One of the Earth's special features is its strong magnetism. The whole planet surrounds itself with the same kind of magnetic field that the magnets around your household have. This geomagnetic field is important to civilization as well as a long-standing puzzle for geoscience.
In ancient times it was known that certain stones had the power of attracting iron to themselves, called magnetism.
Later it was found that these lodestones can be used to point perpetually in the direction of north. Without science, this was simply a magical fact, but starting in the Middle Ages navigators at sea trusted the magnetic compass to show them north when the sun and stars could not be seen.
In England in the late 1500s, William Gilbert reviewed centuries of literature and found it mostly unsound. Previous thinkers said that the compass must be attracted to a star or constellation, or to a remote northerly island made of iron. By conducting many experiments with a large spherical lodestone, Gilbert was able to explain that, instead, the Earth itself is a magnet. His book On the Magnet, published in 1600, is a classic of early science that is still worth reading for its rigorous blend of reasoning and experiment.
In Gilbert's time it was known that the compass is not steady, but fluctuates by a few degrees in various places for unknown reasons. In 1698 another early geophysicist, Edmond Halley, mapped the geomagnetic field of the South Atlantic Ocean and documented its changes.
His map showed that geomagnetic fluctuations were important matters for commerce and operations of the royal navy.
Today we rely on the GPS satellite network for navigation, but magnetic compass bearings are still a mainstay of navigators on the sea and in the air. Airport runways are designated by their magnetic bearing, and occasionally the numbers must be adjusted as the geomagnetic field changes.
In the 1800s researchers learned that electric current generated magnetism all by itself. That led to the invention of electric motors, launching a great wave of technological advances. At this point it was clear that Earth's magnetic field is generated by some sort of internal engine. Around 1840 a worldwide network of magnetic observatories was launched to gather data and aid navigators. Around 1900 it was shown that Earth has a heavy core that must consist of molten iron, and the race was on to figure out the mechanics of geomagnetism.
Your everyday electromagnet runs electricity through a coil of wire, which lines up the resulting magnetism in a nice strong bundle like the magnetic field made by a solid iron magnet. Conversely, moving a magnet through an electric field generates a current. This is how turbines generate electricity in a dynamo and how motors turn electricity into work. Somehow the liquid iron of the Earth's core forms a geodynamo that does that too, but without any wires. The physics of a stirring, electrically conductive liquid, called magnetohydrodynamics, takes gnarly math to model, but in the 1990s some researchers began to succeed in making computer models that act much like the real Earth.
The basics, as we understand them today, are easy to describe. The Earth's core has a central ball of solid iron (the inner core) with a thick layer of liquid iron (the outer core) around it—the higher pressure at the center is what makes iron solidify there. The liquid iron is not pure, but has a fraction consisting of other, lighter elements. Iron freezes against the inner core and leaves the lighter fraction behind, which then rises through buoyancy. And at the top of the outer core, the iron loses heat to the overlying mantle and sinks because of its greater density. The sinking and rising streams of iron keep the outer core stirring, and that's the source of energy behind the geodynamo.
Now take this arrangement and spin it, to match the Earth's rotation. In that setting, the stirring iron organizes itself into several rotating cylinders, pointing north-south and packed around the inner core. Those cylinders create the magnetohydrodynamic equivalent of the wire coil in an electromagnet. Electric currents in the rotating iron generate magnetic fields that wrestle against the mechanical motion of the iron in a natural dynamo. As long as the inner core keeps growing, there's enough mechanical energy to keep this geodynamo running.
The fluctuations of the geomagnetic field, unexplained for centuries, are natural in a dynamo based on complex fluid currents rather than wires and magnets. Parts of the field actually have four magnetic poles rather than two. Over thousands of years, the geomagnetic field wobbles and jerks, sometimes even flipping all the way over. The most recent computer models reproduce this kind of semi-stable behavior, including the reversals.
The Earth's magnetic field extends well above the atmosphere into outer space in a zone called the magnetosphere, where it interacts with the magnetic field from the Sun. The Sun pours out a constant flow of charged particles, the solar wind, that carries solar magnetic field lines with it. The edge of the magnetosphere is where the two fields meet and combat each other. The solar particles are mostly deflected, while some get trapped following the geomagnetic field lines, spiraling around them and bouncing between the north and south magnetic poles. As these penetrate into the upper atmosphere they generate auroras and lose their energy.
As a result, most of the solar wind is prevented from reaching Earth, where it could damage the atmosphere by ionizing gases, breaking apart molecules and driving them away from the planet. The magnetosphere also screens out most cosmic rays, highly charged particles from outside the solar system that create radiocarbon and other interesting isotopes with geologic applications. Without our magnetic protection, we would get dangerous doses of radiation like those that astronauts must protect themselves from with steel armor.
