In a radical rethink of accepted geophysics, new research in the US links variations in the Earth’s magnetic field with the ebb and flow of the world’s oceans. Given the practical importance of these field variations in navigation and atmospheric modeling, the implications of this new research extend far beyond academia. However, the idea has already faced strong criticism from some researchers in the geophysics community.
Given these huge time-scales, sailors and Scouts need not worry about the North Pole suddenly becoming the South, but there is another shorter-term threat to old-fashioned navigation caused by slight drifting of the magnetic field over years-to-centuries. The origin of this “secular variation” is also thought to originate in the molten iron core, due to fluctuations in the established convection pattern. And, although small in comparison with the main dipole field, secular variation can be difficult to predict with effects substantial enough to prompt a revision of the International Geomagnetic Reference Field every five years.
Electric sea salts
Now, Gregory Ryskin of Northwestern University, Illinois, is offering an alternative explanation for the origin of this secular variation. Ryskin believes that electric currents induced in dissolved salts — as ocean waters circulate through the Earth’s magnetic field — can generate secondary magnetic fields strong enough to shift the orientation of the original field. Comparing his own calculations with public geophysical data, Ryskin links circulation in the North Atlantic with observed trends in secular variation over Western Europe.
Scientists have long since known that salt in the ocean can conduct electricity, leading to secondary fields, as the waters chop and change in the presence of the Earth’s magnetic field. In practice, however, it is difficult to gauge the scale of these fields — partly due to the incompleteness of data and the limited precision of computations. Ryskin also suggests that previous measurement of these fields have been somewhat biased by standard theories. “Researchers work backwards — they begin with the assumption that secular variation comes from the core when this is still only a hypothesis.”
Taking a different approach, the physicist looked specifically at the North Atlantic in isolation from other models of the Earth’s field. He calculated the expected variation in magnetic fields between 1995 and 2000 using equations of solute transport and magnetic diffusion, and ocean circulation data from ECCO — a global reference point funded in part by NASA and the National Science Foundation (NSF).
Out of the blue
Ryskin then compared these figures with recorded secular variations in the International Geomagnetic Reference Field (IGRF) — a publicly available resource derived from satellites, observatories and surveys around the world. Publishing his findings in New Journal of Physics, Ryskin finds strong temporal and spatial correlation between his calculated secular variation and the IGRF figures between 1995 and 2000.
The reason this theory is so controversial is that it directly challenges one of the strongest pieces of evidence in the standard model of the Earth’s magnetic field. Secular variation caused by fluid motions in the earth’s outer core is taken by geophysicists as confirmation that the main field also emerges from this region known as the “geodynamo”. As Ryskin asserts in his paper: “If secular variation is caused by the ocean flow, the entire concept of the dynamo operating in the Earth’s core is called into question: there exists no other evidence of hydrodynamic flow in the core.”
Alex Kostinski, an atmospheric physicist at Michigan Technological University told physicsworld.com: “I consider this paper extremely important, although I expect violent opposition from the experts.”
Indeed, some geophysicists believe there are fundamental limitations in this research. “[Ryskin] should compare the required electric currents for the theory with the amplitudes of electric currents that have been measured in the ocean,” said Robert Tyler, an ocean electrodynamics researcher at the University of Washington. Tyler also criticizes the way Ryskin has modeled the spreading of magnetic fields through sea waters. “In a thin conducting shell like the ocean, the diffusion is not through the ocean but along the top/bottom boundaries.”
Despite Ryskin’s bold claims, he is also careful to note that — although he sees strong correlation in his results — this does not prove beyond doubt that all secular variation is due to ocean flow. “In fact, a definitive proof may never be possible, but as the accuracy and completeness of the data continue to improve, and further computations are carried out, sufficient clarity on the issue should be achieved soon,” he writes.
About the author
James Dacey is a reporter for physicsworld.com