The Earth’s magnetic field, or geomagnetic field, forms an extensive, invisible protective bubble called the magnetosphere that extends far into space. This shield is generated by dynamic processes deep within the planet and is responsible for deflecting the majority of the solar wind, a constant stream of charged particles and high-energy radiation emanating from the Sun. Without this magnetic defense, the solar wind would strip away Earth’s atmosphere over time, similar to what is thought to have happened on Mars.
The Source of Earth’s Magnetism
The powerful geomagnetic field originates from a self-sustaining process known as the geodynamo, which takes place in the planet’s outer core. This layer consists of a vast ocean of molten iron and nickel, situated about 2,900 kilometers beneath the surface. Heat escaping from the core drives thermal and compositional convection currents within this electrically conductive fluid.
As the planet rotates, the Coriolis effect acts upon these large-scale fluid motions, twisting the flow into helical columns. This organized movement of the molten metal acts like a giant electric generator, creating powerful electric currents that, in turn, induce a magnetic field. The resulting field is predominantly dipolar, meaning it resembles the field of a large bar magnet tilted approximately 11 degrees from the Earth’s rotational axis.
Where Field Intensity Peaks
Magnetic field intensity, which is measured in units like nanotesla (\(text{nT}\)), is not uniform across the globe; it is generally strongest where the magnetic field lines converge. These areas of maximum strength occur near the magnetic poles, which are distinct from the fixed geographic poles. The magnetic field intensity at the surface typically ranges from about 22,000 \(text{nT}\) to 67,000 \(text{nT}\) globally.
The actual peaks of intensity, however, do not perfectly align with the instantaneous position of the magnetic poles. Current models show that the highest field intensity is found in the Southern Hemisphere, over a region of the Antarctic coast south of Australia, reaching values above 65,000 \(text{nT}\). These peaks occur because the Earth’s magnetic field is not a perfect dipole and contains more complex, non-dipolar components that influence the local field strength. In the Northern Hemisphere, the maximum intensity is located in areas of high latitude, such as over northern Canada and Siberia, where values can exceed 61,000 \(text{nT}\).
The South Atlantic Anomaly
In stark contrast to the polar maxima, the Earth’s magnetic field contains a vast region of minimum intensity known as the South Atlantic Anomaly (SAA). This enormous dent in the protective shield stretches across South America and the South Atlantic Ocean. In this area, the field is so weak that the inner Van Allen radiation belt dips down to altitudes as low as 200 kilometers, much closer to the Earth’s surface than anywhere else.
The SAA is a direct result of the magnetic dipole being offset from the Earth’s center. For satellites in low Earth orbit, such as the International Space Station, passing through the SAA means exposure to higher-than-usual levels of energetic, charged particles.
This increased radiation flux can cause temporary glitches in onboard computers, known as single event upsets, and can lead to long-term damage to sensitive electronic components. Consequently, spacecraft operators often power down non-essential equipment when transiting the anomaly to mitigate these risks.
Why Magnetic Field Strength Changes Over Time
The strongest and weakest points of the magnetic field are constantly shifting due to a phenomenon called secular variation. This refers to the slow, continuous change in the field’s direction and intensity that occurs over decades to centuries. Secular variation is driven by changes in the fluid flow patterns of the molten iron in the outer core, the very engine that generates the field.
A prominent feature of secular variation is the westward drift of certain non-dipolar field components, moving at an average rate of about 0.2 degrees per year. The South Atlantic Anomaly itself is part of this change, currently expanding westward and continuing to weaken in intensity, which makes the continuous monitoring of the geomagnetic field a necessary process.