Rainer Hollerbach

1999 James B. Macelwane Medal Winner

University of Glasgow

Rainer Hollerbach was awarded the 1999 James B. Macelwane Medal at the AGU Fall Meeting Honors Ceremony, which was held on December 15, 1999, in San Francisco, California. The Macelwane Medal recognizes significant contributions to the geophysical sciences by a young scientist of outstanding ability.


“Rainer Hollerbach is one of the leaders of a very talented group of young theoreticians working on the generation of the geomagnetic field by dynamo action. His work is noted for its originality and thoroughness; its presentation is remarkable for its clarity and lack of obfuscation.

“The Earth’s inner body occupies only about 5% of the core’s volume, and for many years most theoreticians felt that its effect on the dynamics of the core and the generation of the Earth’s magnetic field would be equally small, but—largely through Rainer’s work and that of his collaborators (particularly Chris Jones)—this is now known to be untrue.

“Rotation gives a preferred direction to dynamical processes in the fluid core. This is the basic reason why the magnetic compass needle points approximately to the north. Rotation also gives prominence to an imaginary cylinder that touches the inner core on its equator and is parallel to the rotation axis. This is known as the ‘tangent cylinder,’ and it divides the fluid core into three regions: the northern interior of the tangent cylinder, the southern interior, and the exterior. The dynamics of these regions are rather different, and what occurs in the transition layers between them and near the tangent cylinder is extremely complex but appears to be highly significant to the geodynamo mechanism. Rainer has made these layers his special concern, and his mathematical analyses have brought new insights into the way that they function.

“Most theoreticians, believing the inner core to be unimportant, had modeled it as simply as possible—as either an electrical insulator or a perfect electrical conductor. Rainer was one of the first to investigate, using simplified two-dimensional models, the effect of finite electrical conduction. This led to what is perhaps his best known work, which was in collaboration with Chris Jones and which demonstrated the importance of the electrical conductivity of the inner core on the dynamo operating in the outer core. Rainer and Chris noticed something that had been missed by all previous researchers, but which now in hindsight seems, like all good ideas, to be obvious. They realized that the electrodynamic timescale of the inner core would be long enough to influence the dynamics of the overlying fluid, thus lengthening the effective timescale of MHD processes in the fluid. Their proposal was vindicated in subsequent three-dimensional numerical simulations of the geodynamo.

“Rainer displayed numerical skills in the early development of the Exeter geodynamo model, which led to fresh insights into dynamical processes in the core, some of which may explain how polarity reversals of the geomagnetic field can occur. He has worked on fast dynamos; he has contributed to unrelated areas of fluid mechanics. He is making his mark everywhere, but most of all in geomagnetism. He is, in all respects, a worthy recipient of a Macelwane Medal.”

—PAUL H. ROBERTS, University of California, Los Angeles, Calif.


“Thank you very much, Paul, for that generous citation and for nominating me in the first place. It is a great honor to receive this Macelwane Medal, and I am very grateful for it and for the vote of confidence it represents.

“I have to admit that I stumbled into geodynamo theory more by accident than design. In retrospect, it was the obvious choice for me: my favorite subject as an undergraduate in physics had been Gene Parker’s course on electricity and magnetism, and my favorite subject as a grad student in oceanography had been Rick Salmon’s course on geophysical fluid dynamics. However, at the time it involved quite a few twists and turns. Suffice it to say that I am deeply grateful to my two coadvisors, Glenn Ierley, who first suggested I switch from oceanography to geodynamo theory, and Rick Salmon, who was hugely supportive even after I switched. I wouldn’t be standing here today if it were not for both of them.

“After my Ph.D., I was fortunate enough to get postdoc positions with some of the leaders in the field, first in Britain with Andrew Soward, Chris Jones, Mike Proctor, and David Fearn and then in Los Alamos with Gary Glatzmaier. I feel privileged to have had the opportunity to work with these individuals at a time when we were grappling with some of the most fundamental issues in numerical geodynamo modeling. Working with them was a hugely rewarding experience, and I learned a lot, not only about science, but about collegiality as well.

“It was also during my time in Britain that I first met Paul. In his very generous citation just now, he mentioned that I have made certain contributions concerning the dynamics of the tangent cylinder, but he very modestly neglected to mention that he is the one who first told me about this mysterious tangent cylinder. I remember it well: it was our custom to discuss various problems over lunch in Andrew’s office, and that day I was sketching out how I hoped to include the inner core in our latest model, and Paul, who was visiting at the time, pointed out to me that if I really wanted to treat the inner core properly, I’d also have to think about the dynamics of the tangent cylinder, to which my response was ‘the what cylinder???’ So I’ve learned a great deal from him as well.

“Finally, I’d like to mention that my first exposure to geophysics was through an undergraduate summer job at the Geophysical Institute in Fairbanks, Alaska, where Syun-Ichi Akasofu offered me a job as a programmer, despite the fact that I knew nothing about either programming or geophysics. He said that didn’t matter, that I’d soon learn. I’ll always be grateful to him for giving me that opportunity, and I hope that my subsequent career has justified the faith he had in me then. And so I’d like to close by wholeheartedly endorsing the call that David Bercovici made when he received his Macelwane 3 years ago, that we should all strive to involve undergrads in our research wherever possible. In addition to all the benefits that David mentioned then, who knows, maybe someday one of them will be thanking you in their Macelwane acceptance speech.”

—RAINER HOLLERBACH, University of Glasgow, Scotland