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December 2001

TIMED to probe "weather at the edge of space"

The 7 December launch of the TIMED satellite will provide scientists at NCAR and other organizations with an unprecedented view of the mysterious upper regions of the earth's atmosphere.

TIMED (which stands for Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics) is designed to obtain a global picture of the portion of the atmosphere about 40–110 miles (60–180 kilometers) above the earth's surface. This complex region, greatly influenced by the sun, is too high for ground-based instruments to probe in much detail.

"We're looking at weather at the edge of space," says Stan Solomon (HAO), a principal TIMED investigator. "This interface between what some people call the atmosphere and some people call space is an extraordinarily variable and dynamic region. I'm hoping to get a better understanding of how the sun controls it."

Stan and other researchers hope to use data from the NASA spacecraft to learn more about the temperature, wind, and chemical composition of the upper atmosphere. Improved knowledge of this region—known as the mesosphere and lower thermosphere/ionosphere—could bolster communications networks, ensure that satellites stay on course, and provide scientists with greater insight into human influences on the atmosphere.

"There's never been a mission quite like this," says NCAR director Tim Killeen. "We're going to be looking at a fascinating part of the atmosphere, a real crossroads. You change from the lower atmosphere, where collisions are dominant, to a place where collisions are less significant. Because it's a partially ionized region, electric currents are very important. It's also the place in the atmosphere where sunlight in the extreme ultraviolet spectral region is absorbed."

Tim has been looking forward to the launch of TIMED since he chaired a working group of scientists that began designing the mission in the late 1980s. He is the principal investigator for the TIMED Doppler Interferometer (TIDI), which will measure globally the speed and direction of high-atmosphere winds.

Principal investigator Anne Smith (ACD) is analyzing measurements of ozone, hydrogen, and other key upper-atmosphere components. "I'd like to understand the interaction of the energy input of the sun with the chemical composition and how that interaction controls the temperature and the wind vectors," says Anne, who is working closely with ACD colleague Dan Marsh. "The way that energy is transferred to heat affects the composition, and it can be quite complicated. We will use numerical models in conjunction with the TIMED observations to investigate the physical mechanisms that affect the energy and composition."

Anne Smith (ACD) and Ray Roble (HAO) will be using data from TIMED to learn more about the upper atmosphere. (Photo by Carlye Calvin.)

HAO's Ray Roble has spent the last two decades or so constructing a general circulation model of the upper atmosphere that predicts global measurements of the temperature, winds, and chemical composition and their relationship to the energetic dynamics of the region. "I hope to use the model to interpret the TIMED data and to develop the model so it accurately simulates seasonal variations and other variables that are observed by the satellite and supporting ground-based instruments," he says.

Other NCAR scientists working on TIMED include Roberta Johnson (HAO/E&O), Rolando Garcia (ACD), and Art Richmond, Maura Hagan, Hanli Liu, Gang Lu, Qian Wu, and Alan Burns (all of HAO), as well as a number of postdocs and visitors.

The least understood region

The area that TIMED is exploring acts as a bridge between the earth's environment and space. It is profoundly affected by the sun's magnetic field and radiant energy. Scientists expect that data from TIMED will improve their understanding of the sun and its relationship with the earth's atmosphere, as well as the interactions between the lower and upper atmosphere.

"This part of the atmosphere is a crucial link in our understanding of the overall solar-terrestrial system," Tim says. "TIMED will for the first time thoroughly probe that region by exploring the full range of atmospheric parameters—density, temperature, pressure, composition, vector winds, and other variables—that will then allow us to establish a pole-to-pole climatology and really understand the region."

The zone encompassing the mesosphere and lower thermosphere/ionosphere is the least-understood region of the atmosphere. The reason is that ground-based instruments can detect only a small portion of it, and sounding rockets provide just a brief picture of the region before falling back into the lower atmosphere.

To explore the region, the TIMED spacecraft is designed for a 388-mile (625-kilometer) circular orbit around the earth. The satellite's four instruments will measure solar radiation, auroral energy inputs, temperature, pressure, key gases, and other characteristics of the upper atmosphere.

Stan Solomon (HAO) wore a clean-room suit to take a firsthand look at the TIMED satellite at Vandenberg Air Force Base in October. In the background is Sam Yee of Johns Hopkins University. (Photo courtesy Stan Solomon. Inset photo by Carlye Calvin.)

The satellite is scheduled to orbit for at least two years, with an additional two years of data analysis.

Numerous government and private organizations are taking part in the NASA mission. They include Johns Hopkins University's Applied Physics Laboratory, the University of Colorado's Laboratory for Atmospheric and Space Physics and CU/NOAA's Cooperative Institute for Research in Environmental Sciences, the University of Michigan, NOAA's Aeronomy Laboratory and its Space Environment Center, the University of Alaska's Geophysical Institute, Hampton University, Utah State University, the Air Force Research Laboratory, and the University of California at Berkeley.

