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Science Forum Index » Astro Forum » Cluster -- new insights into the electric circuits of polar
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| Andrew Yee |
 Posted: Fri Feb 09, 2007 12:53 pm |
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ESA News
http://www.esa.int
9 February 2007
Cluster -- new insights into the electric circuits of polar lights
Giant electrical circuits power the magical open-air light show of the
auroras, forming arcs in high-latitude regions like Scandinavia. New
results obtained thanks to ESA's Cluster satellites provide a new insight
into the source of the difference between the two types of electrical
circuits currently known to be associated to the auroral arcs.
The deep mechanisms that rule the creation of the beautiful auroras, or
polar lights, have been the subject of studies that are keeping solar and
plasma scientists busy since years. While early rockets and
ground-observations have already provided a few important clues for the
understanding of these phenomena, the real break-throughs in our knowledge
have started with dedicated auroral satellites, such as S3-3, Dynamics
Explorer, Viking, Freja and FAST, and have now come to full fruition with
ESA's multi-point mission Cluster.
The basic process generating auroras is similar to what happens in an old
TV tube. In the TV tube, accelerated electrons hit the screen and make its
phosphore glow; electrons in the atmosphere get accelerated in an
'acceleration region' situated between about 5000 and 8000 kilometres
altitude, and rush down to the Earth's ionosphere -- a region of the upper
atmosphere. Here, they crash into ionospheric atoms and molecules,
transfer to them some of their energy and so cause them to glow, creating
aurorae.
It is today well established that almost-static electric fields, parallel
to the Earth's magnetic fields, play an important role in the acceleration
of the electrons that cause the auroras to shine. The auroral electric
circuits in the near-Earth space involve almost-static 'electric
potential' structures through which electrons and ions are accelerated in
opposite directions -- towards and away from Earth's atmosphere -- at high
latitudes.
It had been observed that these electric potential structures are mainly
of two types -- symmetric (U-shaped) or asymmetric (S-shaped). In 2004,
Prof. Göran Marklund from the Alfvén Laboratory, at the Royal Institute of
Technology, Stockholm (Sweden), noted that the U-shaped and the S-shaped
structures typically occurred at the boundaries between magnetospheric
regions with different properties.
The former type (U-shaped) was found at a plasma boundary between the
so-called 'central plasma sheet', situated in the magnetotail at
equatorial latitudes, and the 'plasma sheet boundary layer', an adjacent
area located at higher latitudes. The latter type (S-shaped) was found at
the boundary between the 'plasma sheet boundary layer' and the polar cap,
further up in latitude.
Marklund was then in the condition to propose a model to explain this
difference. The model suggested that both the asymmetric and symmetric
shape of the potential structures, observed at the different plasma
boundaries, depended on the specific conditions of the plasma (such as
differences in plasma density) in the two regions surrounding the
boundary. According to the 2001 observations, he concluded that the plasma
conditions at the lower-latitude boundary (where U-shaped structures were
observed) are in general more symmetric, while the ones at the polar cap
boundary (where the S-shaped structures were observed) are more
asymmetric.
However, new Cluster measurements did not seem to be consistent with this
picture. On 1 May 2003, one of the Cluster spacecraft crossed the auroral
arc at high altitude in the Earth's magnetotail. As expected, it detected
the presence of a U-shaped, symmetric potential structure when crossing
the boundary between the 'central plasma sheet' and the 'plasma sheet
boundary layer'. Only 16 minutes later a second Cluster spacecraft, moving
roughly along the same orbit and crossing the same boundary, detected an
asymmetric, S-shaped potential structure, 'typical' of the polar cap
boundary and therefore unexpected in that region.
However, within the 16-minute time frame between the crossing of the two
spacecraft, the plasma density and the associated currents and fluxes of
particles decreased significantly in the plasma sheet boundary layer. In
this way this boundary ended up in resmbling the asymmetric conditions
typical of the polar cap boundary.
So, the scientists interpreted that the 'reconfiguration' from a U-shaped
to a S-shaped potential structure, and of the associated electric circuits
that sustain the auroral arcs, reveal the change in the plasma conditions
on the two sides of the boundary.
The results represent a major step forward in understanding the auroral
electrical circuits, but important questions still remain open, such as:
how do the process that accelerate the electrons to form auroras get
triggered and maintained? Cluster measurements in the 'acceleration' area
to be performed in 2008 and 2009 should help us to find out.
The results, by Marklund et al., were published in the 13 January 2007
issue of the Journal of Geophysical Research.
For more information:
Göran Marklund, Royal Institute of Technology, Stockholm, Sweden
Email: goran.marklund @ ee.kth.se
Philippe Escoubet, ESA Cluster Project Scientist
Email: philippe.escoubet @ esa.int
[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaCP/SEM0EZN2UXE_index_1.html ] |
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