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Sun and Earth: Studying A Run-of-the-Mill Star

It turns out that our Sun is very much a run-of-the-mill star. . . . It might be passed over as too ordinary for special study by an astronomer from another galaxy. . . . Notwithstanding the Sun's probable lack of interest to astronomers from other worlds, the fact that we inhabit a piece of rock orbiting this particular star makes it for us unique, and supremely interesting.

The Sun, Our Star, Robert W. Noyes,
Harvard University Press, 1982

Not long after Walter Orr Roberts was appointed UCAR's first president and NCAR's first director a quarter-century ago, the solar research institute that he had headed, the High Altitude Observatory of the University of Colorado, became a division of the new research center. The rationale for making an astronomical institution part of an atmospheric research organization was sound. As Robert MacQueen, director of the High Altitude Observatory (HAO) from 1979 to 1986, puts it: "The radiative input from the sun is the driving force for all atmospheric motions. Anything that alters that radiative input in any way is important for understanding climatic variations and other large- scale changes in the terrestrial atmospheric system."

Robert Noyes, professor of astronomy at Harvard University and senior astrophysicist at the Smithsonian Astrophysical Observatory, concurs. "It may have been a historical accident, but it has worked out very well," he says. "The sun drives the solar-terrestrial environment, and there is abundant evidence that it affects the atmosphere of the earth in many other ways. So it makes a lot of sense for solar research to be a part of NCAR."

The benefits flow in two directions, according to MacQueen. "Both sides of the house—atmospheric science and solar physics—have benefited from our association at NCAR," he maintains. "A lot of solar physics research has to do with small-scale processes on the sun. These processes are important. But people in solar physics are sometimes seduced into working on tiny little fascinating problems and losing sight of the larger scientific context. I think that being here at NCAR has given our solar physicists a unique perspective. We can better appreciate the global sun—the processes that dominate large-scale changes in its gross structure—because we are working in the broad context of the sun-earth system. HAO is one of the strongest institutions in the world in research on global solar processes and efforts to bridge the gap between the sun and earth systems."

Since its earliest days, a great deal of research at HAO has focused on the solar corona—the very hot but extremely dim outer part of the sun's atmosphere. The best observations of the corona are made during total solar eclipses, when the moon blocks off the flood of light from the face of the sun that normally overwhelms structural details of the corona. Over the last 20 years, HAO scientists have photographed eight total solar eclipses, traveling to such remote locations as East Africa, New Guinea, and the high desert of Bolivia to set up their special coronal cameras and other instruments in the path of totality.

Between eclipses, the corona can be observed with a coronagraph, an instrument that uses a metal occulting disc to block off the face of the sun, creating an artificial eclipse. The coronagraph was developed by a French astronomer, Bernard Lyot, in 1930. In 1940, Walter Orr Roberts, then a Harvard graduate student in astronomy, came to Climax, Colorado, to set up the Western Hemisphere's first coronagraph at a station high on the Continental Divide at the crest of the Rockies. That solar observing station evolved into the High Altitude Observatory.

The late Gordon Newkirk, of NCAR's High Altitude Observatory, with Coronascope II, a balloon-borne solar telescope used to test concepts that are now used in satellite solar astronomy.

The quality of ground-based coronagraph observations is limited by clouds, dust, and water vapor in the atmosphere, which act like a dirty windowpane between the observer and the sun. The late Gordon Newkirk of HAO used Coronascope II, a pioneering balloon-borne coronagraph developed in the early 1960s, to observe the corona from an altitude of 100,000 feet, above 80% of the earth's interfering atmosphere. It was a prototype for satellite coronagraphs designed to orbit completely beyond the atmosphere.

"Over the last 25 years, space observations have given us an image and understanding of the sun that could not have been obtained in any other way," MacQueen says. "Solar physics has benefited enormously from man's ability to put instruments into space. Without observations from space, something as basic as the variation of the solar atmosphere at various distances from the sun could not be understood nearly as well as we understand it now. One very important part of that structure—the transition region—was first discovered in the late 1960s through observations made with one of the Orbiting Solar Observatory (OSO) satellites."

HAO's first satellite coronagraph was orbited in 1973 and 1974 aboard Skylab. In eight months of operation, it collected thousands of coronal photographs during the quiet phase of the 11-year solar cycle. These photos revealed that the corona is far more turbulent than scientists had suspected. Another HAO coronagraph was launched in February 1980 aboard the Solar Maximum Mission satellite. It was one of seven instruments carried aloft to observe the sun during its period of maximum activity. The year-long Solar Maximum Mission examined how the corona evolves and tried to determine the sources of energy for these events. After a year this satellite failed, but in 1984 astronauts repaired both the satellite and the HAO coronagraph, extending their lifetimes. Scientists at HAO are studying results from these satellite missions to try to understand the nature and course of ejections of material from the sun's corona.

Another major experiment to study the sun's atmosphere from space is being planned. The space shuttle is expected to put a Spacelab in the earth's orbit, carrying, among other instruments, an HAO coronagraph designed to study the solar wind and the mechanisms that heat the corona to more than 2 million degrees Celsius.

