| | |  | The Earth's magnetotail - Cluster is currently investigating this region | |
Cluster quartet take a trip down Earth’s tail
17 September 2001
Everyone is familiar with animals' tails,
but less well known is the fact that most planets
have tails too – huge, magnetic tails filled with
electrified gas rather than fur, flesh and muscle.
Since the end of August, ESA's four Cluster
spacecraft have been flying along the middle of the
Earth's magnetotail, carrying out the most in-depth
exploration of this region ever undertaken.
The tail that Cluster
is traversing is part
of the invisible
bubble in space that is created by Earth's magnetic
field. If the vast reaches of space between the
planets were completely empty, then the magnetic
bubble would be shaped something like a round
ball. But Earth is continually bombarded by a
supersonic blast of electrically charged particles -
electrons and protons - from the Sun. This so-called
solar wind sweeps around the magnetic bubble,
shaping it so that it resembles a giant windsock.
Trapped inside the tail is a complex mixture of
particles that have escaped from Earth's upper
atmosphere and from the Sun. Scientists know that
these particles can pick up energy and be
accelerated towards the planet's poles, where they
create the auroras - Northern and Southern Lights.
| | Data from the magnetometers track the passage of Cluster's four spacecraft through the plasma sheet | |
"Exactly what causes these substorms is still
uncertain," said Cluster project scientist, Philippe
Escoubet. "Cluster will give us unique sets of data
from 42 instruments that will help us to unravel what
is happening in the magnetotail."
"This is extremely important," he added, "because
substorms can cause power blackouts and
communication breakdowns on Earth, as well as
beautiful auroras."
Although our Earth's tail has been explored by
numerous spacecraft over the past few decades,
many mysteries remain. This is mainly because the
magnetotail is so large - it stretches at least two
million kilometres into space on the night side of
the Earth. Single spacecraft cannot hope to
discover the secrets of this vast region.
However, by travelling up to 120 000 km along the
magnetotail once every 56 hours, the Cluster
quartet should be able to pin down the physical
processes taking place at its heart. As they fly in a
tetrahedral formation, about 2000 km apart, the
spacecraft will obtain the first detailed,
three-dimensional view of this magnetic
powerhouse.
Meanwhile, Cluster's passage through the mid-tail
region on the Earth's night side caused a small
headache for the spacecraft operations team - a
series of three eclipses, the longest of which lasted
for around four hours, between 30 August and 5
September. With no sunlight falling on their solar
cells while they were in the planet's shadow, ground
controllers decided to conserve battery power by
shutting down all of the instruments except one of
the FGM magnetometers.
| | Artist's impression of ESA's Cluster spacecraft | |
This was the first time that magnetic field data had
been obtained during an eclipse since Cluster
became operational. The measurements show that
the magnetic field dropped erratically in the plasma
sheet - the central region of the tail. There were also
some small variations during the eclipse.
"The smaller disturbances during the eclipse were
almost certainly parts of the tail plasma sheet and
not due to lack of sunlight at the time," said
Professor André Balogh of Imperial College London,
principal investigator for the FGM instruments.
"I'm pleased to say that all of the instruments are
now back online and the spacecraft are in fine
shape," commented Dr. Escoubet after the
completion of Cluster's final eclipse.
The spacecraft will stay in the tail until the end of
October, after which they will turn their attention
once more to other regions of near-Earth space.
However, if all goes according to plan, Cluster
should once more plunge into the elongated
magnetotail for a second prolonged period of
exploration towards the end of 2002.
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