Uli Christensen is one of the unique individuals who have contributed to both domains of core and mantle and as such is a fitting recipient of the Lehmann Medal. It is rare for an individual to have had such a level of impact in both domains of geodynamo theory and geodynamics.
Early in his career, in joint work with David Yuen, Uli was the first to determine how pressure-induced phase changes influence mantle convection, demonstrating the viability of circulation across the mantle transition zone.
With Al Hofmann in 1994 he showed how gravitational segregation of ocean crust in the deep mantle resolves isotopic patterns observed in mantle-derived rocks.
Beginning in the late 1990s, Uli has produced a whole host of results that have clarified the behavior of numerical dynamo solutions; along the way he has shown great leadership in the geomagnetism community by instigating the first dynamo benchmark exercise, and he has engaged with observationalists for the common advancement of the subject.
Of the pivotal contributions made by Uli, I will highlight a select few. Uli is the originator of the mapping of regime boundaries for convective dynamos as a function of control parameters such as the Rayleigh, Ekman, and magnetic Prandtl numbers. Further work illuminated the regime boundary between dipolar and nondipolar dynamos, attributed to be controlled by the local Rossby number.
A lasting legacy is work with J. Aubert to create a comprehensive scaling theory for the geodynamo. This showed how the velocities, heat transfer, and magnetic field strengths all scale with the convective power. This analysis was groundbreaking when it was introduced 13 years ago and remains at the very forefront of modern ideas of the geodynamo. A tremendous application of these ideas was to explain the magnetic fields of planets and stars.
Uli has become a much-sought-after keynote speaker at conferences as a result of his prominence in the subject and broad knowledge of the area. It should be mentioned that Uli has freely shared his numerical dynamo database with others so that they can carry out their own analyses. This approach has won him many friends. He is a fitting recipient of AGU’s Lehmann Medal.
—Andy Jackson, ETH Zurich, Zurich, Switzerland
Thank you, Andy, for your kind words, and thank you to all who conspired to get me this prestigious medal. I was fortunate to be born at the right time. A law made by the German government when I was 17 provided generous support to students from low-income families, which allowed me to enter university. In the late 1970s, plate tectonics had come of age as an empirical theory, but its mechanism was not well understood. At the same time, computers became powerful enough for simulating complex nonlinear systems. Both fascinated me. My Ph.D. adviser was not an expert on either topic but was a very open-minded man. I was lucky that he gave me a free hand for working on the numerical simulation of mantle convection on my own. As a postdoc coming from the still somewhat parochial German geoscience community, Dave Yuen taught me, aside from a strong vocabulary in the English language, also the bold American way of tackling cutting-edge problems. Al Hofmann was so kind to host in his geochemistry department a guy who had not the vaguest idea about mantle isotopes. When I had mastered the fundamentals after 10 years, we published a paper together, marrying mantle convection with isotope modeling. In the late 1990s, I looked for something to give my research a new twist. I was lucky again—realistic geodynamo modeling had just become practical. Gary Glatzmaier generously shared his code, and Peter Olson initiated me, coming from the very sticky world of mantle convection, to the airy physics of rotating magnetohydrodynamics. I also profited a lot from working with other colleagues, postdocs, and students. From Neil Ribe I learned that nice numerical models are most useful when coupled with a scaling theory that allows us to extrapolate them to the real world. Carsten Kutzner and I made the first steps toward understanding when a dynamo produces a dipole-dominated field. With Julien Aubert I tackled the question of what actually controls the strength of the magnetic field. I tried to reach for the stars with astrophysicist Ansgar Reiners by showing that the magnetic fields of planets and those of rapidly rotating low-mass stars follow the same scaling rule. It was a great pleasure to collaborate with all these people and many more. I owe them tremendously, and without them my scientific career would certainly not have culminated in receiving the Inge Lehmann Medal.
—Ulrich Christensen, Max Planck Institute for Solar System Research, Göttingen, Germany