University of California, Los Angeles
Paul H. Roberts was awarded the 1999 John Adam Fleming Medal at the AGU Fall Meeting Honors Ceremony, which was held on December 15, 1999, in San Francisco, California. The medal recognizes original research and technical leadership in geomagnetism, atmospheric electricity, aeronomy, and related sciences.
The spectacularly realistic simulations of magnetic field generation in the Earth’s core since 1995 have received so much publicity that most geophysicists are well aware of the remarkable progress that has been made in the last few years. The dynamo theory, however, is not new but, rather, a long-standing problem in geophysics. The idea that the magnetic fields of the Earth and Sun arise from dynamos operating in their interiors was first put forward by Joseph Larmor in 1919, but Thomas Cowling ruled out two-dimensional dynamos by his famous theorem in 1933. The period when Paul Roberts was working for his Ph.D. in Cambridge coincides with the time when Walter Elsasser and especially Edward Bullard were making significant efforts on the homogeneous dynamo problem. Paul’s advisor at Cambridge suggested that he might try to solve the dynamo problem.
“Paul received his Ph.D. from Cambridge University in 1954; his dissertation was on theoretical geomagnetism, but there was nothing on the dynamo. He spent post-doctorate years at the University of Chicago and then returned to England and started to work at Newcastle upon Tyne on the problems of magnetohydrodynamics (MHD) of the Earth’s core. His early results include a paper written with Stan Scot about the possibility of estimating core surface motion using secular variation data and the frozen flux approximation. In 1968, Paul wrote about the convection in a rapidly rotating fluid sphere. This and a later paper by Fritz Busse in 1970 showed that the motions would be predominantly two-dimensional and confined to a set of convection rolls aligned parallel to the rotational axis.
“In 1975, with Subodh Kumar, Paul presented one of the first successful spherical kinematic dynamos. David Gubbins and Chaim Pekeris and his colleagues came up with other successful models at about the same time. Two other papers from 1972 (one with Michael Stix) became classics of the so called $alpha$-effect dynamo theory. Paul returned to MHD dynamos in the 1980s and worked with Gary Glatzmaier and Stanislav Braginsky. The papers with Stanislav on so-called model-Z dynamos demonstrated the subtle balance between Coriolis, Lorentz, and buoyancy forces in the core.
“The recent developments of the fully three-dimensional MHD models have been really remarkable. Gary and Paul presented their first geodynamo models in 1995, at about the same time as Akira Kageyama, Tetsuya Sato, and their colleagues reported on their successful stellar model. Gary and Paul simplified the problem by using the Boussinesq approximation and obtained a spectacular demonstration of a geomagnetic polarity reversal. They also claimed that the inner core is rotating faster than the mantle by a few degrees per year. The seismologists rose to the challenge, and it is now an exciting if controversial branch of their subject. Their work was followed by several more Boussinesq models by other groups: Jeremy Bloxham and Weijia Kuang; Peter Olson, Uli Christensen, and Gary Glatzmaier; Shigeo Kida and Hideaki Kitauchi; and Ataru Sakuraba and myself.
“In 1996, Gary and Paul decided to move toward more realistic models that include both the compressibility of the Earth and the fact that the fluid core is cooled from the top but freezes from the bottom. The necessary theoretical apparatus was at hand. In 1995, Stanislav Braginsky and Paul published the most thorough investigation ever undertaken of core MHD. Gary and Paul made this the basis of the more realistic model that is now central to their work. They have recently, with Robert Coe and Lionel Hongre, shown how the statistics and reversal characteristics of the geomagnetic field depend on the assumed pattern of heat flow from core to mantle.
“There are about 250 papers listed in Paul’s publication list, and they reveal his broad range of interests, from sonoluminescence to superfluidity. These are remote from geomagnetism, and it would be inappropriate for me to dwell on them here. Instead, I would like to draw your attention to two salient features of the list. The first is that Paul is asked to write a review of dynamo theory just about every year! In my opinion, this reflects how people rely on his judgment in evaluating the progress made in the subject. Second, the list of Paul’s coauthors is truly impressive, showing both that he has always been in the mainstream of scientific developments and that his work has had a strong influence on other people.
“In summary, Paul has made an extraordinary contribution to geophysics, especially to the dynamo theory. He is thus a most fitting recipient of the John Adam Fleming Medal of the American Geophysical Union. It gives me great pleasure to introduce Paul Roberts to you.”
—MASARU KONO, Institute for Study of the Earth’s Interior, Okayama University, Tottori-Ken, Japan
“I am grateful to Masaru Kono for his kind words and for initiating the proposal that led to the honor that the American Geophysical Union has conferred upon me. I would also like to thank others who supported his proposal. I can fairly claim to be one of the doyens of geodynamo theory, one of the old fogies of the subject. It all started for me at the outset of my research career when I asked my advisor in Cambridge to suggest suitable thesis topics and he proposed that I should either prove that fluid dynamos could not exist or find a working model. This was a daunting prospect, indeed, for a starting postgraduate student! After a year, I switched topics and advisors; it would be another 6 years before two fluid dynamos were independently devised, albeit ones that were geophysically unrealistic. During my year of failure I learned a lot about magnetohydrodynamics—then a young subject—and was able to demonstrate that the sources of the geomagnetic secular variation did not have to be, as was commonly supposed at the time, within 100 km of the core surface, a result of erroneously applying the electrodynamics of solid conductors to fluids. With hindsight it seems so obvious, but at the time it did not. My new advisor, Keith Runcorn, was so astonished when I told him that he practically fell off his chair The insight helped me to my Ph.D., and I was able to accept the invitation of Subrahmanyan Chandrasekhar to become his postdoc at the Yerkes Observatory, where I learned a great deal before having to return to the U.K. to fulfill my national service obligations.
“My interest in the geodynamo was rekindled in 1965 when news reached the west of the powerful advances being made by Stanislav Braginsky. It was an exciting time for the subject. Steenbeck, Krause, and Radler were revolutionizing ideas about solar, stellar, and galactic magnetism, and I began to develop a passion for the subject and, as Hegel wrote, ‘Nothing great in this world has been accomplished without passion.’ Passion, then, is a necessary state, as many of us know well, and it certainly made me an expert in what everyone else was doing and, very occasionally, like Robert Burton’s ‘dwarf standing on the shoulders of a giant,’ I could see something a giant could not.
“My move to UCLA in 1986 gave me the privilege and pleasure (which I treasure beyond measure) of working with Stanislav Braginsky and Gary Glatzmaier. I am grateful to them and to many colleagues and students (too many to name here) for contributing to my success as a geophysicist. The geodynamo has come remarkably far since I first encountered it almost 50 years ago, although there is (as I hope the funding agencies have noticed) still much to be done. To compare the subject today with where it stood nearly half a century ago is like comparing the Kitty Hawk with a Boeing 747. And, like the Boeing, there is room for many passengers, though what we need is not passengers but, rather, more hustlers and zealots to propel the subject onward. To them, ‘welcome aboard!’”
—PAUL H. ROBERTS, University of California, Los Angeles