Earth's magnetic field

• 1. Introduction

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  Earth's magnetic field begins not in the sky but roughly 3,400 kilometres beneath your feet, in a churning ocean of molten iron and nickel. The planet is mostly rock, yet at its heart sits a region of metal almost as hot as the surface of a star, around 6,000 kelvin at the inner core. From that heat, something invisible escapes into space and wraps the whole world in protection. Without it, the upper atmosphere would slowly be torn away by a stream of particles from the Sun. What is this field, and why does the place on a map labelled the North Magnetic Pole actually mark the south pole of Earth's own magnet? Why do compasses still work even though the poles refuse to sit still? And what happens, every few hundred thousand years, when the planet decides to flip north and south entirely?

• 2. The Geodynamo

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  Inside the liquid outer core, convection currents of molten iron alloy carry electric charge, and moving charge makes a magnetic field. This self-sustaining engine is called a geodynamo. The outer core is liquid, the inner core solid with a radius of about 1,220 kilometres, and heat flows from the inner core to the core-mantle boundary, which sits at roughly 3,800 kelvin. That temperature gradient drives the motion. As the core cools, some molten iron solidifies onto the inner core and leaves lighter elements behind, a process called compositional convection that keeps the fluid buoyant and rising. A dynamo cannot start from nothing; it needs a seed field to amplify. One candidate is the early Sun, which passed through a T-Tauri phase when its solar wind carried a magnetic field vastly stronger than today's, though much of it may have been screened out by Earth's mantle. The Earth's rotation organises this chaos through the Coriolis effect, bending the flow into rolls aligned along the north-south polar axis. The field hidden down there is far stronger than what reaches us. The average magnetic field in the outer core has been calculated at 25 gauss, about 50 times the strength felt at the surface.

• 3. A Tilted Bar Magnet

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  Picture an enormous bar magnet driven through the centre of the Earth at an angle of about 11 degrees from the rotational axis. That is the dipole approximation, and it accounts for 80 to 90 percent of the field in most locations. The geometry hides a famous twist. A magnet's north pole is defined as the end that points north, so it must be attracted to something opposite, which means the place called the North Magnetic Pole is really the south pole of Earth's magnetic field, where the field plunges downward into the ground. There are actually two ways to define the poles. The local definition is the spot where the field points straight down, with an inclination of 90 degrees at the North Magnetic Pole and minus 90 degrees at the South. The global definition comes from a mathematical model, the line through Earth's centre parallel to the best-fitting dipole, which marks the geomagnetic poles. Because the real field has a significant non-dipolar part, these two sets of poles do not coincide, and a compass generally points at neither. The poles also wander on their own. The North Magnetic Pole has been observed moving up to 40 kilometres per year, and over the last 180 years it travelled from Cape Adelaide in the Boothia Peninsula in 1831 to a point 600 kilometres from Resolute Bay by 2001.

• 4. The Shield In Space

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  Out where the field meets the solar wind, the neat dipole gets battered into a teardrop. The solar wind is a stream of charged particles leaving the Sun's corona at 200 to 1,000 kilometres per second, carrying its own interplanetary magnetic field. Where the pressure of that wind balances the pressure of Earth's field, a boundary forms called the magnetopause, marking the edge of the magnetosphere. The shape is lopsided. On the sunward side the boundary sits about 10 Earth radii out, while the far side trails into a magnetotail stretching beyond 200 Earth radii. Just sunward of the magnetopause lies the bow shock, where the solar wind slows abruptly. Trapped inside are two tyre-shaped zones called the Van Allen radiation belts, holding high-energy ions from 0.1 to 10 MeV, the inner belt 1 to 2 Earth radii out and the outer belt at 4 to 7. The field also turns away cosmic rays from beyond the Solar System, which is why standing on the Moon offers no such protection. Anyone on the lunar surface during a violent solar eruption in 2005 would have received a lethal dose. The same physics that protects us once gave Mars no mercy. As the Martian magnetic field dissipated, the solar wind scavenged ions until the planet lost nearly its entire atmosphere.

