Craft flies through Earth's magnetic tail, sees rare event
Posted: July 26, 2001

An artist's concept of Wind. Photo: NASA
Thanks to a fluke encounter while flying through the Earth's magnetic tail two years ago, NASA's Wind spacecraft may have solved a long-standing mystery about how the sun's magnetic field interacts with that of the Earth.

Wind was launched by NASA in 1994 to study the solar wind and its interaction with the magnetosphere - the region in space shielded by the Earth's magnetic fields. On April 1, 1999, the spacecraft was flying through the Earth's magnetotail, the region in Earth's shadow where the magnetosphere is squeezed and stretched by the solar wind into a tail-like structure extending more than 100 times the diameter of the Earth.

Physicists at the University of California, Berkeley, looked closely at the data obtained that day because they suspected that Wind had, by chance, passed through the zone where magnetic field lines from the Earth and sun short circuit, reconnect, and in the process, sling out jets of charged particles.

Called magnetic reconnection, this process has been a mystery of major importance, because such reconnection is thought to take place throughout the universe wherever magnetic fields interact. Reconnection is thought to occur in the atmosphere of the sun, generating solar flares, as well as in interstellar space.

When the magnetic fields of the Earth and sun interact and reconnect, particles from the sun spiraling along magnetic field lines are able to slide like beads onto the Earth's field lines, eventually making their way to the poles and generating the aurorae.

"Most people believe magnetic reconnection happens as a way for solar particles to get inside our magnetosphere, but how?" asked Marit Oieroset, a research scientist at UC Berkeley's Space Sciences Laboratory. "With Wind, we have been very lucky. It went through the magic region in the magnetosphere where field lines actually reconnect and saw the process in action."

Oieroset and her colleagues confirmed many aspects of the reconnection process that theorists had predicted since 1946, when the late Australian physicist Ronald G. Giovanelli first proposed magnetic reconnection to explain the explosive events on the sun called solar flares.

Oieroset (pronounced ur your' o set); Robert Lin, professor of physics at UC Berkeley and director of the Space Sciences Laboratory; UC Berkeley research scientist Tai Phan; Masaki Fujimoto of the Tokyo Institute of Technology; and Ronald P. Lepping of NASA Goddard Space Flight Center report their observations and conclusions in the July 26 issue of Nature.

Magnetic reconnection occurs at the outer fringes of the magnetosphere, where oppositely directed field lines bump, tear and rejoin in a head-to-head hairpin configuration. These twin hairpin bends in the magnetic field snap back like rubber bands, flinging plasma particles in oppositely directed jets at speeds of hundreds of kilometers per second.

Last year, Phan and his colleagues reported observation of jets of particles caused by reconnection at the sunward edges of the magnetosphere. Detected by satellites called Geotail and Equator-S, it was the first time these two oppositely directed jets had been seen simultaneously.

Wind was designed to study such processes near the Earth's bowshock, which takes the brunt of the incessant wind from the sun. In 1999, however, NASA decided to switch the orbit to observe other regions of the magnetosphere, and planned to use the moon to slingshot Wind into a "petal orbit" - one that gradually shifts orientation so that, over many orbits, it traces the outline of a flower petal with the Earth at the center.

During the slingshot maneuver, Wind traveled straight down the center of the Earth's magnetotail and serendipitously passed through the area of magnetic reconnection just before looping around the moon. This was at a distance of about 60 Earth radii, or 240,000 miles from Earth. This area of magnetic reconnection in the magnetotail is thought to be the entryway for most of the solar wind particles that end up inside Earth's magnetosphere.

This area where magnetic reconnection occurs, called the diffusion region, is the source of the oppositely directed jets, and has never before been detected by a spacecraft. Based on Wind's observations, it extends over an area about half the diameter of the moon.

Importantly, the observations showed that the processes involved were collisionless, that is, the plasma particles, mostly hydrogen ions and electrons, moved as if they were alone, unaware of one another's existence.

A collisionless process explains why reconnection happens as fast as it does. Until recently theorists assumed that reconnection involved electromagnetic interactions among the particles - a collisional process. Such a process, however, predicts reconnection at a rate some 10 times slower than that observed.

A collisionless process leads to faster reconnection. Two magnetic field lines can't cross and reconnect until the ions and electrons detach from the field lines. In a collisional process, both ions and electrons detach or diffuse away simultaneously. In a collisionless process, the positive and larger ions diffuse away first while the smaller electrons continue to carry the magnetic field lines toward each other until the final moment, when the electrons detach from the field lines allowing reconnection to occur. According to recent theories, the electron dynamics lead to faster reconnection.

"Typically, in a plasma, charges move around and affect one other electromagnetically, so the whole thing acts like a fluid," Lin said. "But you couldn't explain reconnection at the rate we see it in solar flares or our own magnetosphere using normal fluid theory. These new 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.

"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" Lin said.

Lin headed the group that built Wind's 3-D Plasma and Energetic Particle Experiment, which obtained the data for the current analysis. He also is principal investigator for the High Energy Solar Spectroscopic Imager (HESSI) satellite, scheduled for launch later this year to study details of solar flares and the presumed magnetic reconnection events that generate them.

The research is funded by NASA.