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Basic research

Why are some of the world's top physicists studying solar variability?

It's so magnetic.

This sunspot (bottom) generates complex magnetic fields (above) and a concentration of energy called a filament (red, center). Illustration courtesy Bruce Lites, NCAR/HAO.

The sun pulls us in more ways than one. Large enough to encompass more than a million earths, it keeps us in our yearly orbit. The sun's psychological pull is just as strong in myth, poetry, and music. But it is the sun's variability that attracts scientists.

The outer 30% of the solar sphere is an envelope of hydrogen and helium plasma that generates magnetism through constant, heat-driven churning. Some of this plasma spews outward from time to time in unpredictable storms, called solar mass ejections, that are tied to visible tracers of solar magnetism: sunspots, flares, and prominences. These structures vary in size, shape, and duration, but each reflects an outward push of magnetism from the sun's interior. Occasionally the mass ejections are strong enough to impinge on the earth's atmosphere, where their magnetic energy creates havoc with radio signals and communications satellites while producing spectacular auroras.

Several dozen scientists at NCAR's High Altitude Observatory (HAO) are tracing this epic trek of magnetic energy from the sun's interior to our own atmosphere. Three workshops were held at NCAR in 1995 and 1996 to launch the Solar Magnetism Initiative (SMI). In attendance were 100 of the leading experts in solar physics from North America, Europe, and Asia. "We thought this was the time to pull things together," says HAO director Michael Knoelker.

Plans for the initiative call for five to ten years of focused research at NCAR and collaborating universities and institutes. There may also be a new instrument deployed in space to inspect the sun's surface for the elusive small-scale components of solar magnetism. But a large part of the SMI's work will involve pulling together strands of existing research and coordinating future work to form a coherent whole.



The Solar Magnetism Initiative (SMI)


NCAR's High Altitude Observatory and the National Science Foundation, with involvement from many universities and research centers internationally


To integrate and support research on solar magnetism as the root of solar variability and a major factor in space weather

The solar wind (flowing left to right) affects magnetism and communications in the earth's atmosphere. Illustration courtesy NOAA.


Through a series of workshops and a proposed national research program with strong university participation


Workshops in Boulder; research at HAO and at other participating institutions; observing platforms across the world and perhaps in space


Workshops held in Boulder during 1995-96; research and instrument development to follow

Solar Magnetism Initiative (SMI)

Using special telescopes, scientists can examine images from the solar surface to infer qualities of the deeper levels. These images reveal that solar rotation has an interesting spin. Without the constraint of being a solid body, the sun's outer layer has different rotation rates at different latitudes--more revolutions near the equator, fewer near the poles. As if several adjacent gears were going at different speeds, the result is friction and, literally, noise. Sound waves bounce and reflect inside the sun as do ripples in a pond. Or, as HAO's Thomas Bogdan puts it, "The sun is ringing like a bell."

Over the past two decades, acoustic detectors have been routinely monitoring this noise by observing the undulations it causes on the surface of the sun. Armed with an understanding of how sound waves behave in a fluid that is stratified into discrete sections, like oil and vinegar in a bottle, NCAR scientists have unraveled the data to profile the structure and rotation of the solar interior.

For instance, Paul Charbonneau and Keith MacGregor have carried out computer simulations showing how the sun's rotation appears to have spun down over the 4.5 billion years of its existence, due to the magnetized "wind" of plasma emanating from its corona. When Charbonneau and MacGregor did their first such visualizations in 1993, they only had data on the outer 50% of the sun, encompassing the convection zone (the outer 30%, where plasma, heated from below, constantly mixes) and the fringe of the hotter interior (fueled by nuclear fusion).

Another HAO scientist was soon to delve even deeper. In 1994, Steven Tomczyk placed a new instrument at an NCAR observatory atop Hawaii's Mauna Loa. It measured very-low-frequency sound waves, some bouncing to the very center of the sun. To the surprise of many, Tomczyk found that the solar interior seems to spin more like the earth than the sun--in other words, at a constant rotational rate, regardless of latitude and depth.

Other scientists are training their instruments on the sun's surface. There, the data are more plentiful but the magnetic fields appear more complex. If the sun's outer layer is like a roiling sea of plasma, there are islands within it made of highly magnetized material. Bogdan is studying the largest and best known of these: sunspots.

Using sound-wave measurements, Bogdan studied a typically sized sunspot about as wide as three earths. He, along with colleagues Timothy Brown and Bruce Lites and NCAR affiliate scientist Jack Thomas (University of Rochester), found that the sunspot absorbed about half of the acoustic energy impinging on it during its life cycle of a few days. "Sunspots consume acoustic energy," Bogdan says. "My goal is to try and figure out what they do with this energy." He believes the incoming sound is being converted into magnetic waves that travel upward (ultimately heating the solar atmosphere) and downward (back into the sun itself). NCAR visiting scientist Oskar Steiner (Kiepenheuer Institute, Germany) has noted similar behavior in his computer simulations of a smaller magnetic structure called a solar element.