The space economy

Part of the reason that atmospheric scientists are studying the mesosphere and lower thermosphere/ionosphere is because the region has important impacts on orbiting vehicles and communications systems.

For example, when upper atmospheric temperatures rise, the resulting expansion pushes molecules to higher altitudes. This increased density exerts an added drag on satellites and slows down their orbits. Changes in the ionosphere can disrupt radio waves and affect the Global Positioning System. In addition, bursts of solar radiation can affect astronauts working on the space station.

"We have this large investment in a space-based economy, and we have to have a better understanding of issues like the effects of atmospheric drag on orbits and the effects of the ionosphere on communications," Stan says.

In addition, Stan believes the upper atmosphere may provide a window into the impacts of human-generated emissions of methane, carbon dioxide, and other gases on the lower atmosphere.

Knowledge of the upper atmosphere lags behind that of the lower atmosphere by many decades, Stan says. Scientists lack the data to provide satellite operators with even the crudest of forecasts about winds, temperatures, and other conditions in the mesosphere and lower thermosphere/ionosphere.

"Right now, it's similar to where we were 50 years ago when nobody believed the weather forecasts," he explains.

With TIMED, Stan believes that scientists will move to nowcasting the upper atmosphere—that is, reporting with some accuracy on current conditions. In a few years, scientists may be able to provide the equivalent of one-hour forecasts.

"We'll make some really fundamental advances in understanding this region," he predicts. "Our goal is to take the next step from basic research to being able to provide possible societal benefits."

• David Hosansky

On the Web:
Updates on the TIMED mission

The TIMED satellite

The TIMED satellite is carrying four instruments to study the region of the upper atmosphere known as the mesosphere and lower thermosphere/ionosphere:

  • The Global Ultraviolet Imager (GUVI) is a spatial scanning ultraviolet spectrograph designed to measure composition and temperature profiles, as well as auroral energy inputs.

  • The Solar Extreme Ultraviolet Experiment (SEE) comprises a spectrometer and a suite of photometers designed to measure solar soft X-ray, extreme-ultraviolet, and far-ultraviolet radiation.

  • The TIMED Doppler Interferometer (TIDI) is designed to measure wind and temperature profiles.

  • A multichannel radiometer known as SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) is designed to measure the pressure, temperature, key gases in the oxygen and hydrogen families, infrared cooling, and effects of solar and chemical heating.

Student-built satellite detects extra X rays from the sun

While the TIMED satellite gathers data on the upper atmosphere, information about solar wavelengths that strike that region is coming from another satellite—one that was built with the help of students.

Data from the Student Nitric Oxide Explorer (SNOE) have verified what solar physicists have long suspected: the sun emits far more energy at very low wavelengths than once thought. The finding is not only a step toward improved models of solar output— it also confirms that putting science into student hands can yield benefits for research as well as for education.

About 100 CU students helped bring SNOE to life. The satellite was built at CU's Laboratory for Atmospheric and Space Physics in the mid-1990s under the direction of principal investigator Charles Barth and the deputy PI and project manager, Stan Solomon, who's now with HAO.

Launched from a Pegasus rocket on 26 February 1998, SNOE has collected observations every six hours at several wavelength bands in the poorly monitored soft X-ray range between 2 and 20 nanometers. Within this range, the data show up to four times more solar irradiance than indicated in a standard model. "This confirms contentions developed from several lines of evidence, but to a larger degree than previously suspected," says Stan.

The soft X rays are an important part of what Stan calls space climate. As he points out, "Many people use the term 'space weather' to mean particle-driven changes in the space environment." As these charged particles emerge from solar storms and bombard the earth's outer atmosphere, there's a parallel spike in the level of soft X rays. The spike is an indication of the highly variable nature of the sun's output at such low wavelengths. The total solar irradiance varies by only about 0.1% across the 11-year solar cycle, but the soft X-ray output rises and falls by a factor of five. "These changes have a huge effect on the density and composition of the ionosphere," Stan says.

Although the main goal of SNOE is to monitor nitric oxide levels in the lower thermosphere, the soft X-ray findings are news in themselves. Stan coauthored a paper on the X-ray results with former SNOE student Scott Bailey (now a research assistant professor at Hampton University) and SNOE investigator Thomas Woods (CU). The paper appeared in the 1 June 2001 issue of Geophysical Research Letters.

The names of SNOE students are engraved on the satellite, now stationed 500 kilometers (310 miles) above the earth. Stan is happy that the students are making their mark in the research world. "Our goal was not just to build a student satellite and not just to perform scientific research but to do both. We showed you can do true cutting-edge research on a small budget in a university-laboratory environment with tons of student involvement and excitement."

• Bob Henson

On the Web:
SNOE
SNOE in the making, UCAR Highlights 1996


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UCAR > Communications > Staff Notes Monthly > December 2001 Search

Edited by David Hosansky, hosansky@ucar.edu
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Last revised: Thu Dec 20 17:08:40 MST 2001