Earth's Magnetic Field

Earth has its own magnetic field: an invisible region of magnetic force that extends from deep inside the planet out into space. Although it is too weak to pick up a paperclip, the field is essential to life. It makes compasses work, lets birds and fish navigate huge distances, drives the colourful auroras, and (most importantly) shields life on the surface from harmful radiation from the Sun. The field is generated by molten iron flowing slowly in Earths outer core, around 3,000 km beneath your feet.

  • What it isEarths magnetismActs like a giant bar magnet
  • Generated byMolten iron in outer core3,000 km below surface
  • Field strength at surface25-65 microteslaVery weak by lab standards
  • Protects fromSolar wind, cosmic raysOtherwise atmosphere would erode
  • MakesAuroras at the polesCompass needles point north
  • Flips every200-300 thousand yearsOn average, irregular schedule

Where the field comes from

Earths magnetic field is created deep underground, in the planets outer core. The outer core is a layer of liquid iron and nickel, about 2,200 km thick, sitting around 3,000 km beneath the surface. Temperatures there are around 4,000 to 5,000 degrees Celsius.

The molten metal is constantly moving. As Earth spins, hot metal rises from the inner core and cooler metal sinks back down, creating slow convection currents (like a giant boiling pot). The moving liquid iron carries electric charges with it, and moving charges produce magnetic fields. This process is called the geodynamo.

The geodynamo has kept Earths magnetic field going for billions of years. We know this from studying old rocks: when rocks cool from molten lava, the iron particles in them line up with the magnetic field of the time and lock in place forever, giving us a record of Earths past magnetic field.

What the field looks like

Earths magnetic field has a shape similar to that of a giant bar magnet stuck through the centre of the planet. Field lines come out of the south end of the bar (near Earths geographic North), curve around the planet, and re-enter at the north end of the bar (near Earths geographic South).

The "bar magnet" is tilted at about 11 degrees from Earths spin axis. That is why the magnetic poles are not in the same places as the geographic poles. Both magnetic poles also drift slowly over time. The North Magnetic Pole is currently moving from Canada towards Siberia at about 55 km per year, the fastest movement in recorded history.

Fact Earths magnetic North Pole is actually a magnetic SOUTH pole technically, because the "north end" of any compass needle is attracted to it (and opposite poles attract). The naming has historical roots: the words "north pole" of a compass needle existed long before scientists understood the underlying physics. Confusing but harmless.

The magnetosphere

The space around Earth shaped by our magnetic field is called the magnetosphere. It stretches about 65,000 km outwards on the side facing the Sun (where the solar wind pushes it inward) and trails out into a long "tail" on the opposite side, sometimes longer than the distance to the Moon.

The magnetosphere acts like a giant invisible shield. Without it, the constant flow of charged particles from the Sun (the solar wind) would slowly strip away Earths atmosphere over billions of years. Mars, which lost its magnetic field early in its history, also lost most of its atmosphere as a result.

The auroras

Some solar wind particles still manage to sneak through Earths magnetic shield near the magnetic poles. They spiral down along field lines and crash into the upper atmosphere, exciting nitrogen and oxygen atoms which glow:

  • Green aurora: oxygen at about 100-300 km altitude. The most common colour.
  • Red aurora: oxygen higher up, above 300 km. Rarer.
  • Blue and purple aurora: from nitrogen molecules.

The northern lights (aurora borealis) are best seen in places near the North Magnetic Pole: northern Scotland, Iceland, Norway, Alaska, Canada. The southern lights (aurora australis) are best seen in southern Argentina, southern Chile and Antarctica.

How to detect the field

The simplest device for detecting Earths magnetic field is the compass: a small magnet free to swing on a low-friction pivot. Earths magnetic field gently aligns the compass needle so its "north pole" points to magnetic North.

Compasses have been used for navigation for over a thousand years, since they were invented in China around 200 BC. Before GPS, compasses were essential equipment for ships, expeditions, soldiers and hikers. They still are: GPS can fail, but a compass needs no batteries and always works.

