, to assist in establishing such profiles.
Figure 64 shows the curvature of the Earth and the space station's altitude to approximately true scale. For scale purposes, the typical altitude of a conventional airplane is shown as 11 km.
Photographs from Skylab of moonrise illustrate several features of the Earth's atmosphere. Figure 6-5 shows the illusion that the lower rim of the Moon has been lifted up or flattened. This is because the atmosphere's density increases rapidly toward lower altitudes. A ray of light traversing the dense, lower part of the atmosphere is bent (refracted) more than a ray traversing the less dense higher atmosphere.
In this picture the details of the Moon's surface [82] features are washed out. This is due to the scattering of light in the atmosphere. Scattering diminishes rapidly with altitude as the density decreases; therefore, more detail is visible on the upper part of the Moon than on the lower part, and the upper part is clearly brighter. The air above the horizon appears generally bluish because blue light is more strongly scattered that is red light. Since the Moon is full, the Sun is behind the space station, and the blue light in the atmosphere is back-scattered sunlight.
[83] Figure 6-6 shows the Moon higher on the horizon. The distortion has largely disappeared; the surface details have become visible; and the usual, although slightly reddened, color has begun to appear at the top. The next photograph (fig. 6-7) shows the Moon well above the horizon with no apparent atmospheric effects.
To the jetplane traveler, beautiful sunrises are a common sight. They display layers of various colors and thicknesses, indicating dust and pollutants in the air. Not much is known about local and global distribution of these manmade and natural constituents of the air.
Skylab maintained an enormous global overview and mapped many of the occurrences and changes in these atmospheric phenomena. Examples of such observations are shown in figure 6-8.
The series of photographs in figure 6-8 shows the progress from the first faint glow of sunrise to the point where the Sun can actually be seen underneath the cloud cover near the horizon. These photographs were taken for investigations by Donald Packer of the Naval Research Laboratory in Washington, D.C.
The exact interpretation of the various colors and strata is complicated because absorption in the ozone and oxygen layers cannot immediately be separated from scattering from aerosol or dust layers that may be present. For example, volcanic eruptions inject massive amounts of dust into the atmosphere.
The Earth's atmosphere was also investigated with a Skylab coronagraph T025, a camera with a central disk to occult the Sun so that the region around it (the corona) can be seen without interference from the extremely bright disk of the Sun. Figure 6-9 shows the circular field of view of this instrument. In the center is the occulting disk covering the Sun. Toward the lower right is the illuminated curved Earth showing the Earth's atmosphere as an ill-defined edge. The crescent-shaped feature is an instrumental ghost image, not a real feature of the atmosphere. The object shaped like a bowling pin reaching toward the lower left is the occulting-disk support, which is out of focus.
Figure 6-10 is an enlarged negative print of the Earth's limb and atmosphere taken in 250 nm ultraviolet light with this coronagraph for Mayo Greenberg of the Dudley Observatory, Albany, New York. The atmosphere is seen "edge on," and the amount of radiation passing through each level of the atmosphere can be investigated. It shows that a horizontally layered structure is present. A height scale has been superimposed on the photograph. At 250 nm, atmospheric ozone absorbs much of the radiation. The investigators compare photographs such as figure 6-10 with corresponding photographs at wavelengths (e.g., 360 nm) where the [84] influence of ozone is negligible. From quantitative determinations of the differences, atmospheric composition values can be deduced.
The observed atmospheric radiation at 360 nm is primarily due to scattering by air molecules. A smaller portion may be scattered by very small (less than 0.1 µm) particles in the atmosphere. Since the changes in brightness, as a function of angle from the sun, can be calculated for air molecules, brightness variations from those calculated indicate the presence of very small particles as a function of height. The occulting disk was used in these experiments because the aerosol or particulate matter in the atmosphere is detected more easily when one looks toward the Sun, that is, when the particles are illuminated from behind.
