A note on the title
This article is about Earth’s magnetic field. The field is separate from the atmosphere, which is the layer of air around the planet, but the two interact. Charged particles guided by the field collide with atmospheric gases to make auroras, and the field helps protect the upper atmosphere from being eroded by the solar wind. That connection is why the two topics are often discussed together.
What a magnetic field is
A magnetic field is a region where magnetic forces act on moving electric charges and on magnetic materials. Earth behaves roughly like a giant bar magnet with a north and a south pole, tilted by around 10 degrees from the planet’s spin axis. The field lines emerge from one hemisphere, loop out into space and return in the other. Near the surface the field is weak by everyday standards; a typical refrigerator magnet is far stronger up close. Its importance comes from its enormous size.
Where it comes from: the geodynamo
The field is not produced by a permanent magnet buried in the ground. Iron gets too hot deep inside Earth to remain magnetized permanently. Instead, the leading explanation is the geodynamo.
Earth’s core has a solid inner part and a liquid outer part made mostly of molten iron mixed with some lighter elements. Heat escaping from the core, along with material slowly crystallizing onto the inner core, drives churning convection in the liquid. Because Earth rotates, that flow is organized into spiraling patterns. Moving electrically conducting fluid generates electric currents, and those currents generate magnetic fields, which in turn reinforce the flow of currents. As long as energy keeps driving the motion, the system sustains itself.
This picture is supported by computer simulations and by measurements, though many details remain active research topics. Scientists cannot observe the outer core directly, since the core lies thousands of kilometers below the surface; the knowledge comes largely from studying how seismic waves travel through the planet, a topic tied to Earth structure.
Why compasses point north
A compass needle is a small magnet that lines up with the local magnetic field. The end that points north is attracted toward the planet’s northern magnetic region. Because opposite poles attract, that region is in fact a magnetic south pole in the physics convention, though it is called the north magnetic pole by geography.
Magnetic north and geographic north are not the same place. The difference at your location is called magnetic declination, and it varies from place to place and over time. Navigators correct for it by checking a current map or chart. The magnetic poles also drift, in recent decades at a noticeable pace, so the correction changes over the years. Compasses can also be thrown off by nearby iron objects, electronics or magnets, and become unreliable close to the magnetic poles themselves.
Reversals and changes over time
The strength and direction of the field change. Over short periods, it wobbles and drifts. Over geological time, the field has completely flipped polarity many times, so that a compass would point south. The flips happen at irregular intervals, often hundreds of thousands of years apart or longer, and they take a long time to complete, thousands of years rather than a sudden jolt. Scientists know about past reversals because volcanic and sedimentary rocks record the direction of the field when they formed. Whether another reversal is near is not something science can currently predict.
The magnetosphere: a shield in space
Earth’s magnetic field extends far into space, forming a region called the magnetosphere. The Sun continually emits a flow of charged particles called the solar wind. Charged particles feel a force when moving through a magnetic field, so most of the solar wind is deflected around the planet instead of striking the atmosphere directly. On the side facing the Sun the magnetosphere is compressed, while on the far side it stretches into a long tail.
This shielding is thought to be one reason why Earth’s atmosphere has been retained over billions of years. Mars, which lost its global field early on, has a much thinner atmosphere, and the loss of the field is considered one factor in its Martian climate history, although other factors also played a role.
Auroras
Some solar wind particles do get in, guided along field lines toward the polar regions. When these energetic particles collide with oxygen and nitrogen in the upper atmosphere, they excite the atoms, which then release the energy as light. Oxygen commonly produces green and sometimes red light, while nitrogen can add blue or purple hues. The result is the aurora, called the northern lights in the Northern Hemisphere and the southern lights in the Southern.
Auroras tend to occur in a ring around each magnetic pole and become brighter and more widespread when the Sun is active. Anyone planning to see one should check current aurora viewing conditions, which depend on darkness, cloud cover and solar activity.
Space weather and technology
When the Sun releases bursts of energy, the resulting solar storms can disturb the magnetosphere. Strong events can induce currents in long power lines, interfere with radio, and affect satellites. They are one part of space weather effects that operators of satellites and power grids monitor. Satellite navigation signals can also be degraded during disturbances.
Many animals are thought to sense the field to help them navigate, including some birds and sea turtles, although the exact mechanisms are still being studied.
Key points
- The field is generated by moving liquid iron in the outer core.
- Compasses align with the field but point to magnetic north, not geographic north.
- The field flips polarity at irregular, long intervals.
- The magnetosphere deflects most solar wind, and the particles that reach the atmosphere create auroras.