• 5. Storms And Aurorae

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  The Carrington Event of 1859 remains the largest documented geomagnetic storm. It induced currents strong enough to disrupt telegraph lines, and aurorae were reported as far south as Hawaii. These storms begin when a coronal mass ejection erupts above the Sun and sends a shock wave through the Solar System, a wave that can reach Earth in as little as two days. The damage is not only historical. The Halloween storm of 2003 knocked out more than a third of NASA's satellites. When charged particles do slip inside the magnetosphere, they spiral along field lines and bounce between the poles several times per second. Positive ions drift westward and negative ions eastward, creating a ring current that actually weakens the field measured at the surface. Particles that reach the ionosphere and strike atoms there light up the aurorae and emit X-rays at the same time. The short-term jitter of the field has its own scale, the K-index, used to track instability. Data from the THEMIS mission overturned an earlier assumption, showing the field that interacts with the solar wind is reduced when the magnetic orientation aligns between Sun and Earth, a condition that during future solar storms could bring blackouts and satellite disruptions.

• 6. Reading The Rocks

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  Iron oxides such as magnetite act as the planet's tape recorder, locking in the direction of the field when rock cools or sediment settles. In a lava flow the field is frozen into tiny minerals as they cool, a thermoremanent magnetization. In sediments, magnetic particles tilt slightly toward the field as they settle on an ocean or lake floor, a detrital remanent magnetization. The most dramatic story written this way is reversal. At irregular intervals, the North and South geomagnetic poles trade places, with gaps between reversals ranging from under 0.1 million years to as much as 50 million years. The most recent, the Brunhes-Matuyama reversal, happened about 780,000 years ago. A milder cousin called a geomagnetic excursion swings the dipole axis across the equator and back to the original polarity, as in the Laschamp event during the last ice age, 41,000 years ago. Reversals leave a stunning signature on the seafloor. As crust spreads from a mid-ocean ridge, each band of cooling basalt records whatever direction the field held at the time, producing stripes symmetric about the ridge that a ship towing a magnetometer can read like a barcode. The recording stretches deep into the planet's past. Paleomagnetic work on Paleoarchean lava in Australia and conglomerate in South Africa points to an ancient field, and in 2024 researchers published evidence from Greenland for a magnetic field as early as 3,700 million years ago.

• 7. A Field In Flux

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  Carl Friedrich Gauss measured the field's strength in 1832, and the record since shows a relative decay of about 10 percent over the last 150 years. Over the last two centuries the dipole strength has fallen at roughly 6.3 percent per century; at that pace the field would vanish in about 1,600 years. The alarm is misleading, though. The present strength is about average for the last 7,000 years, and the current rate of change is not unusual, with geomagnetic intensity declining from a maximum 35 percent above the modern value around year 1 AD. The field is best described as heteroscedastic, a fancy word for seemingly random fluctuation. A handful of measurements across decades or even centuries cannot pin down a long-term trend, and the dipole part can shrink while the total field holds steady or even grows. Speed records keep being broken too. A 1995 study of lava flows on Steens Mountain, Oregon seemed to show the field shifting up to 6 degrees per day, until a 2014 follow-up traced that to the lava slowly demagnetizing as it cooled, not the field itself. Then in July 2020, analysis of simulations and a recent field model found maximum directional change reaching about 10 degrees per year, nearly 100 times faster than today. Meanwhile the magnetic north pole keeps drifting from northern Canada toward Siberia, accelerating from 10 kilometres per year at the start of the 1900s to 40 kilometres per year in 2003.

• 8. Mapping The Invisible

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  Gauss did more than measure the field; he gave us the language to describe its global shape, fitting measurements to spherical harmonics. Each harmonic corresponds to an arrangement of magnetic charges at Earth's centre, from the never-observed monopole to the dipole to the quadrupole and beyond. The lowest coefficient, the monopole term, is zero, while the next three set the dipole's direction and magnitude. Two standard models govern modern navigation. The International Geomagnetic Reference Field, maintained by the International Association of Geomagnetism and Aeronomy, is updated every five years and now truncates at degree 13 with 195 coefficients. The World Magnetic Model, produced jointly by the United States National Centers for Environmental Information and the British Geological Survey, truncates at degree 12 with 168 coefficients and is used by the United States Department of Defense, the FAA, NATO, the International Hydrographic Organization and many civilian systems. For finer detail, the Enhanced Magnetic Model reaches degree and order 790, resolving anomalies down to a 56-kilometre wavelength using data including the European Space Agency's Swarm satellite. Satellites such as Magsat and Ørsted probe the three-dimensional structure directly, and the Ørsted comparison hinted at an emerging alternate pole under the Atlantic west of South Africa. On the ground, the International Real-time Magnetic Observatory Network has linked over 100 observatories since 1991. There are still surprises hiding in the field's smallest signals, including ones written by the sea itself, because seawater is a weak conductor and the ocean's heat content can now be inferred from its faint tug on the geomagnetic lines.