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Between Sun and Earth
  • Between Sun and Earth: timeline
  • In their own words: Robert Rosner
  • UCAR at 40
    Who We Are
    Introduction
    One Planet, One Atmosphere
    •Between Sun and Earth
    Measuring and Modeling
    When Weather Matters Most
    Spreading the Word
    Knowledge for All
    Looking toward the Future
    UCAR at a Glance
    List of acronyms


    Walter Orr Roberts, solar scientist and the founding director of NCAR.

    Rising each morning, setting each evening—what could be more constant than the Sun? Observers have known for centuries, though, that the Sun, like everything else in the known universe, is far from constant. Ancient societies marveled at eclipses, when the Sun disappeared from view without warning, only to reappear just as quickly. Renaissance scientists noted the waxing and waning of sunspots visible to the naked eye, making it possible for their modern successors to look back with wonder at the period between 1640 and 1710, when virtually no sunspots were recorded.

    Solar variations are the raison d'etre of the High Altitude Observatory. Founded in 1940, HAO joined UCAR and NCAR within a year of their creation. Solar physics was not in the original plan for NCAR, but the merger made sense. Both labs were located in the same city; Walter Orr Roberts had directed HAO for 20 years and was now the head of UCAR and NCAR. Most important, a truly broad approach to understanding the atmosphere had to include what Robert MacQueen (a former NCAR senior scientist now at Rhodes College) has called "the driving force for all atmospheric motions."

    The rise of global change concerns and the explosion of satellite technology have made NCAR's solar research all the more vital. How much of the 20th century's global warming can be attributed to the Sun? How vulnerable is our space-based communications infrastructure to the wrath of solar storms? While helping to address these and similar questions, HAO has maintained its long-time strength in basic research. Its scientists have created instruments, developed computer models, derived new theories, and worked with colleagues around the world to explore the magnetism and dynamics at the heart of the Sun and other stars like it, as well as to explore the interactions between the Sun and our atmosphere.

    Capturing the corona

    In its first years, HAO's work centered on a tool that studies the Sun by obscuring it. Roberts and his Harvard adviser, Donald Menzel, came to the high country of Colorado in 1940 to operate the Western Hemisphere's first coronagraph. The device employed a metal disk to create an artificial eclipse, which allowed scientists to view the inner part of the solar corona—the faint, hot, magnetized sheath surrounding the Sun and its immediate atmosphere. From their high-altitude station in Climax, Colorado, Roberts and colleagues collected groundbreaking, breathtaking pictures of the corona and the great bundles of magnetized plasma called prominences that occasionally burst from it.

    As HAO expanded, its scientists began trekking around the world to document eclipses. Even the best coronagraphs could not match an eclipse for its ability to block sunlight and unveil the full, or white-light, corona for a few crucial minutes. HAO staff have since been on the scene of almost every major eclipse, 16 in all over the past half-century. In February 1998, the NCAR/NSF C-130 aircraft collected pioneering infrared images of the corona while flying above the Caribbean. The data confirmed a new technique—the most sensitive yet found—to measure the corona's strength.

    Though they are still irreplaceable opportunities, eclipses have been eclipsed themselves by the advent of coronagraphs that can observe far above the obscuring atmosphere of Earth. From the ground, a 1964 team led by HAO's Gordon Newkirk, Jr., collected the first-ever image of the full corona outside of a natural eclipse—and catalyzed decades of work to come. NCAR built the first space-based coronagraph, carried aboard the National Aeronautics and Space Administration's Skylab in 1971, and a coronagraph-polarimeter that collected over 30,000 images aboard the Solar Maximum Mission satellite in 1980. In 1997, scientists at NCAR and elsewhere began drawing data from the Solar and Heliospheric Observatory, operated by NASA and the European Space Agency.

    The Climax Observatory closed its doors in 1972, but its mission lives on at the Mauna Loa Solar Observatory, founded by NCAR in 1965. From the pristine air atop Hawaii's second-highest peak, the MLSO has taken observations virtually every day using a wealth of instruments, including a series of coronameters. The latest in the series, Mark IV, was installed in 1999; it more than doubled the sampling area of its predecessor, spanning nearly three times the diameter of the Sun itself. The instrument is one of several that compose the Advanced Coronal Observing System (ACOS), an important new resource for the world's solar physics community.

    On the surface and beneath

    The close-knit HAO team of the 1940s and 1950s (below) collected breakthrough data from the Climax Observatory on solar prominences and other features emerging from the Sun's surface.

    Impressive as it is, the Sun's corona is but an outward sign of enormous energies roiling within. Recent observing tools, including some pioneered at NCAR, have enabled scientists to peer into details on and beneath the Sun's surface.

    Five years of research and development led by Bruce Lites produced NCAR's Advanced Stokes Polarimeter, which debuted in 1991. The ASP infers the three-dimensional magnetic structure of sunspots and other magnetic features, using imagery from the National Solar Observatory telescope on Sacramento Peak in Sunspot, New Mexico. Sampling the solar disc 60 times a second (faster than the frames of a motion picture), the ASP has helped analyze such phenomena as bright rings around the Sun observed by the Precision Solar Photometric Telescope (PSPT). These rings—predicted for 25 years, but not observed until the late 1990s—appear to help solve an enduring solar puzzle: why sunspots remain darker and some 2,000°C (3,600°F) cooler than the rest of the Sun's surface. The ASP confirmed that only a small fraction of the rings' brightness is due to magnetism, which implies that the rings are actually transferring large amounts of heat to the surface.

