CONNECTION OF SUN'S AND EARTH'S MAGNETIC FIELDS PROVIDES ENERGY FOR
AURORAS, SPACE WEATHER
Space physicists have made the first direct observations of the process
that causes auroras and magnetic disturbances -- or space weather --
around the Earth. Settling a fifty-year-old debate, scientists have
directly measured the transfer of energy from the solar wind into the
magnetic space around Earth, or magnetosphere, and down to the
atmosphere. Such events can affect radio communications, satellite
operations, and the control of electric power systems on Earth.
Relying on observations collected by NASA's Polar spacecraft and Japan's
Geotail spacecraft, scientists associated with the International
Solar-Terrestrial Physics (ISTP) program have gathered the first direct
evidence that a process known as magnetic reconnection occurs naturally
in the Sun-Earth system. Until now, reconnection had only been observed
under contrived conditions in a few physics laboratories.
During reconnection, magnetic fields that are heading in opposite
directions -having opposite north or south polarities -- break and
connect to each other. In space, reconnection between the magnetic
fields of the Earth and Sun allows the solar wind to break through the
planet's magnetic shell and flow into the space around Earth. Along the
way, magnetic energy gets converted to bursts of particle energy that
create auroras - "northern or southern lights" -- and space weather
Indirect evidence of reconnection has provoked debate for more than half
a century, as space physicists could only detect signs of reconnection
after it had happened. But recently, the Polar spacecraft flew through
a region on the sunlit side of Earth where reconnection was in progress,
gathering the first eyewitness account of the process. Using data
collected from Geotail's dozens of passes through Earth's magnetic tail,
scientists also have pinpointed the area on the night side where
reconnection occurs, and have shown for the first time a clear
association between reconnection and auroras.
"Reconnection is the fundamental process for transferring and exchanging
energy in the Sun-Earth system," said Dr. Atsuhiro Nishida, a researcher
with the Japan Society for the Promotion of Science and the recently
retired Director-General of Japan's Institute of Space and Astronautical
Science (ISAS). "Reconnection on the day side of Earth is critical for
allowing solar wind energy to come into the magnetosphere. Night-side
reconnection is critical for the transfer of that energy down to the
Nishida and colleagues presented their results today at the spring
meeting of the American Geophysical Union, held in Washington, D.C.
While crucial for understanding space weather, the direct observation of
reconnection around Earth has implications for many fields of physics.
Reconnection on the Sun likely plays a role in the development of solar
flares and of coronal mass ejections. Similar magnetic activity outside
our solar system may explain some of the galactic X rays that
astronomers have detected. And observations of reconnection in nature
may aid the study of nuclear fusion and other plasma processes in the
laboratory. The magnetosphere is the only place where reconnection has
been observed first-hand as it occurs naturally.
A popular misconception holds that auroras and space weather are caused
when electrically charged particles from the Sun plunge directly into
Earth's atmosphere near the magnetic poles. But in fact, the Sun
provides the energy -- but not necessarily the particles -- to drive
space weather activity around Earth. And rather than a direct trip from
the solar atmosphere to Earth's poles, solar wind and storms from the
Sun must pass through these small and elusive reconnection regions
before they can stir up space weather.
"The magnetosphere acts like a great magnetic cocoon around the Earth,"
said Dr. Jack Scudder, a space physicist from the University of Iowa and
principal investigator for the Hot Plasma Analyzer (HYDRA) on NASA's
Polar spacecraft. "There are often times when the solar wind creates
tears in this cocoon, allowing charged particles and energy from the Sun
to enter the space around Earth. This tearing - reconnection - is what
we directly observed with Polar."
Once these "tears" open up - scientists call them "reconnection regions"
- the magnetic field of the solar wind becomes directly linked to the
magnetosphere. Solar energy floods into the system, overloading and
destabilizing it. The energy excites the particles already trapped
around the Earth and stretches the magnetic tail like taut rubber bands,
forcing reconnection to happen again -- this time inside Earth's space.
As magnetic field lines on the night side snap and reconnect, they shoot
energy stored in the tail down toward the auroral zones near the poles
and into the radiation belts.
"When the solar wind and magnetospheric fields reconnect, it opens a
valve or faucet that lets the solar wind energy cross the magnetopause
and pour into the magnetosphere," said Dr. Jeffrey Hughes, chairman of
the department of astronomy at Boston University. "Without
reconnection, the magnetosphere would be a very benign place."
Over the past eight years, ISAS's Geotail spacecraft has systematically
studied and surveyed the magnetic tail of Earth in search of this
process. As a result, scientists have been able to pinpoint the area
where reconnection happens in the tail, about 140,000 to 160,000 km
(85,000 to 96,000 miles) downwind of the Earth. They have also been
able to show that reconnection frequently occurs in the tail shortly
before auroras and magnetic disturbances begin in Earth's atmosphere.
Nishida and colleagues interpret those results to mean that reconnection
is the source of energy behind the auroras and storms.
The International Solar-Terrestrial Physics program is a joint
scientific study between NASA, ISAS, and the European Space Agency
(ESA), with contributions from Russia's Institute for Space Research and
many other international science institutions. The primary spacecraft
of ISTP include ISAS's Geotail, NASA's Polar and Wind spacecraft, and
the joint ESA/NASA Solar and Heliospheric Observatory (SOHO).
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