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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