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NEWS | Sept. 29, 2014

NRL Researchers Study Solar Disturbances and their Impact on Satellite Navigation Systems

By Donna McKinney

Scientists know that weather in space impacts activity here on Earth, so Dr. Brian Wood, at the U.S. Naval Research Laboratory (NRL) Space Science Division, is analyzing ionospheric disturbances to determine their effect on ground-based satellite navigation systems.

Wood is studying the solar origins of two of the strongest recent ionospheric disturbances over North America using observations from NRL-designed imagers on board NASA's STEREO and SOHO spacecraft. It is important to understand the kinds of eruptions from the sun, Wood explains, that can lead to significant effects at Earth. These two events had a significant effect on positional accuracy for the Wide Area Augmentation System (WAAS) satellite navigation system.

By some measures, the strongest ionospheric disturbances to happen over North America in recent years occurred on September 26 and October 24, 2011, explains Wood. The fury of these two solar disturbances was revealed through their effect on the WAAS system, a network of ground-based reference stations that provide corrections to signals from Global Positioning System (GPS) satellites, leading to improvements in GPS positional accuracy. The WAAS system is widely used in civil aviation within the United States, including for landing approach procedures requiring high positional precision. The September 26 and October 24 disturbances produced the largest degradations in WAAS service since 2007.

Wood traced the origin of these disturbances to coronal mass ejections (CMEs), massive bursts of solar wind and magnetic fields that erupted from the Sun two days prior to the ionospheric storms observed at Earth. He has reconstructed the three-dimensional structure and kinematics of the two CMEs using images from the NRL-designed instruments aboard STEREO and SOHO spacecraft. Wood modeled the two CMEs assuming a magnetic flux rope morphology, which is a tube-shaped structure wrapped in a helical magnetic field.

Kinematically, the September CME reached 1700 kilometers per second near the Sun before decelerating to 1000 kilometers per second. Wood noted that this event was surprisingly geoeffective considering that Earth only received a glancing blow from the shock that formed in front of the CME, with the CME ejecta missing Earth entirely. The October event was slower, only reaching 700 kilometers per second, but it provided a more direct hit on Earth. Both events arrived at Earth during daytime over North America, maximizing their impact on the US.

Looking forward, Wood is hopeful that this research will improve the ability to evaluate and forecast solar events that can significantly degrade navigation systems here on Earth. The ultimate goal, Wood says, is to be able to predict in advance the effects of a solar storm on communications and navigation at Earth. Characterizing the sources of geoeffective solar storms is a necessary step in that direction.

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