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EMBARGOED FOR RELEASE ON DECEMBER 13, 1999 AT 11 A.M. PST
From May 10-12, 1999, the solar wind that blows constantly from the Sun virtually disappeared in the most drastic and longest-lasting decrease ever observed. Dropping to a fraction of its normal density and to half its normal speed, the solar wind died down enough to allow physicists to observe particles flowing directly from the Sun's corona to Earth. This severe change in the solar wind also drastically changed the shape of Earth's magnetic field and produced a rare auroral display at the North Pole.
Starting late on May 10 and continuing through the early hours of May 12, the density of the solar wind dropped by more than 98%. Because of the drop-off of the wind, energetic electrons from the Sun arrived at the Earth in narrow beams, known as the strahl. Under normal conditions, electrons from the Sun are diluted, mixed, and redirected in interplanetary space and by Earth's magnetic field (the magnetosphere). But in May 1999, several satellites detected electrons arriving at Earth with properties similar to those of electrons in the Sun's corona, suggesting that they were a direct sample of particles from the Sun.
"This event provides a window to see the Sun's corona directly," said Dr. Keith Ogilvie, project scientist for NASA's Wind spacecraft and a space physicist at Goddard Space Flight Center. "The beams from the corona do not get broken up or scattered as they do under normal circumstances, and the temperature of the electrons is very similar to their original state on the Sun."
"Normally, our view of the corona from Earth is like seeing the Sun on an overcast, cloudy day," said Dr. Jack Scudder, space physicist from the University of Iowa and principal investigator for the Hot Plasma Analyzer (HYDRA) on NASA's Polar spacecraft. "On May 11, the clouds broke and we could see clearly."
Scudder, Ogilvie, and other scientists affiliated with the International Solar-Terrestrial Physics program (ISTP) presented their findings at the Fall Meeting of the American Geophysical Union in San Francisco's Moscone Center. Researchers working with more than a dozen spacecraft observed various facets of this event.
Fourteen years ago, Scudder and Dr. Don Fairfield of NASA Goddard predicted the details of an event such as occurred on May 11, saying that it would produce an intense "polar rain" of electrons over one of the polar caps of Earth. The polar caps typically do not receive enough energetic electrons to produce visible aurora because those electrons are slowed and depleted by too many collisions in interplanetary space. But in an intense polar rain event, Scudder and Fairfield theorized, the "strahl" electrons would flow unimpeded along the Sun's magnetic field lines to Earth and should precipitate directly into the polar caps, inside the normal auroral oval.
Such a polar rain event was observed as a steady glow in X ray images and confirmed for the first time in May 1999. Aurora were observed at the North Pole, which can only happen if these energetic electrons are coming directly from the solar wind.
"While we saw weak aurora in the south, in the north we saw the effects of intense, energetic electrons on the upper atmosphere in the form of X rays," said Dr. Dave Chenette, a space physicist at Lockheed Martin and principal investigator of the Polar Ionospheric X-Ray Imaging Experiment (PIXIE) on NASA's Polar spacecraft. "These X-ray emissions are the most intense that we have ever seen at the north magnetic pole since Polar was launched in 1996."
According to Chenette and Scudder, the fact that the aurora appeared only at one pole in May 1999 suggests that the North Pole is connected to the end of the magnetic field from the Sun, while the South Pole is connected to the end of the Sun's magnetic field that extends to the outer reaches of the solar system.
"The May event provides unique conditions to test ideas about solar-terrestrial interactions," Ogilvie noted. "It also strengthens our belief that we understand how the Sun-Earth connection works."
Under typical conditions, the Sun emits a tenuous gas of protons, helium, and electrons - the solar wind -- in all directions across the solar system. Carrying energy and magnetic fields from the Sun, the solar wind varies but usually stays within 5 to 10 particles per cubic centimeter (cc) and between 400-600 kilometers per second. The pressure from this solar wind buffets and confines Earth's magnetic field, ramming it up against the planet on the day side and stretching a long magnetic tail on the night side.
But on May 11, the drop in the density of the solar wind (to less than 0.2 particles per cc) allowed Earth's magnetosphere to swell unimpeded to five to six times its normal size. According to observations from the ACE spacecraft, the density of helium in the solar wind dropped to less than 0.1% of its normal value, and heavier ions, held back by gravity, apparently could not escape from the Sun at all. NASA's Wind, IMP-8, and Lunar Prospector spacecraft and the Japanese Geotail satellite observed Earth's bow shock - the region where the solar wind slams into the sunward edge of the magnetosphere - moving out to 238,000 miles from Earth (380,000 kilometers). The event produced the most distant bow shock ever recorded by satellites; the norm is 41,500 miles (67,000 km) from Earth toward the Sun.
In addition, the Earth's magnetic field took on a more dipolar shape - similar to the shape of iron filings spread around a magnet - as Earth's field would appear if there was no solar wind. And data from NASA's SAMPEX spacecraft reveal that in the wake of this event, Earth's radiation belts dissipated and nearly disappeared for several days afterward.
Nearly a dozen spacecraft observed this unusual event, including NASA's Polar, Wind, ACE, IMP-8, SAMPEX, FAST, and Lunar Prospector satellites. Contributions also were made by Interball (Russian Space Agency), Geotail (Japan's Institute for Space and Astronautical Science), and by satellites operated by the National Oceanic and Atmospheric Administration and the U.S. Department of Defense.
A NASA Video File relating to this story will air on December 13 at Noon EDT. NASA Television is available on GE-2, transponder 9C at 85 degrees West longitude, with vertical polarization. Frequency is on 3880.0 megahertz, with audio on 6.8 megahertz. Video File Advisories can be found at
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Author: Mike Carlowicz
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