What satellites reveal to us about the weaker shield above the South Atlantic: eleven years of continuous measurements from ESA's Swarm constellation have provided the most precise picture to date of how the South Atlantic Anomaly (SAA) is expanding, reshaping, and affecting satellites, navigation, and our daily technological life. A cumulative analysis of data for the 2014–2025 period confirms a long-term trend of the magnetic field weakening in this area, along with the simultaneous strengthening and weakening of other "hotspots" on the planet, revealing just how dynamic our geomagnetic shield is.

Earth's magnetic field is neither simple nor static. Instead of a "bar magnet," it is a complex, constantly changing phenomenon driven by the planetary dynamo in the liquid outer core: in an "ocean" of molten iron, about 3,000 kilometers beneath us, an electrically conductive fluid circulates, swirls, and creates electric currents. These currents generate the bulk of the geomagnetic field. The final picture on the surface is formed by the superposition of multiple sources—the core, mantle, crust, and oceans—and electric currents in the ionosphere and magnetosphere. That is why mapping and monitoring changes is only possible through a combination of precise measurements from space and on the ground, along with advanced models that separate and merge these signals into a meaningful whole.

How we know the South Atlantic Anomaly is expanding

Swarm consists of three identical satellites in close, near-polar orbits, launched on November 22, 2013, as part of ESA's Earth Explorer program. Their instrumentation—vector and scalar magnetometers, accelerometers, and electric measurement systems—allows the core signal to be separated from atmospheric and space influences and enables fine spatial and temporal changes in the geomagnetic field to be tracked in real time. Two satellites fly in close formation, with the third at a slightly higher altitude; this geometry increases sensitivity to field gradients and provides in-depth insight from the core to the ionosphere.

By comparing several years of data, signal processing teams build global models of the magnetic field. These models, updated from 2014 to 2025, consistently show the expansion of the weak area over the South Atlantic, with the anomaly itself not behaving as a single "patch." Instead, it appears as a mosaic of multiple minimums whose strength and position change at different rates: one towards southeastern South America, another towards southwestern Africa. In the period after 2020, the fastest weakening is recorded just southwest of Africa, where a more pronounced and rapid change in field strength is observed than further west over the ocean.

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Why the SAA is important for satellites, telescopes, and networks on Earth

The South Atlantic Anomaly is particularly relevant for everything flying low above the Earth—from research satellites to the International Space Station. In this corridor, our magnetic shield weakens, and the inner Van Allen radiation belt comes closer to the planet's surface, increasing the flux of energetic particles. The consequences are very practical: more frequent "bit-flip" errors in memory chips, unexpected software resets, degradation of sensitive detectors (especially UV and X-ray instruments), and occasional interruptions of measurements when passing through the anomaly. Operators solve this with a combination of shielding, redundant electronics, error-correcting codes, and observation schedules that anticipate "quiet" periods when the spacecraft enters the SAA.

Swarm itself has served as a kind of "detector" of environmental risks: ten-year statistics of flyovers and recorded errors show that the impact of radiation in the anomaly differs from the rest of the orbit and requires operational adjustments. The constellation's data has been incorporated into operational space weather and magnetic field models used by space agencies, aviation, maritime, and industry for planning orbits, defining shielding, and extending the operational life of spacecraft. It is precisely because of the SAA that many instruments have "no-go" modes—for example, they shut down or reduce sensitivity during a flyover—to minimize the risk of damage and false readings.

What is happening deep below us: reverse flux patches and the "westward drift"

To understand the anomaly, it is crucial to look at the core-mantle boundary. Measurements and numerical models show the appearance of so-called reverse flux patches—localized zones where magnetic field lines, instead of exiting the core, re-enter it. Their strengthening and migration, often westward, are projected onto the surface as pockets of a weakened field. When such a patch "lingers" under the South Atlantic and then moves towards Africa, the surface field minimum follows its movement—precisely the pattern we have been observing more clearly in the southwestern sector of the anomaly in recent years.

These structures are not a static "defect," but a natural result of turbulent convection in molten iron, modulated by the Earth's rotation and thermal-chemical gradients. As the flows change, the patches strengthen, weaken, or split. This explains why the SAA sometimes takes on a two-lobed geometry—with two more pronounced minimums—and why the intensity and position of the minimums over South America and southwestern Africa do not change synchronously. For operational planning, this means that transits through risky zones become more frequent or longer, even if the global average field strength is seemingly unchanged.