    Scientists began "listening" to the Sun with the Fourier tachometer, built by NCAR and the Sacramento Peak Observatory in the early 1980s. Like the resonant tones of a bell, pressure waves within the Sun reflect off its surface, typically crossing the sphere within five minutes. The new discipline of helioseismology—analysis of the seismic waves within the Sun—got a further boost with the low-degree, or LOWL, instrument deployed at Mauna Loa in 1994. The instrument, developed by Steve Tomczyk, measures the solar surface as it pulses outward and inward, converting those data into low-frequency oscillations. Data show that the inner two-thirds of the Sun appears to rotate at a constant rate regardless of latitude and depth, even though the Sun's outer third (the turbulent convection zone) differs markedly from its interior (dominated by nuclear fusion). Another instrument is now in place on the Canary Islands, half a world away from Mauna Loa, making for a potent duo of vantage points that form the Experiment for Coordinated Helioseismic Observations.

    Since 1965, the Mauna Loa Solar Observatory has collected solar data from Hawaii's second- highest peak. The site now hosts the Advanced Coronal Observing System. (Photo by Kim Streander, NCAR)

    Helioseismology took an unexpected path in the late 1990s. By turning the instruments away from the Sun and tracking low-frequency waves from other, more distant stars, tiny wobbles in those stars' motion could be analyzed—wobbles believed to reveal the gravitational influence of planets. NCAR's Timothy Brown joined a team that included Geoff Marcy and Debra Fischer (San Francisco State University), Paul Butler (Anglo- Australian Telescope), Stephen Vogt (University of California, Santa Cruz), and Robert Noyes and Sylvain Korzennik (Harvard-Smithsonian Center for Astrophysics) to discover, in 1999, the first set of multiple planets around a Sun-like star, Upsilon Andromedae.

    Weathering the solar wind

    NCAR's solar physicists have teamed with atmospheric modelers and chemists to study the rarefied reaches of the upper atmosphere. At a height of 500 kilometers (300 miles), a molecule may travel 30 km (20 mi) before interacting with another one. The ionosphere's thinness and highly charged state make it vulnerable to bursts of solar activity, especially at the peak of the 11-year solar cycle.

    Summer Sands, an undergraduate at Pima Community College, examined supergranules on the Sun's surface with NCAR scientist Mark Rast as part of the Significant Opportunities in Atmospheric Research and Science program.

    Paralleling the long effort to model the lower atmosphere, where weather is focused, HAO's Ray Roble has spent 20 years simulating the upper atmosphere through a computer model. It traces the dynamical, chemical, radiative, and electromagnetic forces shaping the upper atmosphere and ionosphere. Another NCAR model, this one focusing on air chemistry, now extends from ground level up to 85 km (50 mi). The Model for Ozone and Related Chemical Tracers (MOZART) includes over 30 layers and hundreds of chemical reactions, including ozone depletion and other key processes that unfold as sunlight reaches the stratosphere and mesosphere. Roble's model and MOZART have been combined with a middle atmosphere model to create the Whole Atmosphere Community Climate Model. A collaboration among Roble, Rolando Garcia, and Byron Boville, it extends from the Earth's surface up to 140 km (85 mi), a feat accomplished by only a handful of other models worldwide. Eventually it will cover 500 km (300 mi) in altitude.

    In a solar storm, the energy released by coronal mass ejections can vary by factors of 10 to 100 in a matter of hours or days. The charged particle, further energized when passing through Earth's magnetosphere, can trigger beautiful auroras on Earth; at the same time, by heating the sparse upper atmosphere, they can alter the orbits of satellites by increasing the atmosphere's drag on them. The resultant fluctuations of density and temperature in the ionosphere can perturb communication and navigation systems. A new NCAR photometer atop Mauna Loa uses helium absorption to capture, in unprecedented detail, the ejections of mass from the corona that can lead to troublesome space weather. While other institutions provide real-time warnings of impending solar storms for industry and the public, the data from NCAR's instruments are helping to flesh out the knowledge that will guide space weather alerts of the future.

    This sequence of images shows the change in electron densities through Earth's atmosphere, primarily in the ionosphere, during a solar storm on 10 January 1997. Ray Roble and Gang Lu studied the event with their Thermosphere-Ionosphere-Electrodynamics General Circulation Model.

    The total energy emitted by the Sun changes little with the solar cycle—so little, in fact, that for many years it was termed the solar constant. Recent data show that it varies by about 0.1% over a typical solar cycle, with somewhat larger variations over longer time scales. Though tiny, this waxing and waning is enough to make a difference to climate. The 20th century simulation produced by NCAR's Community Climate System Model helped show that a relatively quiet solar period may be behind the global cooling that took place between the 1940s and 1970s, although solar influences account for only about a third of this century's overall warming trend. Future work with the Whole Atmosphere Community Climate Model may clarify how solar phenomena could influence climate and weather. Meanwhile, the PSPT atop Mauna Loa, part of an NSF network, is tracking solar variation at new levels of detail: one part in a thousand at each measurement point. Only with such fine-grained data can scientists hope to follow the gradual, long-term solar changes that could affect climate through the next century and beyond.


    UCAR at 40
    Who We Are
    Introduction
    One Planet, One Atmosphere
    •Between Sun and Earth
    Measuring and Modeling
    When Weather Matters Most
    Spreading the Word
    Knowledge for All
    Looking toward the Future
    UCAR at a Glance
    List of acronyms


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    Executive editor Lucy Warner, lwarner@ucar.edu
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    Last revised: Fri Jan 26 17:18:32 MST 2001