Cluster Hears The Heartbeat of Magnetic Reconnection

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Cluster hears the heartbeat of magnetic reconnection
European Space Agency
02 May 2013

For the first time, scientists have resolved the detailed
structure of the core region where magnetic reconnection takes
place in the magnetosphere of Earth using unprecedented wave
measurements. The study, based on data from ESA's Cluster mission,
has mapped different types of electrostatic waves in this region.
The waves trace populations of plasma particles that are involved
in the different stages of a magnetic reconnection event.

In most cosmic environments, matter is not made up of neutral
atoms and molecules, but rather of electrically charged particles
and ions. This ionised state of matter, called plasma, is
permeated by electric and magnetic fields caused by local
inhomogeneities in the distribution of particles and ions. These
fields in turn influence the dynamics of the plasma on larger
scales, so the distribution of the particles, ions, and fields
changes constantly.

Magnetic reconnection is ubiquitous in the Universe. The
phenomenon, which occurs in plasma, is triggered by microscopic
processes and causes macroscopic effects: magnetic field lines
from different domains collide and later assume a different
configuration. Magnetic reconnection produces rapid and global
changes to the arrangement of a magnetic environment - for
example, the magnetosphere of Earth. This process is an efficient
mechanism to convert energy stored in the magnetic field to
kinetic energy.

Waves play an important role in the transfer of mass and energy
across different plasma layers. Various types of waves develop
during magnetic reconnection and tracing these waves through in
situ measurements in Earth's magnetosphere is a unique way to
investigate the reconnection process. Scientists have now used
data from ESA's Cluster mission to characterise electrostatic
waves in the tail of the magnetosphere and to 'see' into the heart
of a magnetic reconnection region.

"Most of the action during a magnetic reconnection event takes
place at the thin boundaries that separate different layers of
plasma. For the first time, we were able to see through this thin
boundary and identify the different types of waves that arise
there," says Henrik Viberg from the Swedish Institute of Space
Physics in Uppsala, Sweden. Viberg is a PhD student at Uppsala
University and lead author of the paper, published in Geophysical
Research Letters, reporting the new findings based on data from
Cluster.

Magnetic reconnection starts with two colliding flows of plasma
whose magnetic fields are aligned along opposite directions: when
pushed together, these create a thin sheet of current. As plasma
keeps flowing towards this sheet from both sides, particles are
accelerated and eventually released via two jets. This creates an
X-shaped transition region, with a 'separatrix' region that
divides the inflowing plasma from the outflows of highly energetic
particles.

Viberg and his colleagues searched through the vast data archive
of the Cluster mission for an event during which the spacecraft
crossed the separatrix region during magnetic reconnection, and
during which they were collecting data with the Wide Band Data
(WBD) instrument. By making high-resolution measurements of the
electric and magnetic fields, WBD allows scientists to probe the
structure of the plasma through waves, rather than particles.
Although they found only one suitable event in the archive, the
spacecraft had crossed the transition between inflow and outflow
regions several times during this event, providing enough
statistics for a robust investigation.

"Since electrostatic waves are a local phenomenon and don't
propagate over long distances, they allow us to look very closely
into the magnetic reconnection region," explains Yuri
Khotyaintsev, Viberg's supervisor at the Swedish Institute of
Space Physics.

"The Cluster spacecraft detected waves only in the separatrix
region - not in the inflowing or outflowing plasma - confirming
our earlier suspicions. But there's more, because we have also
resolved, for the first time, the structure of this region, as the
spacecraft saw different types of electrostatic waves while flying
across the separatrix."

Close to the boundary between separatrix and inflow regions, the
scientists identified two types of waves: one type with high
frequencies, the Langmuir waves, and another with low frequencies,
known as Electron-Cyclotron waves. Deeper into the separatrix
region, towards the outflowing plasma, they detected Electrostatic
Solitary Waves - single-pulsed waves that span a very broad
frequency range.

