One of the highest sections of the atmosphere is shrinking as a result of low sun activity, U.S. researchers reported Thursday.

That may sound scary, but they say it actually can be good news for satellites. It means they can stay in orbit longer because they have less drag from the gasses they encounter.

On the other hand, of course, space junk orbiting the Earth also can remain up there longer.

According to the study, published in the American Geophysical Union’s journal Geophysical Research Letters, the sun’s energy output was unusually low from 2007 to 2009.

During that period the atmospheric layer called the thermosphere — from 55 miles to 300 miles (90-240 kilometres) above the Earth — cooled and shrank. That reduced the density of gas at high levels where many satellites orbit, explained Thomas Woods of the University of Colorado, a co-author of the report.

The decline in high-level density was as much as 30 percent, Woods said, even less than the last solar minimum.

“Our work demonstrates that the solar cycle not only varies on the typical 11-year time scale, but also can vary from one solar minimum to another,” said lead author Stanley Solomon of the National Centre for Atmospheric Research’s High Altitude Observatory. “All solar minima are not equal.”

Solar energy output tends to vary over 11-year cycles marked by increases and decreases in sunspots. From 2007 to 2009 there were almost no sunspots or solar storms.

“With lower thermosphere density, our satellites will have a longer life in orbit,” said Woods. “This is good news for those satellites that are actually operating, but it is also bad because of the thousands of non-operating objects remaining in space that could potentially have collisions with our working satellites.”

While the reduced solar activity resulted in a cooling of the upper thermosphere the same cannot be said of the Earth’s surface, as 2009 was the fifth warmest year on record and the decade of 2000 to 2009 was the warmest on record.

The research was supported by the National Science Foundation, NASA and the Office of Naval Research.

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