The thermosphere, which blocks harmful ultraviolet rays, expands and contracts regularly due to the sun’s activities. As carbon dioxide increases, it has a cooling effect at such high altitudes, which also contributes to the contraction.

But even these two factors aren’t fully explaining the extraordinary contraction which, though unlikely to affect the weather, can affect the movement of satellites, researchers said.

“This is the biggest contraction of the thermosphere in at least 43 years,” John Emmert of the Naval Research Lab was quoted as saying in NASA news report…

“We cannot explain the abnormally low densities, which are about 30 percent lower” than from previous contractions, Emmert told CNN.com.