Solar researcher K. D. Leka. Photo by Carlye Calvin.

One NCAR instrument has shed new light on the magnetic "islands" at the sun's surface. The advanced Stokes polarimeter (ASP) has been deployed at the National Solar Observatory in Sunspot, New Mexico, for the past several years. It uses polarized light from the solar surface to form a picture of emerging magnetic fields. HAO's Lites, Andrew Skumanich, and postdoctoral researcher K.D. Leka recently discovered small magnetic counterparts to sunspots. "They seem to be short-lived, ranging from minutes to hours, but they also seem to be really numerous," says Leka. "We've seen evidence for these weak, small-scale fields before, but with the ASP we have been able to trace what we think is their birth and what effects they may have on their surroundings."

The new instrument proposed by the SMI is essentially a type of Stokes polarimeter in space. From that vantage point, it will be unencumbered by the earth's atmosphere and thus able to collect a sharper picture of magnetic features from large sunspots to the smallest magnetic elements.

Why should we care about sunspots? By learning more about them, we can anticipate and prepare for their impact. It's the same rationale as for studying hurricanes, tornadoes, and other forms of dangerous weather. The concept of space-weather warnings is now gaining currency through the National Space Weather Program, an initiative that includes NCAR's participation through the National Science Foundation.

In March 1989, a few hours after a flare spewed from one of the most complex sunspot groups ever observed, the resulting plasma reached the earth's outer atmosphere. The magnetism sent power grids into a paroxysm. Much of Quebec was blacked out for hours at a cost of many millions of dollars. Such expense might someday be avoidable, says Bodgan. "Information like we're collecting now can tell us more about the origin of sunspots. The SMI should help us become better able to detect these events as they're developing."

If the study of solar weather can help us avoid short-term trouble, the study of solar climate may help us plan for even bigger change. Longer-term variations in the sun's overall energy output have a lot to do with climatic shifts on earth. For instance, the Maunder minimum of solar activity, centered in the 1600s, and similar variations in solar output are largely blamed for the Little Ice Age that chilled Europe. A medieval maximum appears to have triggered disastrous heat and drought in North America and bountiful harvests in northern Europe for nearly a century.

The solar magnetic machine is at the root of solar variability, according to Knoelker. "Since it's the ultimate source of space weather as well as these longer-term events, we'd better learn to understand it." In the spirit of unification that inspires much of today's science, the SMI may soon lead the way toward understanding that magnetic yellow orb we know--and don't know--so well.

The Atlantic's answer to El Niño

Ozone depletion: the good, the bad, and the uncertain

The El Niño/Southern Oscillation has gained increasing fame for its role in steering global climate via cyclic warming and cooling in the eastern tropical Pacific Ocean. Recent work by NCAR scientists has illuminated the role of a similar climatic dance that, since 1980, has caused persistent wintertime cold over the North Atlantic and corresponding warmth across Europe.

James Hurrell has analyzed decadal trends in the North Atlantic Oscillation (NAO), a north-south oscillation in atmospheric pressure. When the NAO is positive, a persistent westerly flow brings winter warmth to Europe but allows the northwestern Atlantic to cool to below normal. Hurrell found a strong correlation between the NAO pressure signature and the above weather patterns.

What might have been causing the unusually positive phases of the NAO in recent years? One clue comes from NCAR scientist David Erickson. With colleagues Robert Oglesby (Purdue University) and Susan Marshall (University of North Carolina at Charlotte), Erickson put the effects of industrial sulfate aerosol emissions into an NCAR computer model of global climate. The model's results are remarkably similar to the positive NAO pattern: wintertime warming over Europe and cooling across the northwestern Atlantic and eastern United States.

Late in 1995, the Nobel Prize in Chemistry went to three scientists who have worked on ozone research with NCAR's Atmospheric Chemistry Division (ACD): Paul Crutzen, Mario Molina, and F. Sherwood Roland. The world has taken their pioneering research to heart. Thanks to the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, global emission rates of chlorofluorocarbons (CFCs) have decreased in the 1990s.

Still, uncertainty lies ahead. Mount Pinatubo's eruption in 1991 triggered a brief spike in ozone depletion. As shown by Susan Solomon (NOAA Aeronomy Laboratory scientist and 1995-96 ACD director), huge quantities of sulfur dioxide injected into the stratosphere by the volcano formed particles that interacted with the chlorine from CFCs to hasten ozone loss. Pinatubo is now quiet, but future volcanoes could trigger similar temporary spikes, according to Solomon.

Meanwhile, NCAR's Geoff Tyndall and John Orlando have been studying hydrofluorocarbons (HFCs) used in place of CFCs. In the lower atmosphere, some HFCs react to form trifluoroacetic acid (TFA), a potential threat to plant life. However, Tyndall and Orlando have found that the TFA yield from one of the most common HFCs (HFC-134a) is three to four times lower than previous estimates.

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Last revised: Mon Apr 10 13:23:27 MDT 2000