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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.
BASED ON A NASA/GSFC PRESS RELEASE
<../../images/solarwind_gsfc/aurora_big.jpg>*Dec. 13, 1999*: From May
10-12, 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. [larger
image <../../images/solarwind_gsfc/aurora_big.jpg>]
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Starting late on May 10 and continuing through the early hours of May
12, NASA's ACE and Wind spacecraft each observed that the density of the
solar wind dropped by more than 98%. Because of the decrease, energetic
electrons from the Sun were able to flow to 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 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."
Parents and Educators: Please visit Thursday's Classroom
for lesson plans
and activities related to this story.
"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."
<../../images/solarwind_gsfc/northpole_big.gif>*Left:* This pair of
images in visible light from the Polar satellite's Visible Imaging
System compares the northern auroral regions on May 11, 1999, and a more
typical day on November 13, 1999. Credit: University of Iowa/NASA.
[larger image <../../images/solarwind_gsfc/northpole_big.gif>]
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 /
*Left*: This movie, which can span from 2 to 6 hours of actual time,
shows the most recent images of the Earth's auroral region taken from
space by the Ultraviolet Imager (UVI) Instrument on board the POLAR
spacecraft. New movies are generated every 6 hours. Hit reload for the
latest animation or visit the UVI aurora home page
for new pictures every 7
minutes.
Science Data for "The Day the Solar Wind Disappeared"
For more information, please contact:
Dr. John M. Horack , Director of Science
Communications Editor: Dr. Tony Phillips
Curator: Bryan Walls
NASA Official: Frank M. Rose