Spacecraft Finds Spot Where Solar Wind and Earth's Magnetosphere Connect

In a chance encounter, NASA's Wind spacecraft has observed a phenomenon long predicted but never proved called magnetic reconnection. The process occurs when magnetic field lines running in opposite directions come into contact. As a result, magnetic energy becomes kinetic energy and charged particles in the form of plasma jets shoot out into space. Wind happened upon a spot where the sun and the earth's magnetic fields cross in April 1999, and an analysis appears in today's Nature.

"It [Wind] went through the magic region in the magnetosphere where field lines actually reconnect and saw the process in action," says co-author Marit Oieroset, a research scientist at the University of California at Berkeley's Space Science Laboratory. This magic region, the so-called diffusion region, lies 240,000 miles away from the earth in the magnetotail¿a part of our planet's magnetic field that faces away from the solar wind and stretches out into space. The researchers think that most of the solar wind particles entering the earth's magnetosphere come in through this diffusion region, from which plasma jets stream towards the earth and away from it.

The scientists found that the interaction between the plasma particles¿mostly hydrogen ions and electrons¿was collisionless. In other words, the ions and electrons detached from the magnetic field lines simultaneously. (In a slower collisional process, the ions diffuse first.) "Typically, in a plasma, charges move around and affect one other electromagnetically, so the whole thing acts like a fluid," says co-author Robert Lin, director of the Space Sciences Laboratory at UC-Berkeley. "But you couldn't explain reconnection at the rate we see it in solar flares or our own magnetosphere using normal fluid theory. These observations are a confirmation that the reconnection process, at least in this case, was collisionless and that the plasma was not acting like a normal fluid."

In conclusion, Lin notes that "it's very likely that this kind of process plays a role wherever magnetic fields occur in plasmas in the universe, and that magnetic reconnection may have a bigger role to play in the universe than people think."

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