"If we drew a parallel with sound waves, we could associate
Langmuir waves with the high-pitched sound produced by a violin,
while Electron-Cyclotron waves would be closer to the
lower-pitched music from a cello," comments Khotyaintsev. "The
Electrostatic Solitary Waves would be more like the sound of
maracas, consisting of short, individual pulses based on more than
one pitch."

This study provides the first detailed mapping of the types of
waves found throughout the magnetic reconnection region and the
first detection of Electron-Cyclotron waves in such a region.
Resolving the structure of the separatrix region allows scientists
to investigate the mechanisms underlying magnetic reconnection.
Since different types of waves are produced by particles with
different properties, the scientists analysed the correlation
between the populations of particles detected in conjunction with
the various types of waves.

"We find high-energy electrons along with Langmuir waves: this is
consistent with what we believe to be the origin of these waves,
which can be generated by beams of high-energy electrons emerging
from the X-shaped reconnection region. We detected
Electron-Cyclotron waves in the same region, but we were not able
to identify the mechanism that generates them," says Viberg.

"Closer to the outflowing jets, the beam of high-energy electrons
becomes more intense and flows of low-energy electrons streaming
against the beam are also found here. This counter-streaming
distribution is known to give rise to instabilities and,
eventually, to Electrostatic Solitary Waves - which are exactly
the waves we find in these regions," he adds.

In future studies, the scientists plan to investigate if and how
these electrostatic waves, which are confined to the magnetic
reconnection region, might produce electromagnetic waves, able to
propagate over much longer distances. This would allow a
comparison between Earth's magnetic environment and the many
different sites where magnetic reconnection occurs, ranging from
the corona of the Sun, to the accretion discs around forming
stars, to plasma created in the laboratory.

"Working at the peak of its instrumental capabilities, Cluster
has mapped what goes on at the core of the magnetic reconnection
region. This provides an important insight into this fundamental
process that takes place in plasma all across the Universe,"
concludes Matt Taylor, Cluster Project Scientist at ESA.

Notes for editors

The study presented here is based on data gathered by three of the
four Cluster spacecraft (C1, C3 and C4) on 10 September 2001 as
they crossed a magnetic reconnection region in the magnetotail of
Earth's magnetic environment.

Cluster is a constellation of four spacecraft flying in formation
around Earth. It is the first space mission to be able to study,
in three dimensions, the natural physical processes occurring
within and near Earth's magnetosphere. Launched in 2000, it is
composed of four identical spacecraft orbiting the Earth in a
pyramidal configuration, along a nominal polar orbit of 4 × 19.6
Earth radii (1 Earth radius = 6380 km). Cluster's payload consists
of state-of-the-art plasma instrumentation to measure electric and
magnetic fields over a wide frequency range, and key physical
parameters characterizing electrons and ions from energies of
nearly 0 eV to a few MeV. The science operations are coordinated
by the Joint Science Operations Centre (JSOC), at the Rutherford
Appleton Laboratory, United Kingdom, and implemented by ESA's
European Space Operations Centre (ESOC), in Darmstadt, Germany.

Related publications

H. Viberg, et al., "Mapping High-Frequency Waves in the
Reconnection Diffusion Region", 2013, Geophysical Research
Letters, Vol. 40, Pages 1-6. DOI: 10.1002/grl.50227


Contacts

Henrik Viberg
Swedish Institute of Space Physics
and Uppsala University
Uppsala, Sweden
Email: henrik...@irfu.se
Phone: +46-18-4715934

Yuri Khotyaintsev
Swedish Institute of Space Physics
Uppsala, Sweden
Email: yu...@irfu.se
Phone: +46-18-4715929

Matt Taylor
Cluster Project Scientist
Research and Scientific Support Department
Directorate of Science & Robotic Exploration
ESA, The Netherlands
Email: mta...@rssd.esa.int
Phone: +31-71-5658009