Most of us take it for granted that compasses point north. Sailors have relied on Earth's magnetic field to navigate for thousands of years. Birds and other magnetically sensitive animals have done so for considerably longer. Strangely enough, however, the planets magnetic poles have not always been oriented as they are today.
Minerals that record past orientations of Earth's magnetic field reveal that it has flipped from north to south and back again hundreds of times during the planets 4.5-billion-year history. But a switch has not occurred for 780,000 years--considerably longer than the average time between reversals, about 250,000 years. What is more, the primary geomagnetic field has lessened by nearly 10 percent since it was first measured in the 1830s. That is about 20 times faster than the field would decline naturally were it to lose its power source. Is this just a fluctuation in Earths magnetic field, or could another reversal be on its way?
Geophysicists have long known that the source of the fluctuating magnetic field lies deep in the center of Earth. Our home planet, like several other bodies in the solar system, generates its own magnetic field through an internal dynamo. In principle, Earth's dynamo operates like the familiar electric generator, which creates electric and magnetic fields from the kinetic energy of its moving parts. In a generator, the moving parts are spinning coils of wire; in a planet or star, the motion occurs within an electrically conducting fluid. A vast sea of molten iron more than seven times the volume of the moon circulates at Earth's core, constituting the so-called geodynamo.
Until recently, scientists relied primarily on simple theories to explain the geodynamo and its magnetic mysteries. In the past 10 years, however, researchers have developed new ways to explore the detailed workings of the geodynamo. Satellites are providing clear snapshots of the geomagnetic field at Earth's surface, while new strategies for simulating Earth-like dynamos on supercomputers and creating physical models in the laboratory are elucidating those orbital observations. These efforts are providing an intriguing explanation for how polarity reversals occurred in the past and clues to how the next such event may begin.
Driving the Geodynamo
BEFORE WE EXPLORE how the magnetic field reverses, it helps to consider what drives the geodynamo. By the 1940s physicists had recognized that three basic conditions are necessary for generating any planets magnetic field. A large volume of electrically conducting fluid, the iron-rich liquid outer core of Earth, is the first of these conditions. This critical layer surrounds a solid inner core of nearly pure iron and underlies 2,900 kilometers of solid rock that form the massive mantle and the ultrathin crust of continents and ocean floors. The overlying burden of the crust and mantle creates average core pressures two million times greater than at the planet's surface. Core temperatures are similarly extreme--about 5,000 degrees Celsius, similar to the temperature at the surface of the sun.
These extreme environmental conditions set the stage for the second requirement of planetary dynamos: a supply of energy to move the fluid. The energy driving the geodynamo is part thermal and part chemical--both create buoyancy deep within the outer core. Like a pot of soup simmering on a burner, the core is hotter at the bottom than at the top. (The core's high temperatures are the result of heat that was trapped at the center of Earth during its formation.) That means the hot, buoyant iron in the lower part of the outer core tends to rise upward like blobs of hot soup. When the fluid reaches the top of the outer core, it loses some of its heat in the overlying mantle. The liquid iron then cools, becoming denser than the surrounding medium, and sinks. This process of transferring heat from bottom to top through rising and sinking fluid is called thermal convection--the second planetary condition needed for generating a magnetic field.