UC Berkeley analysis of satellite data turns up first in situ evidence that magnetic processes in space can accelerate electrons to near light speed

Berkeley – 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.

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.

=46or about 20 minutes on April Fools’ Day, however, Wind recorded the
first data ever from a region of magnetic connection. Last year,
=D8ieroset, 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. =D8ieroset 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.