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March 1999

Art Hundhausen wins NAS Arctowski Medal for solar research

Art Hundhausen. (Photo by Carlye Calvin.)

The National Academy of Sciences' 1999 Arctowski Medal is going to Art Hundhausen, senior scientist emeritus in HAO. The academy's prestigious award in support of research in solar physics and solar-terrestrial relationships will be presented to Art in a ceremony in Washington in April. Art was chosen "for his exceptional research in solar and solar-wind physics, particularly in the area of coronal and solar-wind disturbances."

This is the second time in a row an HAO scientist has won the Arctowski Medal. Ray Roble received the triennial award in 1996. To division director Michael Knölker, the back-to-back awards are an affirmation of HAO's programs and "an encouragement for the future of coronal research at HAO."

Art's award is for "a lifetime's achievement in coronal physics, which he pursued unbent by fashion or external pressures," says Michael. Art earned a Ph.D. in physics from the University of Wisconsin in 1965. He came to HAO in 1971 from the University of California's Los Alamos Scientific Laboratory. An early interest in interplanetary space evolved into pursuit of questions about the physical structure of the solar corona and its influence on interplanetary magnetic fields and plasma flow.

Mapping the solar wind

By the time Art completed his graduate studies, theory suggested that interplanetary space was not a vacuum, but was filled with ionized gas flowing out from the sun's corona--the solar wind. Its existence had been inferred through geomagnetic activity that recurred on a 27-day cycle in sync with the sun's rotation. But little was known about the coronal structures giving rise to the solar wind. A second major puzzle was the source of sudden, drastic changes in the earth's magnetic field that produced the auroras borealis and australis (the northern and southern lights), disrupted radio transmissions, and occasionally interrupted electrical power supplies. Solar flares, sudden brightenings observable just above the surface of the sun, were thought to be associated with these disruptions.

"It was very exciting when I came into this field in the late 1960s. We were just doing the first space-based observations of the solar wind," says Art. "Everything was new. You'd give ten papers a year and it was never enough. It was a marvelous period." The launch of NASA's science and engineering laboratory, Skylab, in 1973, heralded a new era of observations of the sun's corona from space. This was a major improvement over previous observations from eclipses or ground-based coronagraphs. "You could see things in a new way, and in particular you could see the corona in X rays against the disk of the sun." To Art and his colleagues, "something became so evident that many people discovered little pieces of it at the same time."

What they had discovered was the magnetic field in the corona, with its closed and open regions. In the closed regions, magnetic field lines start at one point and come back to the sun at another to form a closed loop. Open regions have field lines going out into space. Open regions are called coronal holes, because they appear dimmer than the surrounding corona. Art recalls a series of Skylab workshops run by HAO and then-NCAR director Gordon Newkirk in Boulder in the mid-1970s that brought all the pieces together to describe the origin of the solar wind in the coronal holes.

"Suddenly this great puzzle which had been around for 30 or 40 years--with some correct ideas in the literature and some really wacky ideas there--suddenly the whole problem was solved, in the geometric sense [i.e., the location of the sources of the wind]. . . . In a few sessions, things that had puzzled everybody for decades just fell into place."

Art received two NCAR Outstanding Publication Awards recognizing his efforts in the 1970s. The first came in 1972 for his book Coronal Expansion and Solar Wind (Springer-Verlag). In the book, "I drew pictures of [coronal holes] and said this had to be the answer, but I didn't know what they looked like yet." His article "An interplanetary review of coronal holes" in the book reporting on the Skylab workshop, Coronal Holes and High Speed Wind Streams (edited by J. Zirker; University of Colorado Press), was recognized in 1977, the year it appeared.

What about the second question--the sudden, violent disruptions of the earth's magnetic field associated with what were then called solar flares? In the 1980s, the Solar Maximum Mission (SMM) and other second-generation spacecraft began providing data that led to identification of coronal mass ejections (CMEs) as the source of dramatic changes in the earth's ionosphere. Art became principal scientist on SMM, contributing analysis and theory that pointed to the magnetically closed regions in the corona as the probable source of these interplanetary shock waves.

Some researchers still subscribe to the theory that solar flares are the cause of CMEs. But many more, including Art, believe the flares are the aftereffect of much larger mass ejections of solar plasma. Art subscribes to the idea that a sequence where a huge area of plasma (up to one-seventh of the sun's circumference) blows out, dragging previously closed, stable field lines with it. After the explosion, the open lines close back down again, "and that's where the flare happens. . . . The flares are the aftereffect of the mass ejection."

The bottom line, says Art, is that the cause of CMEs is still a mystery. "We're at the point now where I think we know pretty much what the observations presently available can tell us. We need a whole new generation of observations to make the next step."

What's next?

Art plans to use proceeds from the Arctowski Medal to help inspire the next generation of solar physicists. The medal includes a prize of $20,000 plus $60,000 "to an institution of the recipient's choice." If matching funds can be arranged, Art would like to see a postdoc hired in HAO for two or three years "to continue this kind of work. Or to go back and do the part of it right that we haven't done yet."

Reflecting on his long career, Art notes, "There's been real progress, and I've managed to be in on a good bit of it, but nobody ever discovers anything by themselves." He cites the work of NCAR founding director Walt Roberts, Don Billings, and other HAO pioneers beginning in the 1940s as the underpinnings of his research. He also credits Walt and Francis Bretherton for creating a culture at HAO that encouraged long-term approaches to scientific problems.

Art still comes in to work for a few hours each week to advise students, from undergrads to postdocs. The current team working with Art in HAO includes associate scientists Joan Burkepile, Andy Stanger, and Alice Lecinski, who've analyzed coronal data from Skylab, SMM, the Yohkoh and Solar and Heliospheric Observatory (SOHO) satellites, and the Mauna Loa Solar Observatory.

When you ask Art a question, he picks up a marker, strides to the blackboard (or whiteboard), and tells you a story. His gift and passion for teaching were recognized in 1997 with a nomination for the UCAR Outstanding Performance Award for Education (with Joan Burkepile). And if you spend enough time with Art, you learn about his other research passion: Italian history and culture, particularly that of Rome. He leaves for his annual two-week pilgrimage to Italy the day after the awards ceremony. "I can't go to other places until I'm finished with Rome. It's like science--the more you understand, the more things hold together, and the more you enjoy it." •Zhenya Gallon

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