May 2000

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 storms.

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 atmosphere."

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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