In the chapter preceding we looked at the masks of the Sun and the ways we see them: the photosphere in white light, the chromosphere with the spectrograph and spectroheliograph, the corona at eclipse and with coronagraphs, and the magnetic field with a spectrograph-magnetograph. Each method was developed to observe the Sun from the surface of Earth, and in varying degree, each is limited by the ocean of air that lies above.
It was to reduce the effect of the ocean of air that Lyot carried his coronagraph to the summit of the Pic du Midi, and why most solar observatories are built on mountaintops. Astronomers have long known that the ideal observing site lies above the dense and dirty ocean of air, and as methods have been developed to rise above the atmosphere, solar astronomers have been quick to put them to use. Astronomical apparatus was carried aloft in early balloon ascents in the 18th century. Solar and galactic cosmic rays were studied from instrumented balloons in 1911. As early as 1923, astronomers climbed into the open cockpits of aeroplanes to observe a total solar eclipse from above the clouds. In 1935, when the National Geographic Society and U.S. Army sent the manned balloon Explorer II to a record height of 22 km, instruments were on board to measure the solar spectrum. When instrumented rockets came to science at the end of World War II, solar physicists were waiting to put them to use in exploring the unseen ultraviolet and X-ray radiation of the Sun. In keeping with this tradition, the first sophisticated unmanned satellites were dedicated to solar observation-the eight highly successful Orbiting Solar Observatories which have kept watch on the Sun for more than the 11-year solar activity cycle, from 1961 through 1978. On Skylab, the world's first long-term orbital laboratory, solar observation took much of the budgets of space, weight, and time.
TRANSMISSION OF THE ATMOSPHERE
The first and most important reason for ascending above the atmosphere is to push back the limits of the solar spectrum that can be seen and studied. At the surface of Earth, we receive only part of the solar radiation that strikes the top of our atmosphere. The ocean of atmosphere acts like a filter, letting certain wavelengths pass but screening others, some completely. Most of the light in the visible part of the spectrum passes through our atmosphere unabsorbed, but the shorter (ultraviolet and X-ray) and longer (infrared) wavelengths are selectively absorbed by various gases in the upper atmosphere. If we point a ground-based telescope at the Sun and record its complete spectrum, from one end of the wavelength scale to the other, we will find our spectrogram almost blank for wave-.....
....-lengths shorter than violet and longer than red. If we take the same instrument aloft, in a rocket or satellite, we will find the same spectral region covered with intense emission lines and a strong and variable continuum.
These nonvisible parts of the solar spectrum are important because they originate in the Sun's most active layers: the upper chromosphere, the corona, and the region of transition between. These regions radiate most of their energy in the ultraviolet and X-ray wavelengths, as do solar flares. Thus if we "look" at the Sun in these wavelength regions we "see" these layers and these features with great advantage. A spectroheliograph that operates in the ultraviolet region of the spectrum can make full disk pictures of chromospheric layers that cannot be seen in other ways. In the far ultraviolet it can record the otherwise unseen transition region in all its detail over the full disk of the Sun. Because very little X-ray emission comes from the underlying photosphere, an X-ray telescope can make pictures of the corona, not just at the limb of the Sun as at eclipse or with a visible coronagraph, but over the entire disk.
These possibilities offer tremendous advantages over ground-based visible light observation. They offer views of the layers and processes that hold the keys to many of the secrets of the Sun.
We have been able to observe the Sun in detail in these regions for only about two decades. But in the 20 years we have learned more of the Sun than at anytime since the invention of the telescope.
A second advantage in rising above Earth's atmosphere is improved image steadiness, or "seeing." Light rays from the Sun are distorted and bent by changes in the temperature and density as they pass downward through the ocean of air. These distortions make stars twinkle when seen with the naked eye and dance about in the eyepiece of a telescope. They blur an extended object like the Sun, because rays from different parts of it are....
.....distorted in different ways. Atmospheric distortions cheat all astronomers, but they are worse in the daytime when temperature gradients and fluctuations are most severe. Even on bright clear days, atmospheric distortion can blur most of the detail from the solar image, leaving only the largest features. In typical observation, the solar limb appears to boil and seethe, as projected in a telescope.
Certain atmospheric conditions are conducive to good seeing, and astronomers have attempted to locate their telescopes in areas where these conditions are most frequently met. These are not always at high altitude~-some mountain locations have poor seeing, and there are places at sea level where excellent image steadiness is often found. As a rule, however, the most dramatic improvements.....
