ESA Science & Technology11-Jul-2005 15:01:28
 

Science Results

Characteristics of dawn Chorus

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 of the time on the Earth's morning side, but it's not clear why. 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. The multi-spacecraft observations with Cluster have shown for the first time that the sources of Chorus is relatively small (around hundred kilometers) and traveling at very high speed (around 20 000 kms-1). The speed of the source of chorus is comparable with the speed of energetic electrons trapped in the radiation belts, confirming the fact that these electrons are involved in the emission of chorus.

Chorus emission detected by two Cluster spacecraft. Cluster 1 observations are shown in the top-right panel and Cluster 2 in the top-right and bottom-left panels. The Cluster 2 chorus panels are shifted in time (0.2 s) and frequency (2 kHz) when compared to Cluster 1. These shifts give the information about the speed and the size of the Chorus source. The bottom-right panel presents the location of the Chorus emission and the satellites (from U. Inan, 2003).



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