June 21, 2000 The arrival from the Sun of billion-ton electrified-gas clouds that cause severe space storms can now be predicted to within a half-day, a great improvement over the best previous estimates of two to five days.
Scientists at the Catholic University of America, Washington, DC, and NASA's Goddard Space Flight Center, Greenbelt, MD, 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 new model uses recent observations from the European Space Agency/NASA Solar and Heliospheric Observatory (SOHO) and the NASA WIND spacecraft. The model has been validated and made more accurate using historical observations from the Helios-1 (Germany/NASA), the Pioneer Venus Orbiter (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 of Catholic University, a Senior Research Associate at the National Academy of Sciences/National Research Council. "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 and colleagues will present this research today during a meeting of the Solar Physics Division of the American Astronomical Society at Lake Tahoe, Stateline, NV.
Coronal Mass Ejections leave the Sun at various speeds, ranging from 12 to 1,250 miles (about 20 to 2,000 kilometers) per second. Only the CMEs directed at Earth are potentially harmful; estimating when they will arrive is difficult because their speed changes due to interaction with the solar wind, a stream of electrically charged gas blowing constantly from the Sun at about 250 miles (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, Coronal Mass Ejections starting out from the Sun more quickly than the solar wind eventually are slowed by the drag of this "stream." 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 more slowly 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 Coronal Mass Ejections 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.
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