The vacuum of space is hardly a suitable habitat for birds, but someone
tuning in to the signals detected by the Wide Band Data (WBD) experiment on
ESA’s Cluster spacecraft might be forgiven for thinking that this was not
the case. During the first few months of Cluster operations, WBD scientists
have been analysing radio signals which consist of narrowband tones that
rise in frequency over a period of a few seconds. This ‘dawn chorus’
resembles the sound of a rookery heard from a distance and is thought
to be generated by high-energy electrons (atomic particles that have a
negative electric charge) trapped in the Earth’s radiation belts.
Although these bird-like squawking signals have been studied for several
decades, scientists still know very little about how the electrons are
accelerated and how the dawn chorus itself is created. Even with Cluster,
there are few opportunities to observe the intense, but localised outbursts.
“The chorus is detected most on the Earth’s morning side, but it’s not clear
why,” said Craig Kletzing of the University of Iowa, a co-investigator on
the WBD team.
“It appears to be generated at the magnetic equator, and it usually occurs
just outside the region of near-Earth space known as the plasmasphere (*),”
he said. “However, on most of their close orbital passes around the Earth,
the four Cluster spacecraft are travelling inside the plasmasphere when
they cross the magnetic equator.”
Fortunately, although the outer edge of the plasmasphere is typically around
26,000 – 32,000 km above the Earth, it does move inward during periods of
strong magnetic activity caused by gusts in the solar wind. At such times,
Cluster has a good chance of detecting the chorus.
“We have succeeded in seeing chorus activity with multiple spacecraft on
several occasions,” said Kletzing. “For example, on 27 November 2000, real-
time WBD data were received from three Cluster spacecraft for 38 minutes.
Strong chorus emissions from about 5 to 8 kHz were detected during the
entire interval around the magnetic equator crossing.”
“Since the signals radiate out in a cone from the source region, we can use
the Cluster spacecraft to study how the radio waves spread from this region
to different locations,” he explained.
“Just like two cars that start from the same place on a grid but travel at
different speeds, so waves of a given frequency move further and further
apart with distance,” said Kletzing. “Near their source, the radio waves
are close together, so we can tie down their origin quite well if the
Cluster spacecraft are also very close together — less than 200 km apart.”
“By measuring the tiny differences in the arrival times of the signals at
each spacecraft — usually of the order of 10 to 20 milliseconds — we can
calculate the direction of movement of the waves and the distance between
them,” he said. “The largest separation we have seen with Cluster is
around 600 km.”
“The Cluster data have confirmed that the chorus begins at the magnetic
equator and that the waves move in opposite directions either side of the
magnetic equator,” he added. “However, there are still some mysteries
concerning the behaviour of these chorus wave packets that we will
continue to study with Cluster.”
The dawn chorus is just one example of the natural phenomena that are being
studied in unprecedented detail with the aid of the four Cluster spacecraft.
Over the next year and a half, WBD and other instruments on Cluster will
revolutionise our knowledge of the physical processes taking place in the
inner reaches of the magnetosphere — the magnetic bubble that surrounds
our Earth.
“These studies will enable us to learn more about how radio waves propagate
and how they are affected by conditions in space,” said Kletzing. “The
more we understand about how radio waves behave in any kind of plasma or
electrified gas, the better we will be able to understand the physics
behind future applications of radio technology.”
(*) The plasmasphere is a doughnut-shaped region above the Earth’s magnetic
equator. It is distinguished by a relatively high density of plasma-electrons
and protons.
For further information contact:
Dr Craig Kletzing
Dept. of Physics & Astronomy
University of Iowa
USA
Tel: +1 319 335 1904
E-mail: craig-kletzing@uiowa.edu
Dr Philippe Escoubet
Cluster Project Scientist
ESTEC
Netherlands
Tel: +31 71 565 3454
E-mail: philippe.escoubet@esa.int
USEFUL LINKS FOR THIS STORY
* Cluster home page
* The instruments on board Cluster
http://sci.esa.int/content/doc/c6/1990_.htm
* Cluster wideband (WBD) plasma wave investigation
http://www-pw.physics.uiowa.edu/plasma-wave/istp/cluster/
* Sound of plasma waves (45 seconds)
http://sci2.esa.int/cluster/sounds/chorus.wav
IMAGE CAPTIONS:
[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&oid=27799&ooid=27803]
Cluster listens to the dawn chorus. Data from two of the Cluster spacecraft.
The vertical axis shows frequency and the horizontal axis shows time. The
colour indicates the relative intensity of waves. The chorus emissions occur
between 7.5 and 9.5 kHz as indicated. Very similar patterns are seen at both
spacecraft.
[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&oid=27799&ooid=28079]
Region of the Earth’s magnetosphere were Cluster encountered the “Whistler”.
[Image 3:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&oid=27799&ooid=27869]
The Cluster spacecraft detected the chorus at their closest approach to Earth
(perigee). At perigee, the spacecraft are following each other, like a string
of perl, along the orbit (red line). Rumba is leading then follow Samba,
Salsa and Tango. The magnetic field lines are shown in red. Chorus is detected
above the equator.
[Image 4:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&oid=27799&ooid=27889]
Chorus emission point.
