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''SAN FRANCISCO — When the sun is relatively inactive — as it has been in recent years — the outermost layer of Earth's atmosphere cools dramatically, new observations find.''
''Earth's thermosphere is one of the least explored parts of the atmosphere, but it is important because "the thermosphere is where the sun first interacts with our atmosphere," said James Russell III of Hampton University in Hampton, Va.''
''The sun has been in a particularly prolonged and deep solar minimum for the last couple of years, with fewer sunspots and solar storms erupting on its surface. When the sun is in this state, it also sends less energy out in the soft X-ray and extreme ultraviolet parts of its spectrum. These wavelengths of light have a significant impact on the thermosphere, where air molecules absorb their energy and the reradiate it in the form of infrared energy.
The TIMED mission measured both the amount of incoming solar energy in the thermosphere and the amount of energy being sent back out into space from the layer and found a significant decrease in both.
"The sun is in a very unusual period," said Marty Mlynczak, a TIMED team member at NASA Langley in Virginia. "The Earth's thermosphere is responding remarkably – up to an order of magnitude decrease in infrared emission/radiative cooling by some molecules."
Less radiation in both directions means that this layer of the atmosphere also cools substantially. In fact, the thermosphere has cooled by a factor of 10 since the last solar maximum in early 2002.
"I certainly didn't expect to see this eight years ago," Mlynczak said. ''
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