Earth's Magnetic Field
Earth's magnetic field is an invisible shield that extends thousands of kilometres into space. It deflects the solar wind, protects the atmosphere from erosion, and makes life on the surface possible. Understanding how this field behaves — and how it is changing — is central to space weather science.
The Basics: A Giant Magnetic Dipole
To a first approximation, Earth's magnetic field resembles a giant bar magnet (a dipole) tilted about 11° from the rotation axis. The field is generated by convection currents of liquid iron in the outer core — a process called the geodynamo. These currents are sustained by the planet's internal heat and the Coriolis effect from Earth's rotation.
The region of space dominated by this field is called the magnetosphere. On the side facing the Sun, the solar wind compresses the magnetosphere to roughly 10 Earth radii. On the night side, the field is stretched into a long magnetotail extending hundreds of Earth radii downstream. The magnetosphere acts as a barrier: most of the charged particles in the solar wind are deflected around the planet, with only a small fraction entering near the magnetic poles (where they produce the aurora).
Without this magnetic shield, the solar wind would gradually strip away the atmosphere — a process believed to have occurred on Mars after its global field weakened billions of years ago.
A Weakening Field
Since Carl Friedrich Gauss made the first systematic measurements in the 1830s, the overall strength of the dipole field has decreased by roughly 9%. Satellite missions such as ESA's Swarm constellation (launched 2013) have confirmed this trend and mapped it in unprecedented detail.
The weakening is not uniform. Some regions are losing field strength faster than others, while a few areas have actually strengthened slightly. This patchwork behavior is typical of the complex, turbulent flows in the outer core that generate the field.
A weaker global field means that, on average, more cosmic rays and solar energetic particles can reach the upper atmosphere. This is primarily a concern for satellite electronics and high-altitude aviation, rather than for life at the surface, which remains protected by the atmosphere itself.
The South Atlantic Anomaly
The South Atlantic Anomaly (SAA) is a large region over South America and the southern Atlantic Ocean where the magnetic field is significantly weaker than elsewhere at the same latitude. At its centre the field strength is roughly a third of what would be expected for that latitude.
Because the field dips closer to Earth here, the inner Van Allen radiation belt effectively sags lower, bringing energetic protons closer to the surface. Satellites passing through the SAA experience higher radiation doses, which can cause single-event upsets (bit-flips in memory), degrade solar cells, and shorten component lifetimes. The Hubble Space Telescope, the International Space Station, and numerous low-Earth-orbit satellites all encounter the SAA on a regular basis; mission planners routinely schedule sensitive operations to avoid these passes.
Over the past few decades, the SAA has been growing in area and drifting westward. Recent Swarm data shows a second lobe developing southwest of Africa. Whether this growth is connected to a future geomagnetic reversal or is simply a transient feature remains an active area of research.
Magnetic Pole Drift
The North Magnetic Pole — the point where a compass needle would point straight down — has been tracked since James Clark Ross located it in the Canadian Arctic in 1831. For most of the 20th century it drifted slowly, at roughly 10–15 km per year. Since the 1990s, however, the pace has accelerated dramatically to about 50 km per year, and the pole is now heading across the Arctic Ocean toward Siberia.
This acceleration forced the World Magnetic Model (WMM) — used by GPS systems, smartphones, and military navigation — to be updated ahead of schedule in 2019. The WMM is normally revised every five years by NOAA's National Centers for Environmental Information (NCEI) and the British Geological Survey (BGS).
The rapid drift is thought to be caused by changes in the pattern of flow in the outer core, particularly a weakening of a magnetic flux lobe under Canada.
Geomagnetic Reversals
The geological record, preserved in magnetised minerals within volcanic rocks and ocean-floor sediments, shows that the north and south magnetic poles have swapped places hundreds of times over Earth's history. The last full reversal, the Brunhes–Matuyama reversal, occurred approximately 780,000 years ago.
Reversals do not happen overnight. Paleomagnetic evidence indicates that a typical reversal takes between 1,000 and 10,000 years to complete. During the transition the field weakens, becomes complex and multipolar (with magnetic poles appearing at various latitudes), and eventually re-establishes itself in the opposite polarity. Shorter events called “excursions” — where the field weakens and the poles wander significantly but ultimately return to their original orientation — have also been documented, most recently the Laschamp excursion about 41,000 years ago.
During a reversal or excursion, the weakened field would allow more cosmic radiation to reach the atmosphere. While this could increase radiation exposure at high altitudes and affect satellite infrastructure, there is no evidence in the fossil record that reversals cause mass extinctions. Life on Earth has persisted through hundreds of reversals. The atmosphere and ocean provide substantial shielding even when the magnetic field is weak.
Not a Doomsday Scenario
Popular media sometimes portray a magnetic reversal as a catastrophic event, but the science does not support that narrative. The current weakening of the field could be a precursor to an excursion or reversal, or it could simply represent normal variation in the geodynamo — the field has been stronger and weaker many times in the past without flipping.
Even if a reversal were underway, it would unfold over millennia, giving societies ample time to adapt satellite shielding, navigation systems, and radiation protection measures. The primary challenges would be technological rather than biological.