Andrew Yee[_1_]
March 13th 07, 07:31 PM
ESA News
http://www.esa.int
12 March 2007
Cluster opens a new window on 'magnetic reconnection' in the near-Earth
space
Plasma physicists have made an unprecedented measurement in their study of
the Earth's magnetic field. Thanks to ESA's Cluster satellites they detected
an electric field thought to be a key element in the process of 'magnetic
reconnection'.
Thanks to these measurements, obtained by the eight PEACE electron sensors
onboard the four spacecraft, scientists now have their first insight into
magnetic reconnection's detailed behaviour.
Magnetic reconnection is a process that can occur almost anywhere that a
magnetic field is found. In a reconnection event, the magnetic field lines
are squeezed together somehow and spontaneously reconfigure themselves. This
releases energy. When it occurs near the surface of the Sun, such an event
powers giant solar flares that can release thousands of millions of tonnes
of electrically charged particles into space.
The Earth's magnetic field creates a buffer zone, the magnetosphere, between
our planet's atmosphere and the particles released during these eruptions.
The Sun also releases a steadier flow of charged particles called the solar
wind. On the large-scale, any heading this way buffet the magnetosphere, and
are deflected by it. Plasma physicists describe this behaviour with a theory
called 'magneto-hydrodynamics' (MHD).
On smaller scales, however, the picture becomes rather more complicated. The
particles can actually flow across the magnetic field lines.This makes the
mathematics of the behaviour more difficult. First to misbehave are the ions
(positively charged particles). These break away from simple MHD on scales
of less than a few hundred kilometres. On even smaller scales, less than 10
kilometres, the electrons (negatively charged particles) begin playing by
other rules, too.
The new Cluster measurements reveal the electric field on the scale of a few
hundred kilometres. "This is the first ever measurement of this term," says
Paul Henderson, from University College London's Mullard Space Science
Laboratory, UK, who led the investigation.
On 17 August 2003, Cluster was flying high above the night-time hemisphere
of the Earth with an average separation of 200 kilometres between
spacecraft. Data from several instruments shows that at 18:00 CET [1700
UTC], a reconnection event took place and swept across the spacecraft.
Using data from Cluster's Plasma Electron and Current Experiment (PEACE)
Henderson and collaborators calculated the pressure of electrons at each
spacecraft and then calculated the difference between them and the variation
with time. Using these quantities they calculated the electric field present
near a reconnection site.
"This is an impossible calculation to make without four spacecraft," says
Henderson. Now that the scientists can calculate the electric field in such
a way, they have a new window into the process of magnetic reconnection.
Magnetic reconnection within Earth's magnetosphere regularly takes place on
the night-time side of our planet, where the flow of the solar wind
stretches out the magnetic field into a long tail. When the field reconnects
in this region, it triggers jets of energetic particles that can cause
auroral lights but can also damage satellites.
This new Cluster result takes scientists a step closer to seeing the precise
details of magnetic reconnection. "When you think that the magnetosphere
stretches over a million kilometres through space, we are actually looking
at a minuscule part of it," says Henderson.
And that's exactly what plasma scientists want -- the microphysics.
Note for editors:
The original paper, "Cluster PEACE observations of electron pressure tensor
divergence in the magnetotail," by P.D. Henderson at el., is published in
Geophysical Research letters (Vol. 33, L22106, doi:10.1029/2006GL027868,
2006).
For more informaton:
Paul Henderson
University College London's Mullard Space Science Laboratory, UK
Email: pdh @ mssl.ucl.ac.uk
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/SEMZN9Q11ZE_index_1.html ]
http://www.esa.int
12 March 2007
Cluster opens a new window on 'magnetic reconnection' in the near-Earth
space
Plasma physicists have made an unprecedented measurement in their study of
the Earth's magnetic field. Thanks to ESA's Cluster satellites they detected
an electric field thought to be a key element in the process of 'magnetic
reconnection'.
Thanks to these measurements, obtained by the eight PEACE electron sensors
onboard the four spacecraft, scientists now have their first insight into
magnetic reconnection's detailed behaviour.
Magnetic reconnection is a process that can occur almost anywhere that a
magnetic field is found. In a reconnection event, the magnetic field lines
are squeezed together somehow and spontaneously reconfigure themselves. This
releases energy. When it occurs near the surface of the Sun, such an event
powers giant solar flares that can release thousands of millions of tonnes
of electrically charged particles into space.
The Earth's magnetic field creates a buffer zone, the magnetosphere, between
our planet's atmosphere and the particles released during these eruptions.
The Sun also releases a steadier flow of charged particles called the solar
wind. On the large-scale, any heading this way buffet the magnetosphere, and
are deflected by it. Plasma physicists describe this behaviour with a theory
called 'magneto-hydrodynamics' (MHD).
On smaller scales, however, the picture becomes rather more complicated. The
particles can actually flow across the magnetic field lines.This makes the
mathematics of the behaviour more difficult. First to misbehave are the ions
(positively charged particles). These break away from simple MHD on scales
of less than a few hundred kilometres. On even smaller scales, less than 10
kilometres, the electrons (negatively charged particles) begin playing by
other rules, too.
The new Cluster measurements reveal the electric field on the scale of a few
hundred kilometres. "This is the first ever measurement of this term," says
Paul Henderson, from University College London's Mullard Space Science
Laboratory, UK, who led the investigation.
On 17 August 2003, Cluster was flying high above the night-time hemisphere
of the Earth with an average separation of 200 kilometres between
spacecraft. Data from several instruments shows that at 18:00 CET [1700
UTC], a reconnection event took place and swept across the spacecraft.
Using data from Cluster's Plasma Electron and Current Experiment (PEACE)
Henderson and collaborators calculated the pressure of electrons at each
spacecraft and then calculated the difference between them and the variation
with time. Using these quantities they calculated the electric field present
near a reconnection site.
"This is an impossible calculation to make without four spacecraft," says
Henderson. Now that the scientists can calculate the electric field in such
a way, they have a new window into the process of magnetic reconnection.
Magnetic reconnection within Earth's magnetosphere regularly takes place on
the night-time side of our planet, where the flow of the solar wind
stretches out the magnetic field into a long tail. When the field reconnects
in this region, it triggers jets of energetic particles that can cause
auroral lights but can also damage satellites.
This new Cluster result takes scientists a step closer to seeing the precise
details of magnetic reconnection. "When you think that the magnetosphere
stretches over a million kilometres through space, we are actually looking
at a minuscule part of it," says Henderson.
And that's exactly what plasma scientists want -- the microphysics.
Note for editors:
The original paper, "Cluster PEACE observations of electron pressure tensor
divergence in the magnetotail," by P.D. Henderson at el., is published in
Geophysical Research letters (Vol. 33, L22106, doi:10.1029/2006GL027868,
2006).
For more informaton:
Paul Henderson
University College London's Mullard Space Science Laboratory, UK
Email: pdh @ mssl.ucl.ac.uk
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/SEMZN9Q11ZE_index_1.html ]