The South Atlantic Anomaly is a weak patch in Earth's magnetic field over South America and the South Atlantic, where radiation reaches down to low-orbit satellites.
Earth's magnetic field is not equally strong everywhere. Over South America and the southern Atlantic Ocean there is a weak spot. There, charged particles can come closer to the surface than usual. Scientists call it the South Atlantic Anomaly, or SAA.
The magnetic field is made by molten, iron-rich metal flowing in Earth's outer core, about 1,800 miles (2,900 km) below our feet. NASA explains that two features of the core cause the SAA: the tilt of the magnetic axis, and a patch of field with reversed direction that is growing beneath the region and weakening the field there.
The patch is changing. ESA's Swarm satellites show that the field at its weakest point fell from about 24,000 nanotesla in 1970 to about 22,000 in 2020. The area has grown and moved west by about 20 km a year, and a second minimum has formed south-west of Africa, so the anomaly may split into two.
Scientists keep watching it from orbit. NASA's ICON mission crosses the SAA regularly and tracks its position, while data from ESA's Swarm satellites and from ground stations are used to model the field and forecast how it will change.
Inside the SAA a high-energy proton can cause a single event upset — a glitch in a computer, or permanent damage to a key part. Many operators switch off non-essential equipment while their satellites pass through.
The space station is well shielded, so astronauts inside are safe. Outside instruments notice, though. NASA's GEDI laser on the ISS sees detector “blips” about once per million laser shots, and its power boards reset about once a month, each time costing a couple of hours of data.
The SAA has no visible effect on daily life at Earth's surface; it matters only for things in orbit. Nor is it a sign that the magnetic poles are about to flip. ESA notes that reversals happen on average about every 250,000 years and that today's weakening is within normal ups and downs.
When the station's estimated position is over South America or the southern Atlantic Ocean, it is flying through the anomaly. Open the ISS tracker
Put numbers on the change. A fall from 24,000 nT to 22,000 nT over 50 years is a drop of 2,000 nT, or 2,000 / 24,000 ≈ 8.3%, averaging about 40 nT a year. For comparison, ESA says Earth's global average field strength has dropped by about 9% over the last 200 years — so the SAA's minimum has weakened about as much in 50 years as the whole field has in 200.
The westward drift of about 20 km a year adds up to roughly 1,000 km over the same 50 years. NASA's SAMPEX satellite, which observed from 1992 to 2012, saw the region drifting slowly north-west, and recent data show the minimum splitting into two lobes.
Why does a weaker field matter to a satellite? The field steers charged particles. Where it is weak, ESA notes, charged particles reach low-Earth-orbit heights, raising the chance of technical faults. NASA's ICON article puts the bottom of the innermost Van Allen belt at about 400 miles (640 km); over the SAA particles dip closer than that.
The effect is a rate problem. GEDI's detector blips occur about once per million laser shots, and its boards reset about once a month. On NOAA's solar radiation storm scale, single event upsets become likely only in strong (S3) storms — yet inside the SAA, trapped protons produce the same kind of upset even when the Sun is quiet.
Over South America and the southern Atlantic Ocean. It is caused by the tilt of Earth's magnetic axis and a growing patch of reversed field in the outer core, which together weaken the field there.
A glitch caused when one high-energy particle strikes a satellite's electronics; it can flip data temporarily or permanently damage a component.
It is growing, weakening, moving west by about 20 km a year, and a second minimum south-west of Africa suggests it may split in two.