For nearly two years, NASA's TIMED spacecraft has made great strides in its exploration of one of Earth's last atmospheric frontiers during a time when the Sun's activity has been near the peak of its 11-year cycle.

As its initial 2-year orbital mission draws to a close this winter, the team is preparing to embark on the extended mission to study how declining solar activity affects a portion of the upper atmosphere that serves as the gateway between Earth's environment and space, where the Sun's energy is first deposited into our environment.

In a society increasingly dependent on satellite technology and communications, it's vital to understand the variability within a critical region of our upper atmosphere, known as the MLTI (Mesosphere, Lower Thermosphere/Ionosphere). This will help scientists predict this region's effects on communications, satellite tracking, spacecraft lifetimes, degradation of spacecraft materials and on the reentry of piloted vehicles.

"Solar cycle variations strongly affect our upper atmosphere," says Sam Yee, TIMED Project Scientist from The Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

"As solar activity levels transition from maximum to minimum, we see dramatic changes in solar ultraviolet radiation, in the frequency and intensity of magnetic storms and substorms, and in the nature of high energy particles entering the upper atmosphere. TIMED's extended mission is critical to exploring these variations and improving our understanding of the final link in the energy chains connecting the Sun and Earth."

During the next phase of operations, TIMED's science team will focus on how changes in the solar cycle affect solar radiation; the MLTI region's composition, temperature, and wind, and seasonal variations; atmospheric waves; and how changes in the solar cycle affect the variance of geomagnetic disturbances within the upper atmosphere.Scientists will also look at how different radiative, chemical, electrodynamic and dynamic processes within the upper atmosphere vary with solar radiation inputs during times of reduced solar activity.

Since January 2002, TIMED and its ground-based partners have collected unprecedented global observations of the MLTI region's basic structure, temperature, pressure, wind and chemical composition, as well as measurements regarding the region's energy inputs and outputs.

"TIMED is the first mission to simultaneously measure all critical parameters so that we can better understand the processes that control changes in the upper atmosphere," Yee said.As TIMED continues its exploration of one of Earth's last atmospheric frontiers, APL will continue to lead the project's science effort and manage the mission's Science Data Center for NASA.

The Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center, Greenbelt, Md., oversees the TIMED mission for the Office of Space Science in Washington, D.C. TIMED is the first mission in NASA's Solar Terrestrial Probes Program.

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