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July 25, 2001
Wind Spacecraft Flies Through the Spigot Where the Solar Wind Pours into Earth's Magnetosphere

[image-51]Through a combination of good luck and shrewd data analysis, researchers using NASA's Wind spacecraft have made rare direct observations of the mysterious process that allows the solar wind to connect to Earth's magnetic field (the magnetosphere). Known as reconnection, this process allows the magnetic field of the Sun - as carried in the solar wind - to connect to Earth's field, allowing energy and matter to flow from one to the other.

Like water rushing through a pipe and spigot, the solar wind flows into Earth's magnetosphere through a narrow valve-like region a quarter of a million miles downwind of the planet, in a region known as the magnetic tail. Depending upon the orientation of the solar wind and Earth's magnetic field, that valve opens and closes to allow plasma and energy from the Sun to enter Earth's space. In April 1999, Wind passed right through that valve as solar wind poured in.

The research findings were made by Marit Oieroset and colleagues at the University of California-Berkeley, with scientists from the Tokyo Institute of Technology and NASA's Goddard Space Flight Center, Greenbelt, Md. They will appear in the July 26 issue of Nature.

The solar wind is a tenuous, ionized gas (called plasma) that flows out constantly from the Sun in all directions. The wind ebbs and flows, shaping the magnetosphere of Earth and carrying energy, magnetic fields, and matter from the Sun. On the sunlit or "day" side of Earth, the solar wind pushes, stretches, and energizes Earth's magnetic field, but the plasma barely penetrates Earth's magnetic shell. It is in the distant reconnection region of the tail, on the night side of Earth, where the solar wind enters the magnetosphere. The flow of solar wind into the tail of the magnetosphere fills Earth's space with plasma and energy. This energy is stored like a battery until it is eventually released in bursts that cause auroras and other space weather phenomena. Space weather can affect radio communications, satellite operations, and the control of electric power systems on Earth.

Magnetic reconnection was proposed more than forty years ago as the key process that allows this flow of solar wind into the magnetosphere. During reconnection, magnetic fields that are heading in opposite directions -- having opposite north or south polarities -- break and connect to each other. Previous studies have observed the consequences of reconnection after it had occurred, mainly the flow of plasma and energy toward or away from Earth. But eyewitness observations of the process in action have been elusive.

In April 1999, Wind flew right through the reconnection region as the process was occurring. While flying tailward through the magnetosphere (away from Earth), Wind's instruments detected jets of plasma racing toward Earth. Part-way through the journey, the jets stopped and Wind detected unusual electric currents and magnetic signatures that were predicted to occur at the point of reconnection. Some time later, as Wind kept flying away from Earth, the spacecraft detected that jets of plasma flow in the exact opposite direction of the previous flows, racing away from the planet. The event was comparable to a plane flying through the eerie, calm eye of a hurricane.

"Reconnection is one of the fundamental physical processes in the universe," says Oieroset. "It occurs in so many places in the universe, yet the only place we can observe it directly in a natural environment is in our magnetosphere. It is absolutely crucial for understanding how the Sun connects to the Earth. This is how you populate the magnetosphere with plasma."

The direct observation of reconnection 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. Reconnection likely plays a role in the interaction of neighboring stars. 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.

The rare observation of reconnection was made possible due to a decision to move Wind out of its originally planned orbit and into a new pattern that took it on petal-like loops around the Earth and Moon. "When Wind was launched it was not supposed to study the deep tail at all," says Keith Ogilvie, project scientist for the Wind mission and a researcher at NASA Goddard. "But with a new orbit and region comes new discoveries."

Launched in 1994, Wind studies the solar wind both inside and outside of Earth's magnetosphere. The spacecraft is a key component of the International Solar-Terrestrial Physics program, is a joint scientific study of the Sun-Earth system conducted by NASA, Japan's Institute for Space and Astronautical Studies, and the European Space Agency (ESA).

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The solar wind impinges on the Earth's magnetic field, compressing the sunward side and stretching the shadow side into a long tail. On April 1, 1999, the Wind spacecraft passed through the diffusion zone in the magnetotail and observed magnetic reconnection between the fields of the sun and Earth.
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