The U.S. space agency says it has discovered the boundary between the Earth's upper atmosphere and space has moved to extraordinarily low altitudes.

The finding was determined by National Aeronautics and Space Administration instruments aboard an Air Force satellite launched in April. The instruments, including ion and neutral sensors, make measurements of the variations in neutral and ion densities and drifts that can result in disruptions of navigation and communication signals.

But the first discovery was that the ionosphere was not where it had been expected to be. During the first months of the satellite's operations, the transition between the ionosphere and space was found to be at about 260 miles altitude during the nighttime, barely rising above 500 miles during the day. Those altitudes, said NASA, were extraordinarily low compared with the more typical values of 400 miles during the nighttime and 600 miles during the day.

NASA said the discovery by the satellite -- subsequently determined to have been launched during the quietest solar minimum since the space age began -- is providing a unique opportunity to study the connection between the interior dynamics of the sun and the response of the Earth's space environment.