Episode 385 The Magnetic Field Anomaly

Most of my career was in analyzing features of the earth’s gravity and magnetic fields, to infer geologic structures for oil exploration. But that doesn’t mean I really understand the whole earth’s fields – and for some aspects of it, neither do folks much more knowledgeable than I am.

You’ve probably seen the images that show the earth’s magnetic field as like that of a dipole magnet, with north and south poles that don’t coincide exactly with the poles of rotation. That’s fine as a starting point, but in detail, we find that the earth’s field is not smooth and uniform, but it has bumps and changes over time and in space. Today I want to talk about some of the anomalies in magnetic field intensity.

At the earth’s surface, the magnetic field varies from about 23,000 to 65,000 nanoteslas, with a tesla being the standard unit of magnetic field strength. It’s not surprising that a body as complex and heterogeneous as the earth would have variations in its physical properties, but a range of 40,000 out of a maximum of 65,000 might seem to be a wide range.

The highest highs are over Siberia, northern Canada, and the ocean between Antarctica and Australia, while the one big low is over central South America and the South Atlantic Ocean. That South Atlantic Anomaly has gotten some serious study lately.

The weakness of the magnetic field at the South Atlantic Anomaly is enough that increases in radiation – which the magnetic field protects us from – can affect satellites like the Hubble telescope, and the International Space Station has extra shielding just because of the South Atlantic Anomaly. Even at ground level, communications can be disrupted during solar storms.

Jay Shah, a student at Imperial College London, studied rocks on the volcanic island Tristan da Cunha, right in the middle of the anomaly, and found that the magnetic field there has probably been weaker than elsewhere on earth for at least 46,000 to as much as 90 thousand years ago, indicating that the South Atlantic Anomaly is probably a fairly persistent feature of the magnetic field.

One speculation about the nature of the South Atlantic Anomaly had been that it somehow was an expression of an impending reversal of the magnetic field. We know that these inversions happen, and have happened dozens of times in earth’s geologic past, but we know very little about the actual mechanism of a reversal. The finding that the South Atlantic Anomaly is fairly old doesn’t say it’s not related to a reversal, but it maybe reduces the chances. The evidence suggests that reversals probably happen over a fairly short time span, a few thousand years or even fewer, and probably not a time as long as 50,000 years or more.

The earth’s magnetic field is probably generated by electrical currents mostly in the liquid outer core. You can imagine that a fluid, even one as dense and hot and deep as the outer core, would have variations in flow and geometry that would be reflected in the magnetic field generated, and this is almost certainly the case. Models suggest that the South Atlantic Anomaly might be related to some kind of disturbance or variation at the boundary between the outer core and the base of the mantle, but that position is about 2900 kilometers – 1800 miles – beneath the surface. It’s studied mostly by looking at variations in seismic waves, although information about earth’s gravity and magnetic fields also comes from specialized satellites.

This is a field of study that’s very much in flux, with new ideas and models coming out yearly. Stay tuned.

—Richard I. Gibson

 Links:

Interview with Jay Shah

The South Atlantic Anomaly
A really good look at the nature and implications of reversals