A fleet of NASA and European Space Agency space-weather probes recently observed an immense jet of electrically charged particles in the solar wind between the sun and Earth. The jet, at least 200 times as wide as the Earth, was powered by clashing magnetic fields in a process known as "magnetic reconnection.

This graphic represents NASA (ACE and Wind) and ESA (Cluster) spacecraft encountering solar particle jets spanning about 1.5 million miles in the solar wind. The particle jets (indicated by red arrows) are sandwiched between sheets of opposite magnetic fields (blue). Earth's magnetic environment is to the right in the background: The blue area represents a cross-section of the bow shock formed as the solar wind hits Earth's magnetic field, the red area is a cross-section of the magnetic field produced by the Earth, and the blue sphere in the center is the Earth. Credit: Matt Davis, Univ. of California, Berkeley

Similar reconnection-powered jets occur in Earth’s magnetic field, producing effects that can disable orbiting spacecraft and cause severe magnetic storms on our planet, sometimes disrupting power stations. The newly discovered interplanetary jets are far larger than those occurring within Earth's magnetic field. The new observation is the first direct measurement indicating magnetic reconnection can happen on immense scales.

Understanding magnetic reconnection is fundamental to comprehending explosive phenomena throughout the universe, such as solar flares (billion-megaton explosions in the sun's atmosphere), gamma-ray bursts (intense bursts of radiation from exotic stars) and laboratory nuclear fusion. Just as a rubber band can suddenly snap when pulled too far, magnetic reconnection is a natural process by which the energy in a stressed magnetic field is suddenly released when it changes shape, accelerating particles (ions and electrons).

"Only with coordinated measurements by sun-Earth connection spacecraft such as the Advanced Composition Explorer (ACE), Wind, and Cluster can we explore the space environment with unprecedented detail and in 3-D," said Dr. Tai Phan, lead author of the results, from the University of California, Berkeley. "The near-Earth space environment is the only natural laboratory where we can make direct measurements of the physics of explosive magnetic phenomena that occurs throughout the universe." Phan's article will appear as the cover article in Nature on January 12.

The solar wind is a stream of electrically charged or ionized gas that blows continually from the sun and carries magnetic fields in different directions. Magnetic reconnection in the solar wind takes place when "sheets" of oppositely directed magnetic fields get pressed together. In doing so, the sheets connect to form an X-shaped cross section that is then annihilated, or broken, to form a new magnetic line geometry. The creation of a different magnetic geometry produces extensive jets of particles streaming away from the reconnection site.

Until recently, magnetic reconnection was mostly reported in Earth’s magnetosphere, the magnetic shield surrounding Earth. It is composed of magnetic field lines generated by our planet, and it defends us from the continuous flow of charged particles that make up the solar wind by deflecting them. When the interplanetary magnetic field lines carried by the solar wind happen to be in opposite orientation to the Earth’s magnetic field lines, however, reconnection is triggered and solar material can break through Earth’s magnetic field.

Some previous reconnection events measured in Earth’s magnetosphere suggested that the phenomenon was intrinsically random and patchy in nature, extending not more than a few tens of thousands of miles. However, "This discovery settles a long-standing debate concerning whether reconnection is intrinsically patchy or whether it can operate across vast regions in space," said Dr. Jack Gosling of the University of Colorado, a co-author on the paper and a pioneer in research on reconnection in space.

The broader picture of magnetic reconnection emerged when six spacecraft – the four ESA Cluster spacecraft and NASA's ACE and Wind probes – were flying in the solar wind outside Earth’s magnetosphere on Feb. 2, 2002 and made a chance discovery. During an estimated two-and-a-half hour time span, all spacecraft observed in sequence a single huge stream of jetting particles, at least 1.5 million miles (or nearly 200 Earth diameters) wide, caused by the largest reconnection event ever measured directly.

"If the observed reconnections were patchy, one or more spacecraft most likely would not have encountered an accelerated flow of particles," said Phan. "Furthermore, patchy and random reconnection events would have resulted in different spacecraft detecting jets directed in different directions, which was not the case."