Magnetic reconnection in Earth's magnetosphere. Credit: ESA/ATG medialab. (Click here for further details about this animation.)
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.
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.
The magnetic reconnection region in the tail of Earth's magnetosphere
Credit: ESA/ATG medialab
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."
Different types of waves in the magnetic reconnection region: Electron-Cyclotron waves are represented in cyan,
Langmuir waves in blue and Electrostatic Solitary Waves in white. Credit: ESA/ATG medialab
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
Henrik Viberg
Swedish Institute of Space Physics
and Uppsala University
Uppsala, Sweden
Email: henrik.viberg@irfu.se
Phone: +46-18-4715934
Yuri Khotyaintsev
Swedish Institute of Space Physics
Uppsala, Sweden
Email: yuri@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: mtaylor@rssd.esa.int
Phone: +31-71-5658009