Navigation, ionosphere, and communications: why geomagnetism is not just a "compass"

The magnetic field enters navigation on multiple levels. Most directly, through magnetic declination and inclination, which are used for compass orientation on ships, aircraft, and land-based systems. Indirectly, geomagnetism shapes the ionosphere—the electrically conductive layer of the atmosphere crucial for radio wave propagation and GNSS positioning accuracy. When the field lines and the flux of charged particles change, the electron density in the ionosphere also changes, so signals can wander, weaken, or change their path. That is why aviation routes at high latitudes (where changes are most pronounced) require more frequent model updates and greater reliance on multi-sensor fusion (inertial and satellite data alongside magnetometers).

In power grids, strong geomagnetic disturbances can induce currents that overload transformers. The SAA itself is not a "storm" condition, but the fact that the global field is going through phases of strong regional changes (weakening over the South Atlantic, redistribution of strength over Siberia and Canada) is a reason for infrastructure operators to focus on calibrations, space weather monitoring, and adapting protection protocols. In practice, this includes integrating real-time satellite and ground-based measurements into prediction systems that warn of an increased risk of interference in communications and networks.

Figures, trends, and "centers of gravity" of the geomagnetic system

In the Southern Hemisphere, one area of particularly strong field stands out, while in the North there are two—over Canada and over Siberia. Over the last decade, the balance of power has changed: the strength over Canada is weakening, while that over Siberia is strengthening. Consequently, the North Magnetic Pole has moved at an accelerated pace towards Siberia, which has required more frequent updates of navigation models. In addition, maps of strong fields show that the "Canadian maximum" has decreased in area, while the "Siberian" one has expanded. This geographical "spillover" of energy also explains the changes in declination at high latitudes felt by pilots, sailors, and arctic services.

The South Atlantic Anomaly, on the other hand, occasionally "splits" into two noticeable lobes. When this happens, a satellite that previously crossed one narrower patch may now experience two separate intervals of elevated radiation in a single pass. Operational transit tables therefore become more detailed, and instruments (e.g., UV detectors on space telescopes) more frequently pause measurements during a pass. Such patterns are particularly pronounced during periods of high solar activity, when additional particles and currents in the magnetosphere amplify ionospheric variations.

From raw measurements to operational models

The key to Swarm's contribution is the multi-layered combination of data and inversion techniques. Vector magnetometers provide detailed spatial structure, the scalar magnetometer serves as an absolute calibration standard, accelerometers separate non-gravitational forces affecting the orbit (e.g., drag from the thin atmosphere), and electrical instruments monitor currents in the ionosphere. On this basis, global models are built that describe the field by altitude and time, isolate the contribution of the core from that of the atmosphere and space, and allow for comparisons over the years. When such models are "sliced" into time series from 2014 to today (October 14, 2025), it is clearly visible that the SAA has expanded and reshaped, and that the geographical hotspots of the strong field in the north have switched roles.

Swarm was designed as an "Earth Explorer"—a mission that tests new technologies and provides data for fundamental science—but over time it has become the operational backbone for a whole range of services: from global magnetic models used in navigation, through tools for space weather monitoring, to the calibration of other satellites. As the mission is extended, the value of the continuous series grows—a consistent, multi-year record makes it possible to distinguish long-term trends (e.g., core processes) from short-term "spikes" caused by the Sun.

History and lessons for the years to come

The South Atlantic Anomaly was first recorded as early as the 19th century southeast of South America, but it was only high-precision satellites that revealed its internal structure and connection to processes in the core. The bigger picture is that the global field is weakening in the long term, but unevenly: while the maximum over Canada weakens, the one over Siberia strengthens; while the minimum over the South Atlantic expands, counterbalances are forming elsewhere. Such a "mosaic" shows that regional changes are not exceptions but the expected outcome of a chaotic, yet physically driven system in the liquid core.

For practical use, this means more frequent updates of navigation charts, more robust strategies for protecting satellite electronics, and constant monitoring of ionospheric and magnetospheric conditions. As we approach the end of 2025, periods of lower solar activity are helping to further "separate" core signals from solar noise, which will facilitate more accurate forecasting—from local magnetic declination at high latitudes to the likely development of the SAA in the coming years.

What this means for readers, industry, and science

For users of navigation and geolocation services, the most important message is this: modern navigation does not rely on a single compass. Aircraft and ships combine magnetic, inertial, and satellite data; local changes in the geomagnetic map do not cause a "loss of direction," but they do require up-to-date declination tables and correct sensor integration. For the space industry, the expansion of the SAA means greater demands for radiation resistance testing, smarter observation schedules, error correction algorithms, and ever-better use of space weather prediction models. For science, continuous measurements like Swarm's turn abstract processes deep beneath our feet into concrete data that is useful both in orbit and on Earth.