Solar Wind Make Waves; Killer Electrons Go Surfing

http://www.gsfc.nasa.gov/topstory/20...4magwaves.html

SOLAR WIND MAKES WAVES; KILLER ELECTRONS GO SURFING?
Goodard Space Flight Center
September 9, 2003

"Killer" electrons capable of wreaking havoc
on orbiting spacecraft may "surf" magnetic
waves driven by the solar wind, according to a
team of space scientists.

The team from Boston University and the
National Oceanic and Atmospheric
Administration (NOAA) combined
observations from NASA and NOAA
spacecraft to identify a phenomenon that
explains how the solar wind makes waves in
Earth's magnetic field (magnetosphere).
Ordinary electrons orbiting the Earth in the
Van Allen radiation belts may boogyboard
the waves, accelerating to near the speed of
light, with energies 300-500 times greater
than the electrons in a television screen.

The solar wind is a stream of electrically
charged particles blown constantly from the
Sun. The magnetosphere is a cavity formed
when the solar wind encounters the Earth's
magnetic field. When the solar wind density
is high and comes up against the
magnetosphere, the magnetosphere gets
compressed. When the wind density is low,
the magnetosphere expands. The researchers
discovered that the solar wind contains
periodic structures of high and low density,
driving a periodic "breathing" action of the
magnetosphere and the global generation of magnetic waves.

It's known that if the frequency of these
waves matches the frequency of the electrons
in their motion in the Van Allen belt, the
electrons can be accelerated, significantly
boosting their energies. The process is similar
to a boogyboarder catching a wave. Some
electrons "ride the wave" and gain so much
energy that they can then damage expensive
spacecraft.

"If we can confirm this as a significant
mechanism for making the waves that
accelerate 'killer' electrons, then scientists
using data from satellites like Wind could
develop advance warning for spacecraft
operators that their spacecraft may be in
danger of excessive and damaging radiation
exposure," said Dr. Barbara Giles, project
scientist for the Polar spacecraft at NASA's
Goddard Space Flight Center, Greenbelt, Md.

When electrons become this energetic, they
can penetrate to the interior of spacecraft.
Once inside electronic parts, they build up
static electricity that can short circuit a
critical part or put the spacecraft into a bad
operating mode.

"What's new and exciting about this research is that
people had always looked for mechanisms internal to the
magnetosphere for generating these waves," said Dr. Larry
Kepko, research associate at Boston University and lead
author of two papers on this research, one published in
the Journal of Geophysical Research in June 2003 and the
other in Geophysical Research Letters in 2002. "But here
we've found an external mechanism - the solar wind itself."

NASA's Polar and Wind satellites, along with NOAA's
Geostationary Operational Environmental Satellite (GOES),
provided the key observations leading the team to this
conclusion. Polar confirmed that the waves are not local,
but global. The Wind satellite was the primary source for
identifying the density structures in the solar wind that
drive the magnetosphere. GOES provided data about the Earth's
magnetosphere as it increased and decreased in size.

"We already knew that the solar wind has density structures
and that magnetic waves can accelerate electrons," said Dr.
Harlan Spence, associate professor of astronomy at Boston
University and co-author of the two papers on this research.
"What we didn't know was that the solar wind structures can
be periodic and drive magnetic waves. These new observations
may provide a missing link between the two."

The ultimate source of these newly discovered solar wind
structures is still a mystery, but the team speculates that
the Sun may play a direct role. "The solar wind density
variations are partly controlled by the pattern of magnetic
reconnection, the twisting and snapping of magnetic field
lines, on the surface of the Sun," says Dr. Kepko.
"Reconnection occurring in a systematic, periodic manner may
produce the observed periodic density structures in the solar
wind. There is some evidence that this may be occurring, but
further research is required to establish a definitive link."

The Van Allen radiation belts were discovered in 1958 by Dr.
James Van Allen and his team at the University of Iowa with
Explorers 1 and 3, the first satellites successfully launched
by the United States. They are belts of electrically charged
particles trapped by the Earth's magnetic field. Since the
particles are electrically charged (mostly protons and electrons),
they feel magnetic forces and are constrained to spiral around
invisible lines of magnetic force that comprise the Earth's
magnetic field. There are actually two donut-shaped belts in
the Van Allen system, one inside the other with the Earth in
the "hole" of the inner belt. The inner belt, made up of
high-speed protons, is located at altitudes between 430 and 7,500
miles (about 700 to 12,000 km) above the Earth. The outer belt
is made of high-speed electrons and appears at altitudes
between 15,500 and 25,000 miles (about 25,000 to 40,000
km) above Earth. Spacecraft operators try to avoid orbits in
these regions, but sometimes these altitudes are best for a
particular mission, or the spacecraft must pass through the belts
during part of its orbit or to escape the Earth entirely.

NASA's Polar and Wind satellites, together known as the
"Global Geospace Science Program," are dedicated to helping
scientists understand how particles and energy from the
Sun flow through, and interact with, the Earth's space environment.

NOAA is dedicated to gathering data about the oceans, the
atmosphere, space, and the Sun. Its GOES satellite system is
the basic element for U.S. weather monitoring and forecasting.
Dr. Howard Singer from NOAA is a third co-author on the 2002
paper about this research.