Eyes on the corona

For all the glory of the visible Sun, it's the outer atmosphere, or corona—far hotter than the interior, yet invisible to the naked eye—that most intrigued solar scientists during NCAR’s first years. The center inherited and expanded one of the world’s most extensive efforts at coronal observation, with an eye to such mysteries as the corona’s dynamics, the origin of its high temperature, and the causes of geomagnetic effects on Earth.

NCAR’s director-to-be, Walter Orr Roberts, was a Harvard graduate student in the late 1930s when he and his adviser, Donald Menzel, set out to build the first coronagraph in the Western Hemisphere. Their effort followed the lead of French astronomer Bernard Lyot, who in 1930 succeeded in building a telescope that blocked the solar disk in order to observe the thinner, fainter corona from high altitudes. Lyot’s coronagraph could be used with filters that allowed only one particular wavelength, or spectral line, to be studied at a time; each line corresponded to the radiation emanating from a particular element such as iron or hydrogen.

Menzel and Roberts set out to collect similar data to Lyot’s, but this time from the high, clean air of the Rocky Mountains. In 1940, the young Roberts and his wife, Janet, set out from their Massachusetts home in a Graham-Paige automobile and drove to a rugged site on a remote mountaintop near Climax, Colorado, where Roberts established the Fremont Pass Station of the Harvard College Observatory. As World War II raged, the couple embraced a life of solitude punctuated by scientific breakthroughs.

Three men looking at scientific equipment
HAO scientists John Eddy, John Firor, and Robert Lee (left to right) prepare one of two coronagraphs sent to Bolivia for an eclipse expedition in 1966.

The embryonic observing post became the High Altitude Observatory in 1946, with the University of Colorado partnering with Harvard and housing much of the fast-growing group on its Boulder campus. Another new chapter began in 1960, when Walt Roberts became the founding director of UCAR and NCAR; HAO was absorbed into the new institution. Two observers remained at Climax until the site closed in 1972.

Meanwhile, HAO hunted for new ways to unwrap the corona’s sheath of mystery. Total solar eclipses served as natural coronagraphs: tendrils of energy could be seen streaming outward from the obscured disk. HAO helped lead a series of increasingly elaborate expeditions that took instruments and staff to document total eclipses in remote places, including Sudan (1952), the South Pacific (1958), the Canary Islands (1959), New Guinea (1962), and Bolivia (1966). Clouds thwarted some of these efforts, but others succeeded, with the corona profiled at various wavelengths. The late 1960s saw HAO building up to its most ambitious expedition to that point: the successful capturing of a total eclipse on
7 March 1970 at San Carlos Yautepec, Mexico.

Valuable as they were, eclipses provided only a few precious minutes of observing time, and atmospheric interference remained an issue. HAO’s John Evans had devised a streamlined coronal measuring tool in the early 1950s. Gordon Newkirk Jr. picked up that research thread and built a coronascope that could be carried by balloon into the stratosphere, above most of the atmosphere’s confounding effects. Tests and refinements led to Coronascope II, which in 1964 obtained more than 100 images from a perch at 100,000 feet. Newkirk also built a white-light coronal camera that captured the full spectrum of energy on display during eclipses viewed from six continents and three islands.

Space-based coronal observations took off in 1973 with NASA’s Skylab, which carried a coronagraph built at HAO. Since then, increasingly sophisticated sensors have gathered a wealth of coronal data from space, largely supplanting the need for eclipse expeditions. Important roles are still played by ground-based tools, including those at the Mauna Loa Solar Observatory, built by NCAR in 1965.

 

Today — A Hawaiian perch for solar observing

Photo of Steven Tomczyk

"Our coronal observations require pristine skies."

—Steven Tomczyk, NCAR

Located more than two miles above the surrounding Pacific Ocean and hundreds of miles from a major city, the top of Hawaii’s Mauna Loa boasts some of the cleanest air on Earth. Together with a supply of reliable sunshine by day and dark skies at night, this has made the peak a haven for atmospheric and astronomical observations, including the benchmark carbon dioxide readings collected there for more than 50 years by NOAA and the Scripps Institution of Oceanography.

At the Mauna Loa Solar Observatory, established on the NOAA site by NCAR’s High Altitude Observatory in 1965, a three-person staff monitors specialized telescopes that filter and analyze solar energy emissions at several different wavelengths. Countless clues to solar phenomena emerge from the imagery, which is available worldwide via the Web.

Photo of observatory

Prominences are one feature commonly tracked at MLSO. These clouds of warm, dense material are suspended for days of weeks within the much hotter, less dense solar corona.  Most prominences end their lives in eruptions that can produce stormy space weather on Earth. But in late 2007, NCAR’s Giuliana de Toma and Roberto Casini tracked a huge bubble of magnetic material that surged through a prominence in an hour’s time without causing an eruption.

“We observe prominences every day at MLSO, but we had never seen such a large, organized, and rapidly moving disturbance inside a quiescent prominence,” says instrument scientist Joan Burkepile. The discovery revealed that a prominence can have a remarkably dynamic interior without losing its overall stability.

Photo of a man standing next to telescope inside observatory
Peter Nelson adjusts the alignment of NCAR’s Coronal Multichannel Polarimeter shortly after its arrival at Hawaii’s Mauna Loa Solar Observatory. CoMP’s diffusing lens sits within the small black ring atop the instrument’s telescope. (Photo by Andy Watt, NCAR)

Other new insights into solar activity are being provided by the Coronal Multichannel Polarimeter (CoMP). Developed in 2004, the NSF-funded instrument monitored coronal magnetism at the National Solar Observatory in Sunspot, New Mexico, before being moved to Mauna Loa in 2010. Observers can now collect data on more than 300 clear days each year, compared to about 30 in New Mexico. “It’s a huge improvement,” says NCAR principal investigator Steven Tomczyk.

Magnetic fields are difficult to assess within the thin, superheated coronal environment, but CoMP’s highly sensitive infrared sensors do the job well. The instrument’s 20-centimeter (8-inch) telescope tracks magnetic activity around the entire edge of the Sun, gathering data as often as every 15 seconds.

CoMP collected the first-ever coronal observations of Alfvén waves, fast-moving perturbations that emanate from the Sun along magnetic field lines. Previously detected only in interplanetary space, the waves were found to be far more prevalent than expected within the corona, where their influence is a subject of intense debate.

Other changes are in the offing at Mauna Loa. By late 2012, a new coronagraph, part of the forthcoming Coronal Solar Magnetism Observatory (COSMO), will allow for better monitoring of coronal mass ejections, a primary driver of space weather.

 

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The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation. Any opinions, findings and conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.