Satellite finds electrons brought to near-light speed
Posted: November 12, 2002

A chance observation of high-energy electrons emanating from a tiny region of space where the sun and Earth's magnetic fields intertwine provides the first solid evidence that a process called magnetic reconnection accelerates electrons to near the speed of light in the Earth's magnetosphere and perhaps throughout the universe where magnetic fields entangle.

The observations were made April 1, 1999, as NASA's Wind satellite made a chance pass through the magnetic reconnection region in the Earth's shadow. Scientists at the University of California, Berkeley's Space Sciences Laboratory report the results of their data analysis in the Nov. 4 issue of Physical Review Letters.

Diagram of the Earth's magnetosphere, pushed and shaped by the impinging magnetic field of the sun. Credit: NASA/GSFC
Magnetic reconnection should occur wherever magnetic fields clash. As the fields try to bend around one another, the field lines break and recombine like a short-circuit in space, sending out jets of electrons and ions moving at speeds of hundreds of miles per second. In addition to these jets, the process also is thought to produce much more energetic electrons, with energies up to hundreds of thousands of electron volts - equivalent to a speed of more than 100,000 miles per second.

This highly energetic process is thought to occur in explosive solar flares, generating electrons with energies ranging from tens to hundreds of thousands of electron volts that carry away as much as half the energy in the flare. Magnetic fields colliding in interstellar space could just as easily rev particles to nearly the speed of light, as could reconnection in accretion disks around black holes.

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

Magnetic reconnection is considered so intriguing and fundamental that NASA is considering funding the Magnetospheric Multi-Scale mission - four or five satellites placed in Earth orbit before the end of the decade - to study the process.

Despite its presumed importance wherever magnetic fields occur, there has been no direct evidence that regions of magnetic reconnection generate the very energetic particles traveling at near light speed.

"This observation is the first clear-cut, unambiguous evidence that a region of magnetic reconnection is the source of high-energy electrons," said Robert Lin, UC Berkeley professor of physics and principal investigator for the instrument aboard the Wind satellite that detected the electrons.

The idea of magnetic reconnection was originally put forth in 1946 to explain solar flares and the high-energy particles that stream from them.

"The fact that we see energetic particles here in the magnetic reconnection region of the Earth's magnetosphere suggests that the energetic particles you see in solar flares also are produced by reconnection," said Tai Phan, a research physicist at UC Berkeley's Space Sciences Laboratory.

Research physicist Marit Oieroset led the data analysis, along with Lin, Phan, Davin Larson and research physicist Stuart D. Bale. All are scientists at the Space Sciences Laboratory, which Lin directs.

"Electron heating by magnetic reconnection is really fundamentally not understood," added theoretician James Drake, professor of physics at the University of Maryland, College Park, who models the process of particle acceleration by reconnection. "This paper from the Berkeley group is some nice evidence that there is actually direct heating at the center of the diffusion region."

The energetic electrons, traveling at speeds up to 80 percent the speed of light, were observed by Wind as it passed though the region of magnetic reconnection in the Earth's magnetotail. The magnetotail is located downstream of the Earth in the solar wind 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.

Launched in 1994, Wind was designed to study the solar wind and its interaction with the magnetosphere - the region in space shielded by the Earth's magnetic fields. In eight years of operation, the satellite passed only once through the small magnetic reconnection diffusion region, Lin said.

For about 20 minutes on April Fools' Day, however, Wind recorded the first data ever from a region of magnetic connection. Last year, Oieroset, Lin and their colleagues reported in Nature data that confirmed many aspects of the reconnection process that theorists had predicted.

The current paper results from further analysis of the data obtained during this encounter. Oieroset and colleagues were able to measure electron velocities as the satellite traversed the magnetic reconnection diffusion zone, and found a peak energy in the diffusion region of about 300,000 electron volts, equivalent to a speed of about 150,000 miles per second.

"The fact that high energy electrons peak right there, and as you get away from that region, intensities go down and things get less energetic, it really points to this region as being the source of these high energy electrons," Lin said.

Lin, Forrest Mozer and others at the Space Sciences Laboratory have built instruments now flying aboard various Earth-orbiting satellites to gather information about magnetic reconnection. Lin led the group that built the 3-D Plasma and Energetic Particle instrument aboard Wind, which measures the full three-dimensional distribution of energetic electrons and ions.

Earlier this year, Mozer, Bale and Phan reported in Physical Review Letters data obtained when the Polar spacecraft, which carries an instrument built at the Space Sciences Laboratory to measure electric fields, flew though the magnetic reconnection region at the nose of the Earth's magnetosphere, where the solar wind first comes into contact with the Earth's magnetic field. That encounter, coincidentally on April 1, 2001, obtained unprecedented detail confirming theoretical predictions of the structure and dynamics of the region where ions decouple from the magnetic field. The decoupling of ions and electrons from the magnetic field in the diffusion region is a necessary step before the magnetic field lines can change partners, or reconnect.

The satellite RHESSI, designed and built by scientists at UC Berkeley's Space Sciences Laboratory, was launched by NASA on Feb. 5, 2002, on a two-year mission to study high-energy emissions from solar flares, including the production of energetic electrons by magnetic reconnection.

"RHESSI has already obtained direct evidence about energetic particle production, especially electrons, in solar flares, but it is remote," Drake said. "The advantage of studying the magnetosphere is that you can actually get in with satellites and probe what's going on locally and, since you have direct measurements of the magnetic field at the same time, it's much easier to couple theory and experiment. The magnetosphere has become a good laboratory for understanding reconnection in a broader context."

Drake said he is preparing a paper now that "demonstrates for the first time that the reconnection process produces intense electron currents which drive the production of electron holes - areas of low electron density - and these holes then scatter particles and cause heating of electrons."

The research by Oieroset, Lin and their colleagues is supported by NASA.

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