In spite of the unique capabilities of earth-orbiting instruments such as satellite coronagraphs, natural total solar eclipses are still important. "An eclipse gives us one thing we can't get any other way," MacQueen says. "It reveals the detailed form of the corona with a clarity that has not been matched by any other observations from the ground or from space. Although eclipses are no longer the central focus of solar observing programs, they still have a very important role to play."

This giant solar prominence was photographed with a coronagraph at Climax, Colorado, as it erupted from the sun on 4 June 1946.

The High Altitude Observatory's most recent eclipse observations were made in June 1983, when four HAO scientists jointed an NSF-supported eclipse expedition to Indonesia. They used a new technique to measure the density and movement of material in the corona during this relatively long total eclipse, which lasted nearly five minutes. The Indonesian observations were part of a joint observing program with the University of Hawaii's Haleakala Observatory and HAO's Mauna Loa Solar Observatory in Hawaii.

MacQueen says that solar and atmospheric research share a common research approach. "The essence of these observational sciences is this: first you take observations and reduce them to an understandable form. Then you build theoretical constructs or models based on your first limited observations and test those models with more detailed observations and with inferences of your theory," he explains. "Finally you either keep or reject the theory and press on. That's astronomy, that's astrophysics, that's atmospheric science."

One area of common interest to solar and atmospheric scientists is the effect of solar activity on the earth's weather and climate. Climate modelers have speculated that a variation of as little as one percent in the input of solar energy to the terrestrial system could cause substantial climatic changes. But until fairly recently the sun was considered a stable and unchanging source of radiation. The input of energy from the sun to the top of the earth's atmosphere was known as the solar constant.

A natural eclipse of the sun reveals the detailed structure of the corona with a clarity that is unmatched by any other kind of coronal observation.

An important characteristic of the sun that was long considered roughly but reliably cyclical is the rise and fall of sunspot activity. Sunspots appear as dark blotches on the face of the sun. Galileo is usually credited with discovering them in the early 1600s, but at least three other European scientists observed sunspots with telescopes at about the same time. By the mid-nineteenth century, astronomers had discovered that the number of sunspots rose to a maximum and fell to a minimum on a cycle that averaged 11 years, although it sometimes was as short as 8 years, or as long as 15. However, historical studies by HAO's John Eddy in the mid-1970s established that this cycle has been interrupted by several long periods when sunspot activity was either virtually nonexistent or very high.

Although theories that sunspots might be clouds over the sun or solid objects between the earth and the sun were proposed and discarded, nobody really explained their nature satisfactorily until 1890. The American astronomer George Ellery Hale developed an instrument that could measure magnetic fields on the sun and used this magnetograph to determine that sunspots are giant magnetic fields, a thousand times as strong as the earth's magnetic field, covering areas larger than the earth. These magnetic fields, created by motions of the electrically charged particles that compose the gaseous atmosphere of the sun, produce sunspots by blocking the flow of hot, luminous gas from the interior to the surface of the sun. Other manifestations of solar activity—solar flares, prominences, and coronal streamers—are also shaped by the magnetic fields that produce sunspots.

More recently, magnetograph observations have established that the large-scale magnetic field of the sun reverses itself each time the number of sunspots reaches a maximum, so that at one sunspot minimum the sun's magnetic field is parallel to the earth's magnetic fields and at the next one it is opposite to it. Thus there is a 22-year double sunspot cycle superimposed on the 11-year one.

The high temperatures of the solar corona drive the solar wind, made up of electrically charged subatomic particles—protons and electrons—that flow continuously out into space. Storms on the sun—violent disturbances in solar magnetic activity—produce disturbances in the solar wind that result in electrical activity in the earth's upper atmosphere. This causes brilliant displays of the aurora borealis—the northern lights—and disturbances in the earth's magnetic field know as geomagnetic storms, which can affect radio communications, long-distance telephone calls, electric power transmission, and electronic navigation systems. However, the amounts of energy involved in these phenomena of the upper atmosphere have always been considered far too small to affect weather systems.

It seemed reasonable to some scientists to assume that sunspots might block off part of the sun 's energy output and cause climatic variations. As early as 1802, one astronomer suggested that the rainy regions of the tropics showed temperature fluctuations that varied inversely with the number of sunspots. This seemed quite logical at the time, as the sunspots were dark and presumably lowered the sun's radiant output. Later researchers worked to relate patterns of weather and climate to fluctuations in the activity of the sun, but the results were far from conclusive, and, for the most part, the scientific community remained skeptical. Apparent statistical correlations have been proposed between sunspot activity and atmospheric phenomena such as North American storm systems and recurring droughts in the U.S. Great Plains.

One important development in the study of relationships between solar activity and terrestrial climate has come from recent satellite observations. An extremely sensitive photometer on the Solar Maximum Mission measured variations in the sun's output of visible light on the order of 0.1 to 0.3% over periods of days to months. "This level of change, integrated over a long time scale, can be very important climatically," MacQueen says. "Climate modelers at NCAR and other institutions are looking very closely at the implications of these variations in the radiative input."