Discovery of the Geomagnetic Field
In ancient times it was known that certain stones had the power of attracting iron to themselves, called magnetism.
Later it was found that these lodestones can be used to point perpetually in the direction of north. Without science, this was simply a magical fact, but starting in the Middle Ages navigators at sea trusted the magnetic compass to show them north when the sun and stars could not be seen.
In England in the late 1500s, William Gilbert reviewed centuries of literature and found it mostly unsound. Previous thinkers said that the compass must be attracted to a star or constellation, or to a remote northerly island made of iron. By conducting many experiments with a large spherical lodestone, Gilbert was able to explain that, instead, the Earth itself is a magnet. His book On the Magnet, published in 1600, is a classic of early science that is still worth reading for its rigorous blend of reasoning and experiment.
In Gilbert's time it was known that the compass is not steady, but fluctuates by a few degrees in various places for unknown reasons. In 1698 another early geophysicist, Edmond Halley, mapped the geomagnetic field of the South Atlantic Ocean and documented its changes.
His map showed that geomagnetic fluctuations were important matters for commerce and operations of the royal navy.
Today we rely on the GPS satellite network for navigation, but magnetic compass bearings are still a mainstay of navigators on the sea and in the air. Airport runways are designated by their magnetic bearing, and occasionally the numbers must be adjusted as the geomagnetic field changes.
Generation of the Geomagnetic Field
In the 1800s researchers learned that electric current generated magnetism all by itself. That led to the invention of electric motors, launching a great wave of technological advances. At this point it was clear that Earth's magnetic field is generated by some sort of internal engine. Around 1840 a worldwide network of magnetic observatories was launched to gather data and aid navigators. Around 1900 it was shown that Earth has a heavy core that must consist of molten iron, and the race was on to figure out the mechanics of geomagnetism.
Your everyday electromagnet runs electricity through a coil of wire, which lines up the resulting magnetism in a nice strong bundle like the magnetic field made by a solid iron magnet. Conversely, moving a magnet through an electric field generates a current. This is how turbines generate electricity in a dynamo and how motors turn electricity into work. Somehow the liquid iron of the Earth's core forms a geodynamo that does that too, but without any wires. The physics of a stirring, electrically conductive liquid, called magnetohydrodynamics, takes gnarly math to model, but in the 1990s some researchers began to succeed in making computer models that act much like the real Earth.
The basics, as we understand them today, are easy to describe. The Earth's core has a central ball of solid iron (the inner core) with a thick layer of liquid iron (the outer core) around it—the higher pressure at the center is what makes iron solidify there. The liquid iron is not pure, but has a fraction consisting of other, lighter elements. Iron freezes against the inner core and leaves the lighter fraction behind, which then rises through buoyancy. And at the top of the outer core, the iron loses heat to the overlying mantle and sinks because of its greater density. The sinking and rising streams of iron keep the outer core stirring, and that's the source of energy behind the geodynamo.
Now take this arrangement and spin it, to match the Earth's rotation. In that setting, the stirring iron organizes itself into several rotating cylinders, pointing north-south and packed around the inner core. Those cylinders create the magnetohydrodynamic equivalent of the wire coil in an electromagnet. Electric currents in the rotating iron generate magnetic fields that wrestle against the mechanical motion of the iron in a natural dynamo. As long as the inner core keeps growing, there's enough mechanical energy to keep this geodynamo running.
The fluctuations of the geomagnetic field, unexplained for centuries, are natural in a dynamo based on complex fluid currents rather than wires and magnets. Parts of the field actually have four magnetic poles rather than two. Over thousands of years, the geomagnetic field wobbles and jerks, sometimes even flipping all the way over. The most recent computer models reproduce this kind of semi-stable behavior, including the reversals.
How the Geomagnetic Field Protects Us
The Earth's magnetic field extends well above the atmosphere into outer space in a zone called the magnetosphere, where it interacts with the magnetic field from the Sun. The Sun pours out a constant flow of charged particles, the solar wind, that carries solar magnetic field lines with it. The edge of the magnetosphere is where the two fields meet and combat each other. The solar particles are mostly deflected, while some get trapped following the geomagnetic field lines, spiraling around them and bouncing between the north and south magnetic poles. As these penetrate into the upper atmosphere they generate auroras and lose their energy.
As a result, most of the solar wind is prevented from reaching Earth, where it could damage the atmosphere by ionizing gases, breaking apart molecules and driving them away from the planet. The magnetosphere also screens out most cosmic rays, highly charged particles from outside the solar system that create radiocarbon and other interesting isotopes with geologic applications. Without our magnetic protection, we would get dangerous doses of radiation like those that astronauts must protect themselves from with steel armor.