The Earth’s magnetic field is Magnetosphere. The extent of a planet’s dynamic resurfacing due to plate tectonics provides indirect evidence about whether it still has a molten interior. But another, more direct tool for probing the interior of Earth is an ordinary compass, which senses the magnetic field outside our planet. Magnetic field measurements prove to be an extremely powerful way to investigate the internal structure of a world without having to actually dig into its interior. To illustrate how this works, consider the behavior of a compass on Earth.
Source of Earth’s Magnetic Field
The needle of a compass on Earth points north because it aligns with Earth’s magnetic field. Such fields arise whenever electrically charged particles are in motion. For example, a loop of wire carrying an electric current generates a magnetic field in the space around it. The magnetic field that surrounds an ordinary bar magnet is created by the motions of negatively charged electrons within the iron atoms of which the magnet is made. Earth’s magnetic field is similar to that of a bar magnet. The consensus among geologists is that this magnetic field is caused by the motion of the liquid portions of Earth’s interior. Because this molten material (mostly iron) conducts electricity, these motions give rise to electric currents, which in turn produce Earth’s magnetic field. Our planet’s rotation helps to sustain these motions and hence the magnetic field. This process for producing a magnetic field is called a dynamo.
Change of Earth’s Magnetic Field
Studies of ancient rocks reinforce the idea that our planet’s magnetism is due to fluid material in motion (for more information on the origin of the Earth’s magnetic field). When iron-bearing lava cools and solidifies to form igneous rock, it becomes magnetized in the direction of Earth’s magnetic field. By analyzing samples of igneous rock of different ages from around the world, geologists have found that Earth’s magnetic field actually flips over and reverses direction on an irregular schedule ranging from tens of thousands to hundreds of thousands of years. As an example, lava that solidified 30,000 years ago is magnetized in the opposite direction to lava that has solidified recently. Therefore, 30,000 years ago a compass needle would have pointed south, not north! If Earth were a permanent magnet, like the small magnets used to attach notes to refrigerators, it would be hard to imagine how its magnetic field could spontaneously reverse direction. But computer simulations show that fields produced by moving fluids in Earth’s outer core do indeed change direction from time to time.
The rate of reversals in the Earth’s magnetic field has varied widely over time. 72 million years ago (Ma), the field reversed 5 times in a million years. This could weaken Earth’s protective magnetic field by up to 90% during a polar flip. Earth’s magnetic field is what shields us from harmful space radiation which can damage cells, cause cancer, and fry electronic circuits and electrical grids. In fact, smaller fluctuations in field strength and magnetic poles are happening on a regular basis, so scientists are keen to collect as much data as possible on them.
Is the magnetic field useful for anything other than helping Earth’s inhabitants determine direction using a compass? As it turns out, Earth’s magnetic field has important effects far above Earth’s surface, where it interacts dramatically with charged particles from the Sun, a flow of mostly protons and electrons, known as the solar wind, that streams constantly outward from the Sun’s upper atmosphere. Near-Earth, the particles in the solar wind move at speeds of roughly 450 km/s, or about a million miles per hour, considerably faster than sound waves can travel in the very thin gas between the planets, so the solar wind is said to be supersonic. (Because the gas between the planets is so thin, interplanetary sound waves carry too little energy to be heard by astronauts.)
What if, No Magnetosphere!
If Earth had no magnetic field, it would be continually bombarded by this solar wind of charged particles. But our planet does have a magnetic field, and the forces that this field can exert on charged particles are strong enough to deflect them away from us. The region of space around a planet in which the motion of charged particles is dominated by the planet’s magnetic field is called the planet’s magnetosphere.
When the supersonic particles in the solar wind first encounter Earth’s magnetic field, they abruptly slow to subsonic speeds. Still, closer to Earth lies a boundary, called the magnetopause, where the outward magnetic pressure of Earth’s field is exactly counterbalanced by the impinging pressure of the solar wind. Most of the particles of the solar wind are deflected around the magnetopause, just as water is deflected to either side of the bow of a ship.
Some charged particles of the solar wind manage to leak through the magnetopause of the magnetosphere. When they do, they are trapped by Earth’s magnetic field in two huge, doughnut-shaped rings around Earth called the Van Allen belts. These belts were discovered in 1958 during the flight of the first successful U.S. Earth-orbiting satellite. They are named after the physicist James Van Allen, who insisted that the satellite carry a Geiger counter to detect charged particles. The inner Van Allen belt, which extends over altitudes of about 2000 to 5000 km, contains mostly protons. The outer Van Allen belt, about 6000 km thick, is centered at an altitude of about 16,000 km above Earth’s surface and contains mostly electrons.
Sometimes the magnetosphere becomes overloaded with particles. The particles then leak through the magnetic fields at their weakest points and cascade down into Earth’s upper atmosphere, usually in an oval-shaped pattern. As these high-speed charged particles collide with atoms in the upper atmosphere, they excite the atoms to high energy levels. The atoms then emit visible light as they drop down to their ground states, like the excited gas atoms in a neon light. The result is a beautiful, shimmering display called an aurora (plural aurorae). These are also called the northern lights (aurora borealis) or southern lights (aurora australis), depending on the hemisphere in which the phenomenon is observed.
Occasionally, a violent event on the Sun’s surface sends a particularly intense burst of protons and electrons toward Earth. The resulting auroral display can be exceptionally bright and can often be seen over a wide range of latitudes. Such events also disturb radio transmissions and can damage communications satellites and transmission lines.
It is remarkable that Earth’s magnetosphere, including its vast belts of charged particles, was entirely unknown until a few decades ago. Such discoveries remind us of how little we truly understand and how much remains to be learned even about our own planet.