Andrew Yee[_1_]
February 9th 07, 04:56 PM
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.
Gan Marklund from the Alfv 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:
Gan 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 ]
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.
Gan Marklund from the Alfv 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:
Gan 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 ]