Cluster helps to protect astronauts and satellites against 'killer electrons' (Forwarded)

ESA News
http://www.esa.int

22 December 2005

Cluster helps to protect astronauts and satellites against 'killer
electrons'

ESA's Cluster mission has revealed a new creation mechanism of 'killer
electrons' -- highly energetic electrons that are responsible for damaging
satellites and posing a serious hazard to astronauts.

Over the past five years, a series of discoveries by the multi-spacecraft
Cluster mission have significantly enhanced our knowledge of how, where
and under which conditions these killer electrons are created in Earth's
magnetosphere.

Early satellite measurements in the 1950s revealed the existence of two
permanent rings of energetic particles around Earth.

Usually called the 'Van Allen radiation belts', they are filled with
particles trapped by Earth's magnetic field. Observations showed that the
inner belt contains a fairly stable population of protons, while the outer
belt is mainly composed of electrons in a more variable quantity.

Some of the outer belt electrons can be accelerated to very high energies,
and it is these 'killer electrons' that can penetrate thick shielding and
damage sensitive satellite electronics. This intense radiation environment
is also a threat to astronauts.

For a long time scientists have been trying to explain why the number of
charged particles inside the belts vary so much. Our major breakthrough
came when two rare space storms occurred almost back-to-back in October
and November 2003.

During the storms, part of the Van Allen radiation belt was drained of
electrons and then reformed much closer to the Earth in a region usually
thought to be relatively safe for satellites.

When the radiation belts reformed they did not increase according to a
long-held theory of particle acceleration, called 'radial diffusion'.
Radial diffusion theory treats Earth's magnetic field lines as being like
elastic bands.

If the bands are plucked, they wobble. If they wobble at the same rate as
the particles drifting around the Earth then the particles can be driven
across the magnetic field and accelerated. This process is driven by solar
activity.

Instead, a team of European and American scientists led by Dr Richard
Horne of the British Antarctic Survey, Oxford, UK, used data from Cluster
and ground receivers in Antarctica to show that very low frequency waves
can cause the particle acceleration and intensify the belts.

These waves, named 'chorus', are natural electromagnetic emissions in the
audio frequency range. They consist of discrete elements of short duration
(less than one second) that sound like the chorus of birds singing at
sunrise. These waves are among the most intense in the outer
magnetosphere.

The number of 'killer electrons' can increase by a factor of a thousand at
the peak of a magnetic storm and in the following days. Intense solar
activity can also push the outer belt much closer to Earth, therefore
subjecting lower altitude satellites to a much harsher environment than
they were designed for.

The radial diffusion theory is still valid in some geophysical conditions.
Before this discovery, some scientists thought that chorus emissions were
not sufficiently efficient to account for the reformation of the outer
radiation belt. What Cluster has revealed is that in certain highly
disturbed geophysical conditions, chorus emissions are sufficient.

Thanks to the unique multipoint measurements capability of Cluster, the
characteristic dimensions of these chorus source regions have been
estimated for the first time.

Typical dimensions have been found to be a few hundred kilometres in the
direction perpendicular to the Earth's magnetic field and a few thousands
of kilometres in the direction parallel to this.

However, the dimensions found so far are based on case studies. "Under
disturbed magnetospheric conditions, the chorus source regions form long
and narrow spaghetti-like objects. The question now is whether those very
low perpendicular scales are a general property of the chorus mechanism,
or just a special case of the analysed observations," said Ondrej
Santolík, of Charles University, Prague, Czech Republic, and main author
of this result.

Due to our increased reliance on space based technologies and
communications, the understanding of how, under which conditions and where
these killer electrons are created, especially during magnetic storm
periods, is of great iimportance.

For more information:

Philippe Escoubet, ESA Cluster Project Scientist
E-mail: philippe.escoubet @ esa.int

Richard Horne, British Antarctic Survey
Tel: +44 1223 221 542

Umran Inan, Stanford University, California, USA
Tel: +1 650 723 4994

Ondrej Santolík, Charles University, Prague, Czech Republic
Tel: +420 221 912 302

More about...

* This story in depth
http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=38475
* Cluster fact***
http://www.esa.int/esaSC/SEMYN5T1VED_index_0.html

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Related links

* ESA's Space Weather pages
http://www.esa.int/spaceweather

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/esaCP/SEMEHH8A9HE_index_1.html]
Artist's impression of the ESA Cluster mission, with four spacecraft
flying in formation above Earth.

Credits: ESA

[Image 2:
http://www.esa.int/esaCP/SEMEHH8A9HE_index_1.html#subhead1]
The magnetosphere -- a natural protective bubble. This image shows the
main regions of the magnetosphere which are being studied by Cluster.


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