HAO 2010 PROFILES IN SCIENCE: Dr. Stan Solomon
Contact
303-497-2179
stans@ucar.edu
Area of expertise: Sun and Upper Atmosphere
Specialties: physics and chemistry of Earth's upper atmosphere and ionosphere, theoretical modeling and data analysis of impacts of solar output on Earth's atmosphere, solar terrestrial physics, satellite system design
Dr. Stan Solomon is a Senior Scientist at NCAR's High Altitude Observatory. The physics and chemistry of the Earth's upper atmosphere and ionosphere are Stan Solomon's specialty. He works on theoretical modeling and data analysis to investigate the impacts of solar output on Earth's atmosphere. He is Co-Director for the ionosphere-thermosphere of the CISM (Center for Integrated Space Weather Modeling). He is an Interdisciplinary Scientist on the TIMED (Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics) mission. Instruments on this NASA satellite are gathering data to help understand how the Sun controls weather at the edge of space. In addition to his research, Solomon has served as a lecturer at the University of Colorado, teaching topics that range from solar-terrestrial physics to satellite system design.
Upper Atmosphere Cooling Linked to Low Levels of Solar Radiation
Research by Stan Solomon and Liying Qian at the NCAR High Altitude Observatory, Tom Woods at the University of Colorado, Leonid Didkovsky at the University of Southern California, and John Emmert at the Naval Research Lab, shows that solar extreme-ultraviolet (EUV) irradiance declined to unusually low levels during the long and deep solar minimum period of 2008–2009. The Earth's thermosphere cooled during that period, and contracted to the lowest densities observed since the beginning of space exploration.
The thermosphere, a region of the upper atmosphere extending from 100 to 500 kilometers in altitude, generally cools, contracts, and hence becomes lower in density during low solar activity. However, the magnitude of the density change during the recent solar minimum was much larger than expected. Earlier in 2010, scientists from the Naval Research Laboratory, measuring changes in the effects of atmospheric drag on satellite orbits, estimated that the density of the thermosphere declined in 2008–2009 to about 30% less than during the previous solar minimum in 1996. Another study by scientists at the University of Southern California estimated that levels of EUV radiation were about 15% lower during 2008–2009 than in 1996, but uncertainty remained whether the decline in extreme-ultraviolet radiation would be sufficient to have such a large impact on the thermosphere. Also, increasing levels of carbon dioxide and other greenhouse gases, which cool the upper atmosphere, are reducing the density of the outer atmosphere by about 2% to 5% per decade, which could explain some of the observed changes.
To answer this question, Solomon and colleagues used the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model to simulate how changes in the solar EUV between 1996 and 2008 would affect the temperature and density of the thermosphere. They made two simulations of thermospheric conditions in 2008, one including the solar EUV change alone, and a second including the effects of carbon dioxide cooling. The results showed the thermosphere cooled in 2008 by about 40 degrees Kelvin compared to 1996, with only 2 K attributable to increasing carbon dioxide, and that thermospheric density decreased by about 30%, in good agreement with the observed reduction in density indicated by measurements of satellite drag. This confirms that the unusually long recent minimum period was linked to lower EUV irradiance, and implies that long periods of low activity can lead to lower irradiance levels.