The Earth's Unpredictable Magnetic Field

Is it possible to predict the variations of the Earth’s magnetic field? Though a seemingly impossible challenge, scientists are keen to find out. “We can’t take measurements in the Earth’s core,” regrets Gauthier Hulot, from IPGP’s Geomagnetism team.¹ That is where the magnetic field, which protects us from the flux of solar particles, originates. So dangerous is this radiation for the molecules of life that without this magnetic shield, the Earth would probably have remained barren.
“There is continuous convective movement within the liquid core, which carries heat towards the mantle by moving material,” explains Henri-Claude Nataf, from LGIT^,2 also director of OSUG.³ Furthermore, the liquid core, composed of iron and nickel, is an excellent conductor. Convective movement and conductivity enabled a minute magnetic field to appear at least 3.5 billion years ago and initiate a dynamo effect: electric currents were produced within the conducting material once it gathered momentum. The currents produced a magnetic field, which in turn strengthened the currents. This positive feedback continued, eventually giving rise to the Earth’s current magnetic field, which will only cease when the center of the Earth cools down and convection comes to a stop.
If 800,000 years ago Homo erectus had been able to invent the magnetic compass, its needle would have pointed towards the South Pole, not the North Pole. In fact, the intensity and direction of the Earth’s magnetic field varies constantly, and sometimes even reverses altogether. Fortunately for scientists, this is recorded in the rocks. “When the magnetic components in volcanic rocks are hotter than a temperature threshold called the Curie point, they no longer show any magnetization,” Nataf explains. “But as they cool down, the crystals become magnetized in the direction of the Earth’s magnetic field and become fixed in that position.” This lets us keep track of what occurred up to 3.5 billion years into the past. To get more accurate results about more recent events, geophysicists look for archeological artifacts. “Baked clay also has a magnetic memory,” Hulot explains. “When a brick is re-heated–during a fire, for example–its memory is reset to zero. This makes it possible to give a precise date to the magnetic field recorded in the material on that day.” The uninterrupted observation of the Earth’s magnetic field has become increasingly precise, and the orientation of the magnetic field has been recorded since the early 17th century.

Reversals
We now know that the poles have reversed more than 300 times in the last 200 million years. Prior to each reversal, the magnetic field weakens, causing the magnetic shield to become less intense. “The cycles are extremely irregular,” Hulot explains. “Periods of stability lasting 30 million years have been observed, as have reversals at intervals of only a few 10,000 years. If there’s a pattern, it must be probabilistic and very subtle.” The most recent reversal took place 780,000 years ago. We are the living proof that our ancestors–Homo erectus–survived it. On the other hand, if a reversal occurred today, satellites and electrical power grids would suffer frequent disruptions since the Earth would no longer be sheltered from magnetic storms–these sudden surges of charged particles caused by eruptions on the Sun’s surface.
To better understand the Earth’s magnetic field, scientists are studying other planets, like Mars, which has kept traces of a magnetic field that has long since vanished. But they are also attempting to recreate this phenomenon in the laboratory. For instance, geophysicists have managed to reproduce a dynamo mechanism–though still considerably different from the one operating at the center of the Earth. “We use liquid sodium, which provides the best electrical conductivity and the best fluidity,” Nataf explains. “To reproduce the Earth’s magnetic field,” Hulot adds, “we would need a much more conductive and fluid material. We just don’t have one. But we learn a great deal from these experiments.”
In the last few hundred years, the intensity of the field has fallen by 10%. Is another reversal coming? “It could simply be a minor disturbance,” says Hulot. “No deduction can be made about when the next reversal will take place.” We can only estimate variations in the magnetic field over the next five years–already quite an achievement.

Denis Delbecq