Jueves 3 de Abril de 2008, Ip nš 224

First unchanging 'soliton' wave found in space
Por Stephen Battersby

An unusual electrical disturbance has been spotted in space, travelling unchanged through the ionised gas surrounding Earth. A European space mission called Cluster detected a "soliton" wave, a phenomenon similar to the self-contained solitons that can travel along optical fibres and channels of water on Earth. This is the first known soliton in space.

The wave registered as a change in the electrical fields measured by the four spacecraft of the Cluster mission. They orbit Earth, monitoring nearby particles and fields, especially within the magnetosphere, the zone in which Earth's magnetic field is the dominant force.

According to an analysis of Cluster data from 2002, the soliton wave started about 50,000 kilometres from Earth, and travelled towards the planet at about 8 kilometres per second. The authors think it was generated by turbulence in the magnetopause, the edge of the magnetosphere.

So, in a nutshell, what's a soliton?

A soliton is a single, well-defined wave that can travel for a long distance holding the same shape. In water, for example, a soliton can be shaped like a hump or a kink, where the level of water suddenly rises or drops.

How is that different from an ordinary wave?

A localised hump-shaped wave will usually spread out and slump; a soliton is just the right shape to avoid doing that.

An undulating wave of a single, pure wavelength can also travel unchangingly, but most real waves are more complicated than that. An isolated, hump-shaped wave surrounded by flat water, for example, is actually made up of components of many wavelengths. In water, the longer wavelengths usually travel faster than the shorter wavelengths, so they soon get out of step, and the initial wave spreads out.

The shape of the wave also matters, however. In a soliton, the shape is precisely tuned to slow down those impatient longer wavelengths and speed up the sluggish shorter wavelengths, so that they stay in exact step.

(Or, in geek speak, the Fourier decomposition of a soliton is such that linear dispersion is balanced by nonlinear effects.)

When were they discovered?

In August 1834, by John Scott Russell. As he was riding beside the Union Canal near Edinburgh, Scotland, he noticed a strange wave building up at the bow of a boat (scroll down for image). After the boat stopped, the wave travelled on, "assuming the form of a large solitary elevation, a rounded, smooth and well-defined heap of water, which continued its course along the channel apparently without change of form or diminution of speed".

Where else do solitons occur?

In optical fibres, solitons can travel for thousands of kilometres, and can be used to carry information. They might have some fundamental significance in particle physics.

There are also soliton waves within the ocean and the atmosphere, within magnets and super-cooled devices called Josephson junctions, and now, apparently, within the ionised plasma of space.

  18/03/2008. New Scientist Magazine.