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
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 Řieroset 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
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, Řieroset, Lin and their colleagues reported in
Naturedata 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. Řieroset 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
"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, 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
Earlier this year, Mozer, Bale and Phan reported in Physical
Review Lettersdata 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 Řieroset, Lin and their colleagues is supported