A more detailed and sophisticated picture of atmospheric properties can be obtained by studying the extinction, or gradual disappearance, of specific spectral lines of the Sun as it slowly sets beneath the horizon. During a sunrise or sunset, it was possible for Skylab to observe the atmospheric absorption profiles at a number of extreme ultraviolet (EUV) wavelengths using the solar observatory instruments.
The reason for the strong interest in these wavelengths is that the solar EUV radiation at wavelengths below 125 nm is the principal source of energy for heating and ionizing Earth's upper atmosphere. In particular, solar EUV radiation is absorbed at altitudes between 80 and 300 km.
One instrument on Skylab using this technique was an EUV spectroheliometer (S055) that was part of the solar instrument array and used primarily for studies of the Sun. That photoelectric instrument was designed to obtain up to seven simultaneous spectroheliograms in the range 28 to 135 nm with a spatial resolution of 5 sec of arc and also to perform spectral scans with a resolution of 0.16 nm over the same wavelength range. Because of its large size, the instrument had a sensitivity far greater than that of any photoelectric EUV spectroheliometer used in space up to the time of Skylab.
Examples of atmospheric extinction profiles, recorded at sunset on December 6, 1973, are shown in figure 6-11. Each spectral line in the figure is identified with the ionized element in the solar atmosphere emitting the line. The altitude at the horizon is the quantity designated Z in figure 6-3. From these profiles, investigators at the Harvard College Observatory found that it was possible to determine the temperature and composition of the Earth's upper atmosphere over the altitude range from about 100 to 360 km. The quantitative data were obtained by applying sequentially the known absorption characteristics of atmospheric constituents, starting with the longest wavelengths. Since the three longest wavelengths (133.57, 121.57, and 103.19 nm) are absorbed solely by molecular oxygen, which has a different absorption cross section for each wavelength, absorption at these wavelengths can be used to determine its density in the range 100 to 170 km. For example, the 133.57 nm line has the greatest cross section and hence less molecular oxygen is needed to absorb it. Light of this wavelength will therefore be completely absorbed higher in the Earth's atmosphere.
Solar radiation from doubly ionized carbon at 97.7 nm is absorbed both by molecular oxygen and by nitrogen. Once the oxygen density as a function of altitude has been determined, this profile can be used to determine its absorption of the 97.7 nm radiation. Subtracting this from the observed absorption gives the nitrogen absorption, from which can be calculated the nitrogen density in the 170- to 250-km region.
Finally, the two solar wavelengths from triply ionized oxygen (55.4 nm) and nine-times ionized magnesium (62.53 nm) are absorbed principally by atomic oxygen, with a small amount of absorption by nitrogen. These profiles can thus be used to determine the density of atomic oxygen over the altitude range 250 to 360 km. The temperature of the neutral gas can then be determined from the variation of density with altitude.
Figure 6-12 shows the number density of molecular oxygen determined from a set of absorption profiles taken during one Skylab sunset on September 15, 1973. The densities predicted by a theoretical model of the upper atmosphere are shown for comparison with the measured data.
The composite illustration in figure 6-13 .was reconstructed from data obtained with a solar ultraviolet spectrograph (S082) by investigators from the Naval Research Laboratory. These spectrograms of the setting Sun were taken on August 18, 1973, as Skylab's line of sight passed through progressively lower and more dense layers of the Earth's atmosphere. The minimum altitude Z of the light path above the Earth's surface is given in the scale at the left.
Molecular oxygen is the principal absorbing species, although the absorbance varies a great deal over the spectral region from 121.6 to 192.5 nm shown in figure 6-13. At 155 nm, absorption by molecular oxygen is the strongest, and the line pair (154.8 and 155.1 nm) due to triply ionized carbon is completely obliterated at an altitude of 145 km. Wavelengths above 190 nm are least affected, and the Sun's emission at these wavelengths penetrates much deeper into the atmosphere, down to altitudes that can be reached by high-altitude balloons.
The Lyman-alpha radiation from the Sun is broadened by turbulence and pressure in the Sun's atmosphere. The sharp absorption line seen at the center of this line is due principally to absorption by Earth's geocorona of hydrogen. Similar absorption "notches" can.....
