Jean-Louis LeMouel

1997 John Adam Fleming Medal Winner

University of Paris, France

Jean-Louis Le Mouël was awarded the John Adam Fleming Medal at the AGU Fall Meeting Honors Ceremony, which was held on December 10, 1997, in San Francisco, California. The Fleming Medal recognizes original research and technical leadership in geomagnetism, atmospheric electricity, aeronomy, and related sciences. The citation and response are given here.


“Jean-Louis Le Mouël is among our century’s most effective geomagneticians. It reflects much credit on the AGU that he is now a Fleming Medalist. He has made many significant contributions to the collection, analysis, and interpretation of magnetic data and has solved several long-outstanding theoretical problems. There is space here to describe only some of the highlights of his research: (1) Field and theoretical work to establish the observability of local regions of high conductivity in the crust (channeling and lensing). (2) With Vincent Courtillot and Joel Ducruix, the first observations suggesting a magnetic impulse in 1969, and the most complete discussion of the data and their possible implications for lower mantle conductivity. Based on what appeared to be another impulse over a decade later, Le Mouël, Courtillot, and Dominique Jault predicted a kink in the length-of-day curve. Its timely appearance made a strong case for magnetic impulses. (3) With Courtillot, derivation from the Maxwell equations of a previously ignored physical constraint that helps to estimate the lower-to-upper mantle transfer function at the 11-year sunspot period. (4) The independent discovery that Hill’s hypothesis of tangential geostrophy is defensible in the upper core and removes a large part of the ambiguity in inferring the fluid motion at the top of the core from the magnetic secular variation. (5) With D. Jault and C. Gire, the observation that two purely mechanical torques between core and mantle are probably so large as to preclude detection of magnetic torque in the variations of length of day. These torques arise from geostrophic pressure on Hide’s core-mantle boundary bumps and the gravitational attraction between density inhomogeneities in the core and mantle. (6) With Y. Cohen and M. Menvielle, the use of unmanned balloons to make random magnetometer surveys with a crustal resolution that will never be obtainable from satellites. (7) With M. Alexandrescu and V. Courtillot, the assembly of a geomagnetic time series at Paris going back four centuries. (8) With J. Zlotnicki, the use of magnetic ground surveys in volcanology. (9) With G. Hulot and A. Khoklov, mathematical proofs that the external field can be recovered from complete directional or intensity data on the surface of the Earth. Directional data leave an undetermined multiplicative constant in the recovered field, and the observed direction field on the surface must have only two dip poles. Intensity data must be supplemented by observing the location of the dip equator, and the sign of the recovered field remains ambiguous.

“Jean-Louis’ interest in the details of the data makes him a very effective scientist. For example, while director of the French National Magnetic Observatory at Chambon-le-Foret, he was led by various suspected anomalies in the observations to discover an iron spike left by accident in the concrete magnetometer pier by its original builders, and an electrolytic contact between lead and copper drain pipes activated seasonally by changes in the water table. Both had contributed errors of several nanotesla to the observatory’s data for nearly a century.

“Jean-Louis is a public-spirited scientific citizen and a conscientious administrator. During his 10-year term as director of the Institut de Physique du Globe de Paris, he fostered a stimulating and productive environment for staff, students, and visitors and contributed significantly to IPGP’s success as one of the world’s foremost centers of geophysics. Vincent Courtillot, the current director, continues this tradition for IPGP as a whole, while Jean-Louis focuses his efforts again on the geomagnetism section, which he heads. He has been president of the scientific council of Geoscope (the French program in seismology), of the committee on scientific programs of the French National Space Center (CNES), and of SEDI. He has been a member of the directorate of the international program for magnetic observatories (INTERMAGNET) and of the administrative council of the French National Institute for the Sciences of the Universe (INSU).

“Jean-Louis Le Mouël’s contributions to science have been widely recognized. He is a fellow of the AGU and the Royal Astronomical Society, was president of the Geological Society of France, and is a Chevalier of the French Legion of Honor. In 1988 he was elected to the French Academy of Sciences. The AGU and the holders of the John Adam Fleming Medal are very pleased to welcome him as the newest Fleming Medalist.”

—GEORGE BACKUS, IGPP, University of California, San Diego


“Mr. President, members of the American Geophysical Union, and honored guests. “It is a great and unexpected honor for me to have been elected the 1997 John Adam Fleming Medalist and to receive an award that went to J. A. Van Allen, W. M. Elsasser, K. Runcorn, and other eminent scientists. This statement sounds trite, but I will try to persuade you that it is not. “I came to geophysics by chance, attracted by my former physics teacher in Lycée de Pontivy, E. Le Borgne, who joined Institut de Physique du Globe after writing a thesis on the magnetic properties of soils. Again, after a first work on thermoelastic stresses with G. Jobert, I chose geomagnetism through Le Borgne; at the beginning of the 1960s, E. Le Borgne received the charge of realizing the aeromagnetic surveys planned by the Institut National d’Astronomie et de Géophysique (INAG) and invited me to join him in this enterprise. I also enthusiastically accepted the position of observer-in-charge at the Chambon-la-Forêt Magnetic Observatory, in the heart of the Forêt d’Orléans. It was planned that calibration and verification of the brand-new optical pumping magnetometers to be used in the surveys, an important first step of the program, would be performed at the Observatory.

“E. Thellier was then director of the Institut de Physique du Globe de Paris. His pioneering contribution to rock magnetism and archeomagnetism was well known. However, most of the aspects of geomagnetism were simply ignored in France. Comparing this situation with the one prevailing in Britain, J. Coulomb and E. Thellier wanted to develop geomagnetism and encouraged me in my choice. I spent some years in the Observatory, working at the realization of the INAG aeromagnetic surveys. I gained a deep interest for the measurement of the magnetic field (I am all the more happy to receive the Fleming Medal) and maybe some familiarity with his “character somewhat closed, even for its intimates” (E. Thellier). This is undoubtedly why I am so fond of our Intermagnet project, since I lead it with colleagues sharing the same feelings.

“It was later that I decided, strongly encouraged by C. Allègre, to build in IPGP a team involved in geomagnetism at large, while V. Courtillot started a paleomagnetic laboratory. I could find inspiration, especially in British and U.S. examples. Also, I got interested in the various topics generously enumerated by my eminent citationist G. Backus, among which I could not chose, since I found all of them fascinating. The only rule in our IPGP group was to keep close to the data.

“I was then lucky enough to meet the great names of the discipline at the AGU, IUGG, and SEDI meetings. I will only mention G. Backus and S. Braginsky, two John Adam Fleming Medalists, with whom I had the privilege to do some work. I am also proud to be counted among the founding members of SEDI, and I felt honored when I was elected President, after the premature deaths of E. Benton and D. Doornbos. It was a very light chairmanship, since D. Loper was the one who made things work.

“I am happy to seize the opportunity to thank all the former students and colleagues with whom I have shared great moments, most of whom have been quoted in the citation by G. Backus.

“Mr President, I express my heartfelt gratitude to AGU. The John Adam Fleming Medal is the most splendid award I could dream of.”

—JEAN-LOUIS LE MOUËl, University of Paris, France