It has long been known that space-based navigation systems often suffer disruption in equatorial regions after sunset due to disturbances in the upper atmosphere. Now scientists have used Swarm data to show that these disturbances fluctuate according to a well-known mathematical law of nature. They hope this finding will enable better forecasts of navigation outages.
 

Ionospheric irregularities

Global Navigation Satellite Systems (GNSS) – such as the US GPS and the European Galileo constellations – provide vital services to millions of people every day, but they are not without their vulnerabilities.

These systems rely on signals travelling smoothly through the ionosphere, which is an atmospheric region – extending from 80 km to 1000 km above Earth’s surface – where electrons and ions play a crucial role in influencing radio wave propagation. Under certain conditions, ionospheric instability can lead to “loss of lock” events where the position fix of a receiving device is temporarily lost.

Hossein Ghadjari, University of Calgary physicist and lead author of the study, explained: “After the Sun goes down, the sudden drop in solar radiation causes the ionosphere’s lower layer to lose its charge faster than the upper layer. The result is an electrical gradient that draws low density bubbles of plasma to higher altitudes, where they disrupt GPS signals. This effect is most pronounced over the magnetic equator – which is why we focused on this region.

“Swarm enabled us to undertake a long-term and detailed statistical analysis of these phenomena to better understand their severity and frequency.”
 

Swarm investigates

Hossein and his colleagues drew on nearly a decade of ionospheric data from ESA’s Swarm constellation. The research team included scientists from the University of Calgary and the National Observatory of Athens.

Launched in 2013, Swarm consists of three satellites named Alpha (Swarm-A), Bravo (Swarm-B) and Charlie (Swarm-C). Each satellite hosts sophisticated sensors that monitor ionospheric disturbances associated with loss of lock events. They also carry GPS receivers to track their locations.

Swarm-A and Swarm-C cruise at a lower altitude than Swarm-B, allowing the constellation to deliver information on different parts of the ionosphere.

Hossein said: “The different orbital positions of the Swarm satellites proved very useful for our study. We found that the GPS receivers on Alpha and Charlie suffered loss of lock more frequently than those on Bravo, showing that disturbances are stronger in the lower ionosphere.

“We went on to fit a range of statistical models to the Swarm data to try to explain the pattern behind the observed ionospheric irregularities.”
 

Mathematical law of nature

The team reported that disturbances followed a well-understood pattern that is observed in many natural systems called ‘power law distribution’. This means there is a consistent mathematical relationship between the size and frequency of ionospheric irregularities: small disturbances are common, medium disturbances are less common, and large disturbances are rare.

These ionospheric disturbances also showed signs of self-organised criticality, where the system naturally reaches a point at which a minute shift in conditions can trigger far larger changes.

Hossein added: “The theory of self-organised criticality was introduced by Per Bak and his coworkers in 1987. It can be explained by using an example of a sand pile: if you drop single grains of sand randomly on a flat surface, the pile will grow gradually – eventually the gradient of the sides of the pile will be so steep that the addition of one grain will cause an avalanche of sand.”

In addition, the patterns of ionospheric disturbances observed by Swarm were multifractal and intermittent in nature, which means they appeared in sudden bursts rather than forming gradually.

Hossein concluded: “Thanks to more than 10 years of Swarm data, we have uncovered important statistical clues that, in future work, could help generate predictions of loss of lock events.

“Building on our work on the equatorial ionosphere, we also plan to investigate ionospheric disturbances at higher latitudes.”
 

References:

Intermittency in the integrated power of ionospheric density fluctuations; Hossein Ghadjari, David Knudsen, Georgios Balasis, Susan Skone; J. Space Weather Space Clim. 15 30 (2025); DOI: 10.1051/swsc/2025026