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CUA Researchers Improve Space Weather Forecasting

Activity on the Sun Affects Satellites in Space, Cell Phones on Earth

Scientists can now predict the arrival of billion-ton electrified-gas clouds from the sun that cause severe space storms to within a half-day of the event, a great improvement over the best previous estimates of within two to five days.

Researchers at The Catholic University of America and NASA's Goddard Space Flight Center have created a model that reliably predicts how much time it takes for these clouds, called Coronal Mass Ejections (CMEs), to traverse the gulf between the sun and the Earth, based on their initial speed from the sun and their interaction with the solar wind.

The model, called the Empirical CME Arrival Model, uses recent observations from the European Space Agency/NASA Solar and Heliospheric Observatory (SOHO) and the NASA WIND spacecraft, and has been validated and made more accurate using historical observations from the Helios-1 (Germany/NASA), the Pioneer Venus Orbiter (PVO) (NASA), and the Space Test Program P78-1 (United States Air Force) spacecraft.

Earth-directed CMEs cause space storms by interacting with the Earth's magnetic field, distorting its shape and accelerating electrically charged particles (electrons and atomic nuclei) trapped within. Severe solar weather is often heralded by dramatic auroral displays (northern and southern lights), but space storms are occasionally harmful, potentially disrupting satellites, radio communications, and power systems.

"The new model more accurately predicts the arrival of Coronal Mass Ejections, and will greatly benefit people who operate systems affected by space storms," said lead author Dr. Natchimuthuk Gopalswamy, a research professor in CUA's Institute for Astrophysics and Computational Sciences. "The improved forecasts let operators of sensitive systems take protective action at the proper time and minimize the unproductive time when systems are placed in a safe mode to weather the storm."

Gopalswamy, who also is a senior research associate at the National Academy of Sciences/National Research Council, and colleagues presented the research at a press conference June 19 during a meeting of the Solar Physics Division of the American Astronomical Society at Lake Tahoe, Stateline, NV.

CMEs are ejected from the sun at various speeds, ranging from 12 to 1,250 miles per second (about 20 to 2,000 kilometers per second). Only the ones directed at Earth are potentially harmful, and estimating when they will arrive is difficult because their speed changes due to interaction with the solar wind, a stream of electrically charged gas that blows constantly from the sun at about 250 miles per second (about 400 kilometers per second). Just as a motorboat heading downstream will slow to the speed of the river's current if its motor is turned off, drag from the solar wind slows down CMEs that are initially ejected from the sun faster than the solar wind. If a boat pulls up anchor, it will gradually accelerate until it is moving at the speed of the current. Similarly, CMEs that start out slower than the solar wind are pulled along until they match the solar wind's speed.

Using data from solar-observing spacecraft, Gopalswamy and his team discovered how much the solar wind sped up or slowed down various CMEs according to their initial speeds. If the initial speed of a CME is known, the new model accurately accounts for the influence of the solar wind on the CME speed, and the CME arrival time at Earth can now be precisely estimated.

The model is expected to permit even more accurate arrival time predictions once the next generation of solar-observing spacecraft capable of measuring the true initial speed of Earth-directed CMEs is in place. Because it is directly in line between the Earth and the sun, the SOHO spacecraft can't see the true three-dimensional extension of an Earth-directed CME. Thus, SOHO can't tell how close the front of the CME cloud is, or how quickly the front is approaching Earth. The WIND spacecraft detects the magnetic field carried by CME clouds, but WIND's location in space only allows about an hour's notice. (However, by measuring the direction of a CME's magnetic field, WIND can estimate how severe the resulting space storm will be.)

Currently, scientists using SOHO estimate the CME's initial speed by measuring how quickly it expands across SOHO's field of view as it is ejected into space towards Earth. Historical observations validating the model used data from the P78-1 spacecraft (also located on the sun-Earth line) and from Helios-1 and PVO, which were off the sun-Earth line. Earth-orbiting spacecraft like P78-1 can measure the true initial speed of CME clouds heading sideways to the sun-Earth line, because the distance from the sun to the front of the CME cloud can be seen. P78-1 measured the true initial speeds of 17 sideways-directed CME clouds, while Helios-1 and PVO measured their speeds far from the sun in the solar wind at Earth distances. The combined measurements gave the change in speed due to the influence of the solar wind. Because the influence of the solar wind on CME speed is similar in any direction from the sun, the measurements can be applied to Earth-directed CMEs.

Planned solar observatories include NASA's Solar-Terrestrial Relations Observatory (STEREO) mission, comprising two spacecraft positioned at angles to the sun-Earth line, which will provide views of Earth-directed CMEs in three dimensions and measure their true ejection speed from the sun.

Gopalswamy is a research professor with CUA's Institute for Astrophysics and Computational Sciences, based in the physics department. The institute was established at CUA in 1996 to enhance research, educational and employment opportunities for CUA faculty, research staff, and students.

For images and more information, refer to: http://www.lmsal.com/spd/Press/



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