Swarm consists of three identical satellites in low Earth orbit. Alpha and Charlie fly in tandem about 450km up, descending slowly over time, while Bravo sits a little higher and drifts slowly away from the orbital plane of the others
It sounds like the plot of a low-budget sci-fi movie: satellites in orbit are subject to a mysterious navigational blackout when they pass over the equator between South America and Africa – and it only happens at night.
This is just what the Swarm experiment has experienced since it was launched in 2013.
Swarm is a European Space Agency (ESA) mission in which three identical spacecraft fly in formation around the globe, mapping the Earth's magnetic field with 'unprecedented precision and resolution'.
That precision demands continual fixes of position from GPS satellites – and this is how the mystery blackouts were noticed. Data being downloaded to mission HQ at Potsdam, Germany, showed no position information at certain times and locations.
For years, experts were left baffled by these blackouts. Now, the culprit turns out to be thunderstorms in space – or more specifically the ionosphere, between 300 and 600km above the Earth (the official boundary between atmosphere and space is 100km).
"These ionospheric thunderstorms are well known, but it’s only now we have been able to show a direct link with the loss of GPS signal," said Swarm project scientist Professor Claudia Stolle of GeoForschungsZentrum Potsdam in Germany.
"The storms typically occur for one or two hours between sunset and midnight, and cause loss of the GPS signal from the satellites for some minutes".
The cause of the blackouts could be traced because the Swarm satellites have both magnetic and GPS detectors on board – and the discovery could fortuitously pave the way to better space weather forecasting and better GPS back here on the surface.
The scientists report that the link between GPS blackouts and ionospheric thunderstorms has provided a new way to investigate the systematic irregularities in the ionosphere, and link these to changes in solar magnetic activity.
With civilisation's increasing reliance on satellites and growing awareness of their vulnerability to solar activity, this could prove to be a literal live-saver.
"GPS has become an unexpected scientific tool for us" added Professor Stolle. The findings are published in the journal Space Weather.