Upper atmosphere shrinking: scientists

THE upper reaches of Earth's atmosphere are unexpectedly shrinking and cooling due to lower ultraviolet radiation from the sun, US scientists said on Thursday.

The sun's energy output dropped to unusually low levels from 2007 to 2009, a significantly long spell with virtually no sunspots or solar storms, according to scientists from the National Centre for Atmospheric Research in Boulder, Colorado.

During that period, the thermosphere, whose altitude ranges from about 90 to 500km, shrank and contracted from the sharp drop in ultraviolet radiation, said the study published in the American Geophysical Union's journal Geophysical Research Letters.

The thermosphere cooled by 74 degrees Fahrenheit (41 Celsius) in 2008 compared to 1996, and shrank by 30 per cent, "more than at any time in the 43-year era of space exploration", the researchers said.

"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 study lead author Stanley Solomon.

A narrower, less dense thermosphere is good news for satellites orbiting Earth, including the International Space Station, since reduced friction means they can remain aloft longer, said University of Colorado professor and study co-author Thomas Woods.

"This is good news for those satellites that are actually operating." Prof Woods said the research shows the sun could be going through a period of relatively low activity, as it did in the early 19th and 20th centuries.

"If it is indeed similar to certain patterns in the past, then we expect to have low solar cycles for the next 10 to 30 years," he added.

AFP