What Is the South Atlantic Anomaly and Why Is Earth's Magnetic Shield Weakening?

The South Atlantic Anomaly (SAA) is a large, expanding region over the South Atlantic Ocean where Earth’s protective magnetic field is significantly weaker than the rest of the planet — and after 11 years of satellite data, scientists have confirmed it is still growing and has now split into two distinct lobes.

What ESA’s Swarm Satellites Discovered

In November 2013, the European Space Agency launched three satellites — Alpha, Bravo, and Charlie — collectively known as the Swarm mission. Their sole purpose was to map and monitor Earth’s magnetic field with unprecedented precision. After more than a decade of continuous measurements, scientists confirmed that the South Atlantic Anomaly has expanded by approximately 2 million square miles since 2014 — an area larger than the entire continental United States.

More striking still, the anomaly is no longer one unified weak zone. It has fractured into two separate lobes: one centred over South America, and a second emerging off the southwest coast of Africa. The split suggests the underlying dynamics driving the anomaly are intensifying, not stabilising.

How Weak Is the Magnetic Field Inside the Anomaly?

Magnetic field strength is measured in nanoteslas (nT). Near Earth’s poles, the field reaches around 60,000 nT — robust enough to deflect most incoming solar and cosmic radiation. Inside the South Atlantic Anomaly, that figure drops to roughly 22,000 nT. That is barely one-third of normal protection.

This is not an isolated trend. Earth’s overall magnetic field has weakened by approximately 9 percent globally over the last 200 years. The SAA represents the most extreme expression of that broader decline.

Why Satellites and Spacecraft Are Affected

The reduced magnetic shielding inside the anomaly allows higher-energy particles from the Sun and deep space to penetrate far closer to Earth’s surface than they otherwise would. Every satellite and spacecraft that passes through this region is exposed to elevated radiation bursts, leading to documented hardware glitches, memory errors, and sensor malfunctions mid-orbit.

The International Space Station actively schedules operations around the anomaly — astronauts are advised to avoid spacewalks and non-essential computer activity when the station crosses this zone. It is one of the most operationally significant hazards in low Earth orbit.

What Is Causing the Anomaly?

The root cause lies approximately 1,800 miles beneath Earth’s surface, at the boundary between the mantle and the outer core. Scientists have identified a patch of reversed magnetic flux at this depth — a region where the local magnetic field points in the opposite direction to the rest of the planet. This reversed patch effectively punches a hole in the shielding effect of the broader field above it.

The outer core, composed of liquid iron and nickel, is in constant turbulent motion. That motion generates Earth’s magnetic field through a process called the geodynamo. Irregularities in that flow — possibly linked to complex interactions at the core-mantle boundary — are believed to be responsible for the anomaly’s location and behaviour.

This Has Happened Before

Volcanic rocks preserve a record of Earth’s past magnetic field, and what they reveal is remarkable. Evidence from ancient lava flows shows that a nearly identical anomaly existed in the same South Atlantic region at least 11 million years ago. The weak patch has appeared, faded, and re-emerged over geological timescales — long before humans existed.

Earth’s last complete magnetic pole reversal occurred approximately 780,000 years ago. No human civilisation has ever experienced one. Whether the current weakening is a precursor to another reversal, or simply part of a long natural cycle, remains an open and actively debated scientific question.

A Shield Older Than Life Itself

Earth’s magnetic field is estimated to be at least 3.5 billion years old — predating complex life by billions of years and serving as one of the foundational conditions that allowed life to evolve at all. The South Atlantic Anomaly is not a catastrophe, but it is a measurable, accelerating change in one of Earth’s most critical systems. Two lobes at a time, a new chapter in that ancient story is being written right now.