Space Forum / Astronomy / March 2006

ESA News
http://www.esa.int

30 March 2006

Cluster and Double Star witness a new facet of Earth's magnetic behaviour

Five spacecraft from two ESA missions unexpectedly found themselves
engulfed by waves of electrical and magnetic energy as they travelled
through Earth's night-time shadow on 5 August 2004.

The data collected by the spacecraft are giving scientists an important
clue to the effects of 'space weather' on Earth's magnetic field.

Shortly after 15:34 CEST [1334 UTC], something set the tail of Earth's
natural cloak of magnetism oscillating. "It was like the waves created
by a boat travelling across a lake," says Dr Tielong Zhang of the
Austrian Academy of Sciences, Graz.

Only in this case, the identity of the 'boat' is unknown. It might be
the fast flow of particles often observed in the central part of the
magnetotail. Whatever it was produced waves that travelled from the
centre of the tail to its outer edges.

The five spacecraft caught in this event were the four units of ESA's
Cluster mission and the first unit of the joint CNSA/ESA mission Double
Star. The Cluster quartet fly in formation, passing through Earth's
magnetotail at distances of between 16 and 19 times Earth's radius.

One of the two spacecraft of Double Star, the TC-1 spacecraft, orbits at
between 10 and 13 Earth radii. All five spacecraft are designed to
collect data on the magnetic bubble surrounding our planet, called the
'magnetosphere'.

Earth's magnetic field is generated deep inside the planet and rises
into space where it constantly interacts with the solar wind, a
perpetual stream of electrically charged particles released by the Sun.

The stream pulls Earth's magnetic field into a tail that stretches
behind the planet for tens of thousands of kilometres. Gusts and storms
in the solar wind are known as 'space weather' and can make Earth's
magnetic field quake.

On 5 August 2004, Cluster and Double Star satellites found themselves in
the right place at the right time. The readings showed that the
oscillations took place simultaneously across an area over 30 000 km in
length. This is the first time that the true extent of the oscillations
has been revealed.

Previous Cluster measurements, before the launch of Double Star, could
only reveal the movement across a restricted location surrounded by the
four satellites.

Understanding the way Earth's magnetic field interacts with the solar
wind is the space-age equivalent of a meteorologist investigating the
way a mountain range disturbs airflow, creating weather systems.

In the case of space weather, storms consist of fluctuating magnetic and
electrical fields that can damage satellites and pose health risks to
astronauts. If we are to fully exploit the potential of space, we have
to understand the effects of space weather and be able to predict them.
That's where missions like Cluster and Double Star come in.

"By studying the August oscillations, we may be able to develop a
unifying theory for all the various motions of the magnetotail," says
Zhang, who is heading the investigation into what happened that day.

Note to editors:

The paper 'Double Star/Cluster Observations of Neutral Sheet
Oscillations on 5 August 2004' is published in Annales Geophysicae, 23,
2909–2914, 2005.

The ESA/NASA Cluster mission is the first magnetospheric mission
composed of four spacecraft flying in formation, and was launched in
summer 2000.

The CNSA/ESA Double Star Programme is the first Chinese space science
mission composed of two spacecraft (TC1-1 and TC-2) launched in 2003 and
2004. Their orbits are designed to have good conjunctions with Cluster.
Some of the scientific instruments on both spacecraft have been provided
by Europe and are similar experiments to those on board the four Cluster
spacecraft.

For more information:

Dr T Zhang, lead author
Österreichische Akademie der Wissenschaften (Austrian Academy of Sciences)
Institut für Weltraumforschung, Graz, Austria
E-mail: tielong.zhang @ oeaw.ac.at
Tel: +43 316 4120 552

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

More about...

* This story in depth
  http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=38993
* Double Star overview
  http://www.esa.int/esaSC/120381_index_0_m.html
* Cluster overview
  http://www.esa.int/esaSC/120383_index_0_m.html

Related articles

* Cluster and Double Star see star crack during massive 'starquake'
  http://www.esa.int/esaSC/SEMERY7X9DE_index_0.html
* Second Double Star satellite successfully launched
  http://www.esa.int/esaSC/SEMIQMV4QWD_exploring_0.html
* Surfing and diving in the Earth's magnetosphere, Cluster celebrates
one year of science excellence in orbit
  http://www.esa.int/esaCP/ESAN3D0VMOC_index_0.html
* Cluster's new view of near-Earth space
  http://www.esa.int/esaCP/ESA6VTTM5JC_index_0.html
* Solar storm blasts Cluster
  http://www.esa.int/esaCP/ESAB7E8VTTC_index_0.html
* ESA's Cluster sees 'squashed' magnetosphere
  http://www.esa.int/esaSC/SEMGZ8WLDMD_exploring_0.html
* How the Sun affects us on Earth
  http://www.esa.int/esaSC/SEML7BS1VED_foryou_0.html

Related links

* More on Cluster operations
  http://www.esa.int/spacecraftops/index.html
* ESA's Space Weather pages
  http://www.esa.int/spaceweather

[NOTE: Images supporting this release are available at
http://www.esa.int/esaCP/SEM92N59CLE_index_1.html ]