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
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]
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."
"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."
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]
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.
