Geomagnetism is the magnetic field generated by Earth itself, stretching from deep inside the planet’s core out into space, where it forms an invisible shield around the entire globe. This field is what makes compass needles point north, protects life from harmful solar radiation, and guides migrating animals across thousands of miles. At the surface, its strength ranges from about 22,000 to 67,000 nanoteslas, roughly 1,000 times weaker than a refrigerator magnet but powerful enough to deflect streams of charged particles from the Sun.
How Earth Generates Its Magnetic Field
Earth’s magnetic field originates in the outer core, a layer of liquid iron and nickel roughly 2,200 kilometers thick that sits between the solid inner core and the rocky mantle above. This molten metal is in constant motion, driven by heat escaping from the inner core and by the gravitational energy released as the planet slowly cools and heavier elements sink toward the center. As Earth rotates, it organizes these churning flows into spiraling columns of liquid metal.
Moving electrically conductive fluid generates electric currents, and those currents in turn produce magnetic fields, which then influence the flow of the liquid, sustaining the whole cycle. This self-reinforcing loop is called the geodynamo. It has kept Earth’s magnetic field running for at least 3.5 billion years, though the field’s strength and direction have shifted continuously over that time.
Magnetic North vs. Geographic North
Geographic north is the fixed point at latitude 90°N where Earth’s rotation axis meets the surface. Magnetic north is the spot where the planet’s magnetic field lines point straight down into the ground, and it doesn’t sit in the same place. Right now, magnetic north is in the Canadian Arctic, drifting toward Siberia. Since the 1970s, that drift has accelerated from less than 10 miles per year to more than 30.
The angle between true north and the direction a compass needle points is called magnetic declination. Depending on where you stand on the planet, your compass could be off by anywhere from zero to more than 20 degrees east or west. Navigators, surveyors, and mapping software all correct for this offset. Declination values are updated regularly using models like the International Geomagnetic Reference Field, now in its 13th generation.
The Magnetosphere: Earth’s Radiation Shield
Earth’s magnetic field doesn’t stop at the surface. It extends tens of thousands of kilometers into space, forming a region called the magnetosphere. This is the planet’s primary defense against the solar wind, a constant stream of electrically charged particles blasting outward from the Sun at hundreds of kilometers per second. When those particles hit the magnetosphere, the magnetic field forces them to curve sideways rather than slamming into the atmosphere head-on. Most of the solar wind simply flows around the planet like water around a rock in a river.
Without this shield, the solar wind would gradually strip away the atmosphere, much as it appears to have done on Mars, which lost most of its global magnetic field billions of years ago. The magnetosphere also traps some charged particles in doughnut-shaped zones called the Van Allen radiation belts, keeping them well above the altitudes where people live and breathe.
The South Atlantic Anomaly
Not every part of the shield is equally strong. Over South America and the southern Atlantic Ocean, the field dips to unusually low intensity in a region known as the South Atlantic Anomaly. Here, charged particles from the Sun can reach closer to the surface than anywhere else on the planet. Satellites passing through this zone regularly experience computer glitches, data corruption, and sensor interference from the elevated particle radiation.
The anomaly is growing. Recent NASA observations show it expanding westward and weakening further. Between 2015 and 2020, it began splitting from a single weak spot into two distinct low-intensity cells, creating additional headaches for satellite mission planners who need to account for increased radiation exposure on each orbit.
How the Field Is Measured
Scientists describe Earth’s magnetic field using seven parameters. Two capture direction: declination (the compass offset from true north) and inclination (how steeply the field lines tilt into the ground). The remaining five capture intensity, broken into a total field strength, a horizontal component, a vertical component, and the north-south and east-west parts of the horizontal component. Together, these seven values give a complete picture of the magnetic field at any point on or above the surface.
On the ground, magnetic observatories around the world take continuous readings. From space, ESA’s Swarm mission, a constellation of three identical satellites, provides the most detailed survey of the geomagnetic field ever attempted. Swarm measures the field’s strength, direction, and changes over time with enough precision to tease apart the different sources: the deep core dynamo, magnetized rocks in the crust, electric currents flowing in the ionosphere, and the influence of the solar wind pressing on the magnetosphere.
Pole Reversals
Earth’s magnetic poles have swapped places many times. Over the last 170 million years, roughly 540 reversals have occurred, meaning north became south and south became north. The intervals between flips are irregular, sometimes as short as a few hundred thousand years, sometimes stretching to tens of millions.
A reversal isn’t a sudden event. Detailed records from both ocean sediments and sequential lava flows show the process unfolds in three phases: a precursor phase lasting about 2,500 years, a main transition of roughly 1,000 years when the poles actually swap, and a rebound phase of another 2,500 years as the field settles into its new orientation. The total duration runs about 9,000 to 11,000 years. During the transition, the overall field weakens significantly, which could increase radiation exposure at the surface, though life on Earth has survived every reversal so far. The last complete flip, called the Brunhes-Matuyama reversal, happened about 770,000 years ago.
Geomagnetic Storms and Infrastructure
When the Sun erupts with a massive burst of plasma, the resulting wave of charged particles can temporarily overwhelm parts of the magnetosphere, compressing and distorting the field. This is a geomagnetic storm. NOAA rates these events on a five-level G-scale, from G1 (minor) to G5 (extreme).
At the low end, a G1 storm produces weak power grid fluctuations and makes the aurora visible across northern Michigan and Maine. A G3 (strong) storm can trigger false alarms on grid protection devices, cause intermittent satellite navigation problems, and push aurora visibility as far south as Illinois and Oregon. At G5, the most extreme level, widespread voltage control failures can cause complete grid blackouts, transformers can be physically damaged, high-frequency radio communication may go dark for one to two days, and aurora has been seen as far south as Florida and southern Texas. On average, about four G5-level days occur per 11-year solar cycle.
Pipelines are also vulnerable. During severe storms, induced electrical currents can reach hundreds of amps in long metal pipelines, accelerating corrosion and requiring special protective measures.
How Animals Use Earth’s Magnetic Field
Dozens of species navigate using geomagnetism. Birds, sea turtles, salmon, bees, and sharks all appear to sense the magnetic field in some way, using it as an internal compass during migration or while searching for food. Even at the lowest level on NOAA’s storm scale, migratory animals can be affected by geomagnetic disturbances.
Scientists have identified three leading explanations for how animals pull this off. The first involves tiny crystals of a magnetic mineral called magnetite, found in organisms ranging from bacteria to birds to humans. Chains of these crystals could function like microscopic compass needles, physically tugging on surrounding cells when aligned with the field. The second mechanism involves a light-sensitive protein called cryptochrome, found in bird eyes. When light hits cryptochrome, it triggers a chemical reaction that produces pairs of molecules whose behavior changes depending on the orientation of the surrounding magnetic field, essentially letting birds “see” magnetic direction overlaid on their normal vision. The third hypothesis, most studied in sharks and rays, involves electromagnetic induction: as an animal moves through Earth’s magnetic field, tiny electrical voltages are generated in its body, and specialized sensory organs (like the ampullae of Lorenzini in sharks) detect those voltages and translate them into directional information.
No single mechanism explains magnetoreception across all species. Different animals likely rely on different systems, and some may use more than one at the same time.