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Cluster sees tsunamis in space
04/18/2007
From http://www.physorg.com
Cluster is providing new insights
into the working of a ‘space tsunami’ that plays a role in
disrupting the calm and beautiful aurora, or northern lights,
creating patterns of auroral dances in the sky.
Generally seen in high-latitude regions such as Scandinavia or
Canada, aurorae are colourful curtains of light that appear in the
sky. Caused by the interaction of high-energy particles brought by
the solar wind, with Earth’s magnetic field, they appear in many
different shapes.
Early in the evening, the aurora
often forms a motionless green arc that stretches across the sky in
the east-west direction. Colourful dancing auroral forms are the
results of disturbances known as ‘substorms’ taking place in Earth’s
magnetosphere. These perturbations can affect our daily lives, in
particular by affecting the reception of GPS signals. Thus,
understanding the physical processes involved is important to our
routine life and security.
These substorms typically last one to two hours and are
three-dimensional physical phenomena spread over altitudes from 100
to 150 000 kilometres. Trying to understand such complex physical
processes with a single scientific spacecraft is like trying to
predict the behaviour of a tsunami with a single buoy in an ocean.
That is why, the simultaneous use of several satellites like the
Cluster constellation is necessary to understand these events.
Currently, there are two competing theoretical models to describe
these substorms or space tsunamis. The first one is called the
‘Current-Disruption’ model, while the second one is the ‘Near Earth
Neutral Line Model’. Using data from the four Cluster spacecraft, a
group of scientists from both sides of the Atlantic were able to
confirm that the behaviour of some substorms is consistent with the
Current Disruption model.
A substorm develops and builds up in different stages, and it is the
detailed study of one of these stages that helps us to understand
which of the two models apply. For example, in the late stage of
substorm development, auroral disturbances move towards the poles,
suggesting that the energy source for auroras and substorms moves
away from Earth.
Previous satellite observations have found that, during this late
stage, the flows of plasma (a gas of charged particles populating
Earth’s magnetosphere) in the magnetotail exhibit a reversal in
direction. In recent years it was generally thought that a flow
reversal region is where magnetic reconnection takes place, that is
where the energy of the magnetic field is converted into particle
energy (dissipation effect), resulting in high-speed plasma flows
that hurl towards Earth, like space tsunamis.
Detailed analysis of data obtained by the Cluster satellites while
crossing such a region in the magnetotail, where flows of plasma
exhibit a reversal in direction, has been reported by the team of Dr
Tony Lui, a scientist of the Applied Physics Laboratory at the John
Hopkins University, Maryland, USA, Co-Investigator of the Research
with Adaptive Particle Imaging Detectors (RAPID) high-energy
particles experiment on Cluster, and lead author of the study.
Thanks to the unique capability of Cluster to perform simultaneous
multipoint measurements, the scientists were able to derive several
physical parameters never before estimated for such a flow reversal
region.
By comparing the directions of the
electric current and the electric field in the magnetosphere it is
possible to understand whether the cause of the flow reversal is a
dissipation effect (where magnetic field energy converted to
particle energy) or a dynamo effect (where particle energy is
converted to magnetic field energy). For this case study, the
Cluster scientists observed that features associated with flow
reversal are actually very complex, consisting of both dissipation
and dynamo effects in localised sites.
This result shows that the plasma turbulence disrupts the local
electric current. "The features we observed are consistent with the
current disruption model. However, it is unclear how general these
findings are. More events will be examined in the future," said Dr
Lui.
"The magnetic substorm phenomenon is a hot topic of research," added
Philippe Escoubet, Cluster and Double Star project scientist for ESA.
"This new Cluster result will certainly contribute to the on-going
scientific debate and foster research cooperation with scientists
involved in the newly launched NASA Time History of Events and
Macroscale Interactions during Substorms (THEMIS), a mission
specifically dedicated to studying substorms."
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