As far as apparent sun-weather statistical correlations are concerned, MacQueen is among the skeptics. "I am absolutely convinced that the only way we're going to make any progress in studying sun-weather questions is through a long-term, concerted attack on the physical processes that would be involved," he declares. "Some of the statistical correlations are intriguing, and they may give us clues as to what part of the physical system we should concentrate on. But I think the only way we can really get a handle on sun-weather relationships is through a detailed understanding of physical interactions between the solar wind and the upper atmosphere and how they couple with radiative processes—a whole hard system approach, in which each step is physically meaningful. And that is going to be really tough."

MacQueen says that one of the most exciting areas of solar physics research today is solar seismology, or helioseismology. "It's a new technique, developed over the last six or seven years, that lets us do something that was beyond the wildest imagination of anyone working in astrophysics a couple of decades ago—probe the interior of a star."

Scientists at HAO and Sacramento Peak Observatory in New Mexico have designed an instrument that is used to observe and analyze the oscillations of pressure waves propagating outward from the interior of the sun. "At least in theory, you can tell something about the material a bell is made from by analyzing its resonant mode—the tone that it emits when it is rung," MacQueen says. "That's what we're trying to do with the sun's pulsations, using the new instrument at Sacramento Peak. We call it seismology—an analogy with the technique used to probe the earth's interior structure by analyzing seismic waves produced by earthquakes."

Robert Noyes, who has been deeply involved in helioseismology, says that these new measurements of motions in the interior of the sun can lead to a better understanding of the solar activity cycle. "This is one of the most important issues in solar physics," Noyes asserts, "especially in terms of solar-terrestrial relationships. Almost every effect of the sun on the earth is magnetically induced through the sunspot cycle, whether it's the influence of solar flares on the upper atmosphere and the aurora or the effect of fluctuations in the ultraviolet flux from the sun on ozone formation in the upper atmosphere."

The observing station near Climax, Colorado, established in 1940, housed the Western Hemisphere's first coronagraph.

A fundamental solar-physics phenomenon that is currently getting a good deal of attention at HAO, the acceleration of the solar wind, was first identified in 1958, at about the same time that the National Academy of Sciences Committee on Meteorology was recommending that NCAR be established. Eugene Parker, a University of Chicago physicist, concluded that the solar corona was continually expanding outward in an outflow of subatomic particles. This outflow, Parker theorized, should have a velocity that was undetectably small near the sun but that increased with the distance from the sun. The reality of the solar wind was confirmed in the early 1960s by observations made by the first scientific satellites. In 1962, the Mariner 2 spacecraft, on it way to Venus, clocked great gusts in the solar wind that doubled its velocity in a few hours.

"The acceleration of the solar wind is an enormously important astrophysical question," MacQueen says, "because the flow of material away from the sun takes angular momentum with it. Ultimately, like every star, the sun will spin down as it loses angular momentum. So in terms of understanding stellar evolution, it's important to understand how much material is flowing out, the role of the magnetic field and other details of how the solar wind really works."

Noyes points out that the solar wind is important in solar-terrestrial relations as well. "The earth is surrounded by the outer corona of the sun," he says. "We need to understand how much material is being ejected, where it comes from, and what triggers it. These things are fundamental to solar-terrestrial physics."

"The whole question of mass ejections from the sun has been getting a lot of attention at HAO," MacQueen says. "Skylab and the Solar Maximum Mission provided us with excellent data sets. We know now that mass ejections occur much more frequently than anyone imagined, and that they transport 5 to 10% of the solar wind outward in discrete belches, as it were. When a mass ejection from the sun reaches the earth's upper atmosphere, it causes perturbations in the magnetic field, interference in radio communication, and so on. We're trying to determine whether the mass ejections are magnetically driven or result from a pressure pulse at the base of the solar atmosphere. For the last five years or so, we've been using observational evidence to generate theoretical models that we can play back against the observations to refine our physical concepts."

Much of HAO's work is done in close collaboration with scientific visitors, many of them from Europe and Asia as well as U.S. universities. "Some HAO visitors come from Japan and China," Noyes says, "and this is very important, because few other U.S. institutions have interacted with solar researchers from those countries."

"Roughly 40% of our visitors over the years have come from other countries," MacQueen confirms. "This high percentage reflects strong ties with foreign research institutions as well as individual scientists. We've had a number of visitors from the Max Planck Institute in Germany, the Meudon Observatory in France, and the University of Tromsø in Norway. We have had a series of visitors for five or six years now from the Beijing Observatory in the People's Republic of China, and we have a long-term visitor from the Wroclaw Observatory in Poland. We support a graduate assistantship at the University of Sydney, in Australia, that allows the student to work here for six months."

"HAO has pursued a very strong scientific visitor program," Noyes says, "and it has been very successful. I think that success can be attributed to a widespread perception that there is no better place in the United States to do solar research."

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