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Hellis and his colleagues have been using a suite of NASA instruments called CINDI, which fly on the U.S. Air Force Communication/Navigation Outage Forecast System (C/NOFS) satellite between 250 miles and 530 miles around the planet's equator. models based on previous research had predicted the ionosphere to be about 370 miles above Earth at night and about 620 miles up during the day -- the variation due to temperature and other factors.
Instead, the CINDI team discovered that the transition between the ionosphere and space was about 250 miles above Earth at night and about 500 miles up during the day.
The researchers never expected to encounter the fringe of Earth's ionosphere at those altitudes, but that is exactly what happened during the summer of 2008, a time when the solar activity was unusually quiescent.
"It was a real fortuitous combination of low solar activity and the satellite's [range]," Hellis said. "We didn't expect to be able to look at the top of the ionosphere in all places."