The Day the Solar Wind Disappeared
For two days in May, 1999, the solar wind that blows
constantly from the Sun virtually disappeared -- 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 changed the shape of Earth's magnetic field and produced an unusual auroral display at the North Pole.
Right: Data visualization of the X ray emissions over the North Pole during the "polar rain" of electrons on May 11, 1999. The emissions were detected by the PIXIE instrument on NASA's Polar spacecraft. [ ]
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"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 NASA's Goddard Space Flight Center, Greenbelt, MD. "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."
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"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 on NASA's Polar spacecraft. "On May 11, the clouds broke and we could see clearly."
Fourteen years ago, Scudder and Dr. Don Fairfield of 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. 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 precipitate directly into the polar caps, inside the normal auroral oval. Such a polar rain event was observed for the first time in May when Polar detected a steady glow over the North Pole in X-ray images.
In parallel with the polar rain event, Earth's magnetosphere
swelled to five to six times its normal size. NASA's Wind, IMP-8,
and Lunar Prospector spacecraft, the Russian INTERBALL satellite
and the Japanese Geotail satellite observed the most distant
bow shock ever recorded by satellites. Earth's bow shock is the
shock front where the solar wind slams into the sunward edge
of the magnetosphere.
Right: Animation -The Day the Solar Wind Disappeared (Berry animation 9.4MB) As the solar wind dissipates on May 11, 1999, the magnetosphere and bow shock around Earth expand to five times their normal size. The aurora, which usually forms ovals around Earth's poles, fills in over the northern polar cap. Credit: NASA [Quicktime movie]
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 the Sun's gravity, apparently could not escape from the Sun at all. Data from NASA's SAMPEX spacecraft reveal that in the wake of this event, Earth's outer electron radiation belts dissipated and were severely depleted for several months afterward.
"The May event provides unique conditions to test ideas about solar-terrestrial interactions," Ogilvie noted. "It also strengthens our belief that we're beginning to understand how the Sun-Earth connection works."
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 ftp://ftp.hq.nasa.gov/pub/pao/tv-advisory/nasa-tv.txt
Solar Cycle Update Oct. 14, 1999. Updated predictions from NASA scientists place the solar maximum in mid-2000.
Surfing Magnetic Waves in the Solar Atmosphere July 8, 1999. How the Solar Wind Gets Up to Speed
Solar Flares Show Their True Colors June 2, 1999. New research points to a common mechanism for spectral behavior in Solar Flares
"Cool" microflares could be solar hot spots May 31, 1999. Secret of coronal heating may be multitude of tiny blasts.
Finding the smoking gun before it fires March 9, 1999. Physicists discover a new tool for predicting solar eruptions.
Links Related to ISTP Fall 1999 AGU Meeting News
- Science Data for "The Day the Solar Wind Disappeared"
- News and Info about Sun-Earth Connections Science
- Simple Background on the Sun-Earth System
- International Solar-Terrestrial Physics (ISTP) Program
- NASA's Wind Spacecraft
- NASA's Polar Spacecraft
- Hot Plasma Analyzer (HYDRA) on Polar -- Lead by University of Iowa
- Polar Ionospheric X-ray Imaging Experiment (PIXIE) -- Lead by Lockheed Martin Space Physics Department
- Japan's Geotail Spacecraft
- NASA's Advanced Composition Explorer (ACE)
- NASA's Solar, Anomalous and Magnetospheric Particle Explorer (SAMPEX)
- NASA's Interplanetary Monitoring Platform (IMP-8)
- NASA's Lunar Prospector
- NASA Goddard Space Flight Center
- NASA Sun-Earth Connections Program
- Sun-Earth Connections Education Forum
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