Shrinking Atmospheric Layer Linked to fluctuations in Solar Radiation
A shrinking Atmospheric Layer Linked to Levels of Solar Radiation may be driven by large changes in the sun's energy output, and may in turn drive unexpectedly dramatic fluctuations in Earth's outer atmosphere.
Results of a study published recently link a recent observed shrinking of a high atmospheric layer with a sharp drop in the sun's ultraviolet radiation levels; the shrinking is presumed to be temporary.
The research, led by scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado 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.
The results, published this week in the American Geophysical Union journal Geophysical Research Letters, are funded by NASA and by the National Science Foundation (NSF), NCAR's sponsor.
"This research makes a compelling case for the need to study the coupled sun-Earth system," says Farzad Kamalabadi, program director in NSF's Division of Atmospheric and Geospace Sciences, "and to illustrate the importance of solar influences on our terrestrial environment with both fundamental scientific implications and societal consequences."
The findings may have implications for orbiting satellites, as well as for the International Space Station.
"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 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 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 becomes cooler and less dense during low solar activity.
But the magnitude of the density change during the recent solar minimum appeared to be about 30 percent 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 percent to 5 percent per decade.
Research results 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 percent 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 percent 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, or K (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 percent, with just 3 percent attributable to carbon dioxide, and closely approximated the 30 percent 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.
CitationAdministration, N. (2012). Shrinking Atmospheric Layer Linked to fluctuations in Solar Radiation. Retrieved from http://www.eoearth.org/view/article/51cbef7f7896bb431f69df49
SECOND HALF OF ARTICLE: "THEY BUILT THEIR WORK...." 5th para: "the model": Need a very brief two sentence summary of the way this key model works 7th para: need to know absolute temperatures of the thermosphere to see how much this temperature change means overarching comment: this research could have major implications for climate impacts relative to CO2 driven changes. needs a summary indicating the possible implications for the lower atmosphere...even if one needs to speculate as to various possible implications end of review comments.
7th para: more reference to "varying" is this the same discussion as started in para 3. If so the sentences should be brought together for cohesion ....not having discussion extraneous to this critical core issue in between para 3 and 7. 12th para. "prolonged solar minimum" discussion. This is really the heart of the article and deserves much more quantitative treatment. even a layman needs to know how much the flares deviatie from historical in a numerical way. 13th para: "shrank more": does that mean on a year to year change or to an absolute new low? or both? 18th para: CO2 caused shrinkage: need to summarize basis for CO2 induced shrinkage phenomenon
Comments to enable article to get to a publishable form: 1. Clearer structure is needed One might imagine the following sections: =Overview= =Definition of the thermosphere and solar flare background= =The new observations= =Confirming model runs revealing flares responsible= =Operational implications (eg. for satellite drag, etc)= =Climate implications and liklihoods= Places needing clearer prose: 2nd para: word "temporary" is puzzling. who says this is temporary? this seems to go to the crux of the matter. 3rd para: "may vary more than previously thought." vary in magnitude or vary in recurrence period? needs clarifying.