.....in daytime seeing come with altitude. This was impressively demonstrated by Martin Schwarzschild and his colleagues of the Princeton Observatory who, in a series of pioneering unmanned balloon flights in 1957 and 1959, sent an automated 25 cm telescope aloft to photograph extremely fine details of photospheric granulation and sunspots.
A third advantage is a darker sky, which is of great benefit to coronal observation. In chapter 2 we found how coronagraphs are limited by the background sky brightness and how this is overcome in part by climbing to thinner air and darker skies. The deep blue skies of the mountains are a part of their striking beauty and appeal; in the mountains the Sun looks whiter and the shadows sharper and darker. On the clearest days on high mountains the sky looks blue-black, but even then it is still brighter by far than the corona, except at the very edge of the Sun. A coronagraph on a mountaintop must wait for these clearest days to see the corona really well, and even then it has to pick it out against a background "glare" as bright as the corona itself. The outer corona, several hundred times dimmer, is unobservable from mountaintops with present coronagraphs.
At balloon altitudes of 25 to 30 km (six times higher than mountaintop observatories) the sky in the visible region of the spectrum is nearly 100 times darker, but even there the sky is so bright that most of the outer corona cannot be seen. At an altitude of about 50 km above sea level the daytime sky is as dark as that seen on Earth during a total eclipse, which still limits practical observation of the corona to 5 or 6 solar radii. At satellite altitudes of 200 to 400 km, so little air remains that the daytime sky near the Sun is truly black offering no limit to observation of the faintest details of the white light corona.
A fourth and final advantage in extraterrestrial observation of the Sun is freedom from clouds and the interruption of the long terrestrial night. Many of the important questions of the Sun-especially those that deal with solar activity and change- require a more or less continuous patrol of the solar surface for their ultimate answers. This was the basis for an extensive collaboration of world solar observatories during the 1957-58 International Geophysical Year. During that time efforts were made to keep a coordinated and continuous patrol of the Sun by the organized cooperation of a chain of solar observatories around the world. As the Sun set on solar telescopes in southwestern United States, it was kept in view by stations farther west-in California and Hawaii and then Japan and later India, the Soviet Union, and the countries of Western Europe. The afternoon solar patrol in Europe was overlapped by the morning watch of the Sun in America, and so on.
This system is kept up today, informally, be  cause solar observatories around the world cooperate and share their data fully. But problems of two sorts make this worldwide chain of surface stations less than perfect. The first is meteorological. Key stations, or sometimes all stations, can be blocked from seeing the Sun by persistent weather patterns, resulting in long data gaps. In addition, subtle differences in sky conditions from station to station make the combined data patchy and nonuniform. The second problem is instrumental. Different instruments at diverse stations around the world are not alike and are not uniformly calibrated. Different observing procedures, and even different observers, portray a slightly different Sun. Even the simple matter of recording the number of sunspots on the Sun requires individual correction factors for specific telescopes and operators to be brought into agreement. For some problems these differences are not important; but for others they are crucial.
A single telescope in orbit around Earth can keep a nearly continuous watch on the Sun-a procedure that offers many advantages, including economy. The data are uniform and uniformly good. There are no clouds or other variations in the background sky; it is always clear and monotonously black. Observations are interrupted only when the spacecraft moves across the night side of Earth, but the spacecraft night is short. In the most unfavorable orbit the cycle for an Earth-orbiting spacecraft is about a 45-min day, followed by a 45-min night. In more polar orbits the Sun can be watched continuously for much longer periods with instruments that are always "on station and ready."
Skylab brought together, in a single orbiting observatory, solar instruments to capitalize on each of the advantages of observation above Earth. Three sophisticated solar spectrographs watched the Sun in the full span of the ultraviolet, with almost no absorption between their apertures and the Sun itself. Two X-ray telescopes brought to focus the full and unattenuated X-radiation of the Sun. White light and telescopes kept a constant patrol of the Sun through the clear and steady vacuum of space, unhindered by vacillating currents and clouds in the ocean of air beneath them. An Evans white light coronagraph took continuous pictures of the corona against a background of black sky. For 251 days and nights Skylab kept its arsenal of telescopes directed at the Sun from a vantage point 435 km above the surface of Earth.