baffling shrinkage...,

CNN | An upper layer of Earth's atmosphere recently shrank so much that researchers are at a loss to adequately explain it, NASA said on Thursday.

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.

Emmert is the lead author of a paper announcing the finding in the June 19 issue of the journal Geophysical Research Letters.

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

The thermosphere lies high above Earth's surface, close to where the atmosphere ends and space begins. It ranges in altitude from 55 miles (90km) to 370 miles (600km) above the ground -- the realm of meteors, auroras, space shuttles and the international space station.

The thermosphere interacts strongly with the sun and hence is greatly influenced by the sun's solar activity, which occurs in cycles.

When solar activity is high, solar extreme ultraviolet rays warm and expand the thermosphere. When it's low, the opposite occurs.

The collapse occurred during what's known as a "solar minimum" from 2007 to 2009, during which the sun plunged into an unprecedented low of inactivity. Sun spots were scarce and solar flares were nonexistent, NASA reported.

Still, the collapse of the thermosphere was bigger than the sun's activity alone can explain.