.....be seen in the atomic oxygen lines near 130 nm; they are caused by the atomic oxygen in the Earth's atmosphere.
Measurements of spectral absorption by the Earth's atmosphere, such as those shown in figure 6-13, are the starting point for detailed atmospheric composition determinations. Results on the molecular oxygen distribution, for example, show large variations with time and geographic location.
Photographs of aurorae were made as part of a study (S063) of the upper terrestrial atmosphere by Donald Packer of the Naval Research Laboratory. Auroral displays are produced by charged particles that precipitate downward along geomagnetic lines of force into the high atmosphere in a zone circling each magnetic pole.
The formation of aurorae is related to geomagnetic disturbances, which in turn are often triggered by solar events and resulting changes in the solar wind.
One such solar event was a spectacular solar flare that began at 1140 GMT on September 7, 1973. About 2 1/2 days later, the particles ejected from the Sun during the flare began impinging on the magnetosphere, as indicated by a sudden decrease in the cosmic ray flux and rapid rises in geomagnetic activity indices, quantities measured throughout the world on a routine basis. Skylab's orbits carried it into the southern auroral zone at night during this period of atmospheric disturbance. Owen D. Garriott, the scientist pilot of the second manned mission, photographed some of the spectacular aurorae that occurred. Two pictures taken during this period are shown in figures 6-14 and 6-15. These photographs were made with a hand-held camera with a [87] 55-mm lens on Kodak High Speed Ektachrome. Exposure times ranged between 1 and 4 sec.
Figure 6-14 was taken at 2140 UT on September 9, 1973. It shows a very intense emission from the 557.7-nm green atomic oxygen line extending in a band parallel to Earth's horizon. Some discrete auroral forms are also present. Emission from atomic oxygen at 630.0 to 636.4 nm is also prominent as a red glow extending upward from the green band to beyond 250 km. Auroral rays above the main layer are parallel to the geomagnetic lines of force.
The photograph shown in figure 6-15 was taken at a shorter exposure I min later and slightly displaced from it. The aurora has evolved somewhat, now showing folds extending toward and away from the observer.
Approximately 2 days later, a discrete rayed arc appeared at night (fig. 6-16), with a thin shell of quiet luminescence viewed "edge on" at the left. The sharp upper boundary of this layer is characteristic of airglow, but the overall intensity appears to be excessive for airglow and probably includes radiation induced by particle bombardment. The full Moon illuminates the Earth in the foreground.
Approximately 1 min later, the complete rayed arc....
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....was seen (fig. 6-17). It extended over 400 km in length. The lower border at 120 km coincided with that of the luminous layer seen edge on at the right. Not visible in figure 6-17, but visible in the original, is another band extending to the left and 20 km lower in altitude. The lower band is at the same altitude as the luminous layer seen in figure 6-15.
Figure 6-18 was taken two orbits later on the same day, when Skylab was back close to the same position relative to the Earth and the Sun, but the Earth had rotated about 45° to the east (Skylab was therefore about 45° further west than it had been when the photograph shown in figure 6-17 was taken). Figure 6-18 shows an intense, green-rayed arc folded back on itself and merging into a thin, less bright homogeneous band extending toward the horizon.
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Figure 6-19, taken 20° farther west at early dawn, shows two distinct luminous layers above the brightly illuminated lower atmosphere. These layers are similar to those shown in figures 6-16 and 6-17, and are at 100 and 120 km. No discrete auroral forms are present. The top layer is whitish, and its intensity is twice that of the lower layer, which appears generally reddish.
The series of photographs in figure 6-20, taken during the second manned mission, show a fairly constant northern polar cap layer of aurora with a surprisingly persistent pattern of aurora over the horizon. The bright spots in the foreground are lightning; the yellowish light patterns are city lights showing diffusely through clouds. The location of the lightning flashes varies from picture to picture, whereas the cities form a constant pattern on the ground.
The atmosphere is very complex and changeable; despite the many techniques that have been used in the past to study it, it has been very imperfectly understood. Skylab has permitted observation from a new perspective and also the use of new techniques.
