Cluster tunes in to the dawn chorus
09 Aug 2001The 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
Dr Philippe Escoubet