European Space Agency (ESA) scientists have discovered the reason why low-orbit GPS satellites experience periodic navigational blackouts while flying over the equator between Africa and South America. 

The answer is "thunderstorms" in the ionosphere, according to an ESA announcement.

ESA tracked down the cause through its Swarm trio of satellites, which aim to measure various Earth magnetic fields. The satellites carry GPS receivers so operators can maintain the correct orbits.

The GPS link was broken 166 times during the first two years of Swarm, of which 161 coincided with ionospheric thunderstorms.

"These thunderstorms occur when the number of electrons in the ionosphere undergoes large and rapid changes," ESA said. "This tends to happen close to Earth's magnetic equator and typically just for a couple of hours between sunset and midnight.

"As its name suggests, the ionosphere is where atoms are broken up by sunlight, which leads to free electrons. A thunderstorm scatters these free electrons, creating small bubbles with little or no ionized material. These bubbles disturb the GPS signals so that the Swarm GPS receivers can lose track."

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