Did you know? Many animals can detect Earths magnetic field and use it for navigation. Sea turtles use it to return to the exact beach where they were born, even after years swimming across oceans. Birds use it during long migrations. Salmon use it to find their birth river. Even some bacteria have tiny magnetic particles in their cells that line them up with the field. How animals sense it remains an active area of research.

Field reversals

Earths magnetic field is not stable forever. Every few hundred thousand years, the field flips completely: the magnetic North becomes the magnetic South and vice versa. The last full reversal happened about 780,000 years ago.

Reversals are slow on a human timescale (each one takes about 1,000 to 10,000 years), but very fast geologically. During the transition, the field weakens dramatically (perhaps to 10 per cent of normal) and may temporarily have several pairs of poles in different places at the same time.

Reversals do not appear to cause mass extinctions. Life survived all the previous ones we know of. But a future reversal would make compasses unreliable, cause spectacular auroras worldwide, and force redesigning of satellites and power grids to cope with extra space-weather damage during the weak-field period.

Space weather

Solar storms (sudden eruptions of solar wind from the Sun) can briefly weaken Earths magnetic field. Major storms can cause:

  • Damage to satellites in orbit.
  • Disrupted radio communications.
  • Power surges in long electricity grids, sometimes large enough to cause blackouts.
  • Brilliant auroras visible much further south (or north) than usual.
  • Slight problems for GPS accuracy.

The biggest known solar storm was the Carrington Event of 1859, when telegraph systems worldwide were knocked out, some catching fire from the surges. Aurora was visible as far south as the Caribbean. A modern Carrington-class event would cost trillions of pounds in damage to power grids, satellites and electronics. Scientists now monitor space weather constantly and can shut down vulnerable systems if a big storm is forecast.

Try this Make a simple compass at home. Magnetise a steel sewing needle by stroking it about 30 times in the same direction with a strong magnet. Float the needle on a small piece of cork in a bowl of still water. Slowly the needle will turn until it points north-south, aligned with Earths magnetic field. You can also balance the needle on a thread (tied around the middle) and let it dangle. A real compass is essentially the same thing in a more durable case.
Deeper dive: how Earths magnetism was discovered

People have used naturally magnetic stones called lodestones for thousands of years. The ancient Greeks knew that lodestones (magnetite ore from the region of Magnesia in modern Turkey, where the word "magnet" comes from) could attract iron. Around 200 BC, the Chinese discovered that a lodestone needle floating on water would consistently point in the same direction. The compass was born.

By the 1100s, Chinese sailors were navigating by compass on long sea voyages. The technology spread to Europe by the 1200s and revolutionised navigation, enabling the great age of discovery in the 1400s and 1500s. But for centuries no one knew WHY compass needles pointed north.

The first clue came from English physician and scientist William Gilbert. In 1600 he published De Magnete ("On the Magnet"), based on years of careful experiments. Gilbert showed that Earth itself acted like a giant magnet, with the North Magnetic Pole near the geographic North. His insight was revolutionary: the Earth was not a passive ball, but a kind of huge spinning magnet.

Two centuries later, German mathematician Carl Friedrich Gauss measured the field strength precisely and proposed mathematical models for its shape. The unit of magnetic field strength is now called the gauss in his honour.

It was not until the 1940s and 1950s that scientists figured out the source of the field: convection of molten iron in Earths outer core. The full mathematical theory of the geodynamo was only worked out in the late 20th century, with the help of large computer simulations.

Today we know that Earths magnetic field reversed sign many times in the past, that it has weakened slightly over the last 200 years, that it varies with the Suns activity and that it is slowly drifting. Each new instrument (modern satellites, geomagnetic ground stations, deep-sea sediment cores) tells us more about one of the most important and least visible features of our home planet.

For more, see what is a magnet and electromagnets.