For solar observations it is essential to note that our atmosphere is opaque to portions of the sun’s radiation. Only the so-called visible wavelengths, from about 400 to 800 nanometers (4,000 to 8,000 Angstroms) can pass through
the atmosphere without significant absorption. It is presumably no coincidence that this just happens to be the wavelengths at which our eyes are sensitive to light; while it is presumably a coincidence that the sun’s major radiation band is passed through the atmosphere to warm our planet. But the sun also radiates substantially at even shorter wavelengths in the ultraviolet, extreme ultraviolet, and x-ray ranges. It is again fortunate that the atmosphere does absorb these rays as they would otherwise be lethal to all forms of life here on the Earth’s surface. Without the thick atmosphere (and the Earth’s magnetic field for protection against arriving charged particles) we might all be living in caves, underground, or in the water.
So the Skylab was equipped with eight large telescopes in a large canister called the Apollo Telescope Mount, most of which had to be above almost all of the atmosphere to function properly. Two of the telescopes looked at the sun at visible wavelengths called “hydrogen-alpha,” radiation coming from excited hydrogen atoms in the solar atmosphere. As this wavelength can be seen with ground-based telescopes, it provided a means of coordinating observations with terrestrial observatories all around the globe to look at a particular feature and compare observations. Three more telescopes were sensitive to the extreme ultraviolet and two to the x-ray portion of the spectrum. The sun looks quite different at each of these wavelengths, and solar astronomers use these differences to deduce many things about the sun. For example, each wavelength of emission is directly related to a different temperature of the emitting atom and therefore to a different altitude in the solar atmosphere, which permits a much better picture of the sun’s structure to be constructed.
The eighth telescope was a White Light Coronagraph, which could have worked on the ground except for the fact that the sun’s disk is about one million times brighter than the faint corona. Light scattering in our atmosphere from the bright disk completely swamps the faint coronal light here on Earth. The one exception to this occurs at times of a total solar eclipse, when the moon passes directly in front of the sun and blocks out its bright light. So only for a few minutes every year or so, at some places in the world (usually remote, it seems), it is possible to see the solar corona. But on Skylab an occulting disk was placed in front of the telescope to replace the moon, and the crew could then see the corona continuously whenever they were not on the dark side of the orbit.
So the ground astronomers and the crew remained in close contact every day. The ground teams worked up plans for how the crews could best optimize their limited observation time. There could be at most periods of about fifty minutes of sunlight, coming about ten times each twenty-four hours. Although there were almost sixteen orbits each Earth day, the crew was sleeping or otherwise occupied on some of them. This still permitted substantially more observational time than in the original flight plan. The ground team noted what the likely interesting solar features might be, argued within their own ranks about which instruments should have priority on each occasion, then had to argue with other research disciplines when special circumstances arose, sometimes taking their case all the way up to the flight director who controlled the whole mission for a decision. Then when their plan was sent up to the crew on a teleprinter, the crews would plan the orbit’s activity and usually follow the ground’s suggestions. But the final pointing of instruments always had to be done onboard, and whenever solar conditions changed or the crew saw evidence of even more interesting phenomena, they might well change the ground plan and proceed with an alternative observational program.
With the extreme ultraviolet telescopes the view presented was of gases at tens of thousands of degrees Celsius, coming from the chromosphere above the photosphere, which is the part of the sun the naked eye sees. With the x-ray telescopes the crew was viewing gases at millions of degrees Celsius, where the emission comes from even higher in the corona. All of these features differ when the sun is very active as compared with a cooler and very quiet sun. The sun’s activity varies over a cycle of about eleven years. When Skylab was first planned, it was hoped to have a very active sun, which would be especially interesting to most of the solar astronomers. But flight delays of several years pushed the launch date out into the quiet portion of the cycle. Then nature came to the rescue. As it happened during the second mission, the sun became very active, at least on one side, and the sunspot number (a measure of the number of visible black dots and groups on the surface of the sun) varied all the way from io to over 150 at other times. So very good observations could be made from quiet to very active solar conditions.
The White Light Coronagraph stood apart from the other experiments in that it looked at the very faint light coming from far above the disk of the sun. Of particular interest were phenomena then called “coronal transients,” but now called “coronal mass ejections,” or cmes. In these cases a long magnetic strand lifts off the lower atmosphere and expands into an enormous loop far out into the corona like a stretched rubber band. Occasionally, the band is stretched so far that it breaks apart and the confined gases escape into interplanetary space.
When this happens a coronal mass ejection occurs and if it is pointed in the correct general direction, it will eventually arrive in the vicinity of the Earth. This usually takes two to three days. When it gets here, it frequently produces not only the beautiful aurora but also not-so-desirable power line fluctuations or outages and sometimes damage to sensitive satellites in Earth orbit. Even though satellites in Earth orbit may be damaged, it could be far worse for space probes or manned spacecraft far outside the magnetic envelope (the magnetosphere) of the Earth. In this case very energetic solar protons could be more dangerous to space probes and people, whereas within the magnetosphere, the magnetic field largely turns away the charged particles streaming outward from the sun.
Garriott reported one of the most exciting observations of his mission was the day the first major cme was observed. “As I recall, the ground called us and alerted us to the possibility that one of the low magnetic loops appeared to be rising. When we first looked at the White Light Coronograph, indeed, the loop was already extended roughly a solar radius or half a million miles above the photosphere. So I promptly took a Polaroid picture of the screen while we also activated others of the telescopes to record relevant data as well. Then when we came back around the Earth in about an hour and a half, I took another Polaroid photo, and sure enough the loop had expanded by perhaps another two solar radii into the corona. A quick calculation told us that the outward velocity of the cme was at least 500 kilometers per second, a phenomenal speed it seemed to us! As far as I know, this was the first visual observation of a cme and the first real-time measurement of its minimum speed. When analyzed more carefully on the ground, the same number was calculated.
“Also of interest to us were small spots visible in the extreme ultraviolet, but not on the hydrogen-alpha visible images. For lack of better terminology, we called them ‘xuv bright points.’ We didn’t really know what they were or their relevance to other solar events. We hoped they might even be precursors to a solar flare, for which a full study was a very high priority. They didn’t seem to live too long, maybe thirty minutes, then faded away. We spent quite a lot of time studying them in space, and subsequent analysis on the ground revealed that they are the source of much of the solar magnetism. In addition we felt that they could be the location of new solar flares, so we watched them carefully. Indeed, Ed used this clue to provide images of the very early stages of a flare by following the development of one of the ‘bright points.’”
Another feature studied at considerable length was coronal holes, which are areas of very low emission. They can be discerned all across the spectrum, but especially in hydrogen-alpha emission and at xuv wavelengths. Both the ground and the space crew could see them, permitting close cooperation in selecting targets and timing. What was less expected was to see the close association between areas on the sun’s face where there was very little emission (the “holes”) and the solar wind, which permeates all of interplanetary space. It appears that the “holes” are areas where any local magnetic fields do not form loops, but instead are essentially open. For this reason, any hot gases from the sun, which would otherwise be trapped by the solar magnetic field, can now flow straight out into space.
While the results of Skylab solar observations seemed to exceed even the ground investigators’ expectations, one of its most valuable benefits was the instruction it provided to later experimenters who wanted even longer continuous observations of particular features. The corona and cmes? Now the researchers knew roughly their frequency and speed. They knew to connect them to appearance of coronal holes visible on the ground. Bright points? Solar physicists have now been studying them for decades building from the pioneering work on Skylab. Much of the next phases of solar research have been done with automated satellites based on the results found on Skylab.