Tuesday, August 31, 2010

Shrinking Atmospheric Layer Linked to Low Levels of Solar Radiation

Here is a recent press release from the National Center for Atmospheric Research.

Large changes in the Sun’s energy output may drive unexpectedly

dramatic fluctuations in Earth’s outer atmosphere, new research

indicates. A study published today links a recent, temporary shrinking

of a high atmospheric layer with a sharp drop in the Sun’s ultraviolet

radiation levels.

The research, led by scientists at the National Center for Atmospheric

Research (NCAR) and the University of Colorado at Boulder (CU),

indicates that the Sun’s magnetic cycle, which produces differing

numbers of sunspots over an approximately 11-year cycle, may vary more

than previously thought.

“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,” says lead author Stanley Solomon, a scientist at NCAR’s High

Altitude Observatory. “All solar minima are not equal.”

The findings may have implications for orbiting satellites, as well as

for the International Space Station. The fact that the layer in the

upper atmosphere known as the thermosphere is shrunken and less dense

means that satellites can more easily maintain their orbits. But it also

indicates that space debris and other objects that pose hazards may

persist longer in the thermosphere.

“With lower thermospheric density, our satellites will have a longer

life in orbit,” says CU professor Thomas Woods, a co-author. “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.”

The study, published this week in Geophysical Research Letters, was

funded by NASA and by the National Science Foundation, NCAR’s sponsor.

-----Radiation or carbon dioxide?-----

The Sun’s energy output declined to unusually low levels from 2007 to

2009, a particularly prolonged solar minimum during which there were

virtually no sunspots or solar storms. During that same period of low

solar activity, Earth’s thermosphere shrank more than at any time in the

43-year era of space exploration.

The thermosphere, which ranges in altitude from about 55 to more than

300 miles (90 to 500 kilometers), is a rarified layer of gas at the edge

of space where the Sun’s radiation first makes contact with Earth’s

atmosphere. It typically cools and becomes less dense during low solar

activity. But the magnitude of the density change during the recent

solar minimum appeared to be about 30% greater than would have been

expected by low solar activity.

The study team used computer modeling to analyze two possible factors

implicated in the mystery of the shrinking thermosphere. They simulated

both the impacts of solar output and the role of carbon dioxide, a

potent greenhouse gas that, according to past estimates, is reducing the

density of the outer atmosphere by about 2% to 5% per decade.

Their work built on several recent studies. Earlier this year, a team of

scientists from the Naval Research Laboratory and George Mason

University, measuring changes in satellite drag, estimated that the

density of the thermosphere declined in 2007–09 to about 30% less than

during the previous solar minimum in 1996. Other studies by scientists

at the University of Southern California and CU, using measurements from

sub-orbital rocket flights and space-based instruments, have estimated

that levels of extreme-ultraviolet radiation—a class of photons with

extremely short wavelengths—dropped about 15% during the same period.

However, scientists remained uncertain whether the decline in

extreme-ultraviolet radiation would be sufficient to have such a

dramatic impact on the thermosphere, even when combined with the effects

of carbon dioxide.

To answer this question, Solomon and his colleagues turned to an NCAR

computer tool, known as the Thermosphere-Ionosphere-Electrodynamics

General Circulation Model. They used the model to simulate how the Sun’s

output during 1996 and 2008 would affect the temperature and density of

the thermosphere. They also created two simulations of thermospheric

conditions in 2008—one with a level that approximated actual carbon

dioxide emissions and one with a fixed, lower level.

The results showed the thermosphere cooling in 2008 by 41 kelvins (about

74 degrees Fahrenheit) compared to 1996, with just 2 K attributable to

the carbon dioxide increase. The results also showed the thermosphere’s

density decreasing by 31%, with just 3% attributable to carbon dioxide.

The results closely approximated the 30% reduction in density indicated

by measurements of satellite drag.

“It is now clear that the record low temperature and density were

primarily caused by unusually low levels of solar radiation at the

extreme-ultraviolet level,” Solomon says.

Woods says the research indicates that the Sun could be going through a

period of relatively low activity, similar to periods in the early 19th

and 20th centuries. This could mean that solar output may remain at a

low level for the near future.

“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,” Woods says.

The University Corporation for Atmospheric Research manages the National

Center for Atmospheric Research under sponsorship by the National

Science Foundation. Any opinions, findings and conclusions, or

recommendations expressed in this publication are those of the author(s)

and do not necessarily reflect the views of the National Science Foundation.

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