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NASA/ESA space probes detect enormous magnetic annihilation event

– Four years ago, a fleet of NASA and European Space Agency space-weather probes observed an immense jet of electrically-charged particles in the solar wind between the sun and Earth.

 

Animation showing how magnetic field lines of opposite direction break and reconnect. Such reconnection events in space create jets of high-speed particles, such as the one recently captured by the ACE, Wind and Cluster spacecraft in orbit around Earth. (Copyright: Center for Visual Computing, University of California Riverside)

A subsequent analysis now reveals that the jet, at least 200 times as wide as the Earth, was powered by clashing magnetic fields in a process called "magnetic reconnection."

Similar reconnection- powered jets occur in Earth's magnetic shield, producing effects that can disable orbiting spacecraft and causing severe magnetic storms on our planet that disrupt power stations.

The newly discovered interplanetary jets, however, are far larger than those occurring within Earth's magnetic shield. The new observation is the first direct measurement indicating magnetic reconnection can happen on immense scales, according to lead author Tai Phan, an associate research physicist at the University of California, Berkeley's Space Sciences Laboratory.

Phan and his colleagues report the discovery in a cover story appearing in the Jan. 12 issue of Nature.

Understanding magnetic reconnection is fundamental to comprehending explosive phenomena throughout the universe such as solar flares, which are billion-megaton explosions in the sun's atmosphere; gamma-ray bursts, which are intense bursts of radiation from exotic stars; and laboratory nuclear fusion. Just as a rubber band can suddenly snap when twisted 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 charged particles (ions and electrons).

"Only with coordinated measurements by Sun-Earth connection spacecraft like ACE, Wind, and Cluster can we explore the space environment with unprecedented detail and in three dimensions," said Phan. "The near-Earth space environment is the only natural laboratory where we can make direct measurements of the physics of explosive magnetic phenomena occurring throughout the universe."

The solar wind is a dilute stream of electrically-charged, or ionized, gas that blows continually from the sun. Because the solar wind is electrically charged, it carries solar magnetic fields with it. The solar wind arising from different places on the sun carries magnetic fields pointing 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 has been almost exclusively reported in Earth's "magnetosphere," the natural magnetic shield surrounding Earth. It is composed of magnetic field lines generated by our planet, and defends us from the continuous flow of charged particles that make up the solar wind by deflecting the particles away from Earth. However, when the interplanetary magnetic field lines carried by the solar wind happen to be in the opposite orientation to Earth's magnetic field lines, reconnection is triggered and solar material can break through Earth's shield.

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 kilometers. However, "this discovery settles a long-standing debate concerning whether reconnection is intrinsically patchy, or whether instead it can operate across vast regions in space," said Dr. Jack Gosling of the University of Colorado, a co-author of the paper and a pioneer in research on reconnection in space.

The broader picture of magnetic reconnection emerged when six spacecraft - the four European Space Agency Cluster spacecraft and NASA's Advanced Composition Explorer (ACE) and Wind probes - were flying in the solar wind outside Earth's magnetosphere on Feb. 2, 2002. During a time span of about two and a half hours, all spacecraft observed in sequence a single, huge stream of jetting particles, at least 2.5 million kilometers wide (about 1.5 million miles or nearly 200 Earth diameters), caused by the largest reconnection event ever measured directly.

"If the observed reconnection was patchy, one or more spacecraft most likely would have not 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."

Another 27 large-scale reconnection events were identified by ACE and Wind, so the team could conclude that reconnection in the solar wind is to be looked at as an extended phenomenon.

The 2002 event could have been considerably larger, but the spacecraft were separated by no more than 200 Earth diameters, so its true extent is unknown. Two new NASA missions will help gauge the actual size of these events and examine them in more detail. The Solar Terrestrial Relations Observatory (STEREO) mission, scheduled for launch in May or June of 2006, will consist of two spacecraft orbiting the Sun on opposite sides of the Earth, separated by as much as 186 million miles (300 million kilometers). Their primary mission is to observe coronal mass ejections - billion-ton eruptions of electrically charged gas from the sun - in three dimensions. However, the spacecraft will also be able to detect magnetic reconnection events occurring in the solar wind with instruments that measure magnetic fields and charged particles.

The Magnetospheric Multi-Scale mission (MMS), planned for launch in 2013, will use four identical spacecraft in various Earth orbits to perform detailed studies of the cause of magnetic reconnection in the Earth's magnetosphere.

Co-authors with Phan and Gosling are Matt S. Davis, Marit Řieroset and Robert P. Lin of UC Berkeley; R. P. Lepping of NASA Goddard Space Flight Center; D. J. McComas of the Southwest Research Institute in San Antonio, Texas; R. M. Skoug of Los Alamos National Laboratory; C. W. Smith of the University of New Hampshire; H. Reme of France's Centre d'Etude Spatiale des Rayonnements; and A. Balogh of the Imperial College, London.

Wind, ACE and Cluster research in the United States is supported by NASA. Cluster research in France and the United Kingdom is supported by the Centre National d'Etudes Spatiales and the Particle Physics and Astronomy Research Council.

Links:

NASA press release and graphics