Cluster spies reformed shock wave

Twenty years ago, physicist Vladimir Krasnoselskikh predicted that the Earth's bow shock wave, formed by the solar wind building up against our magnetosphere, would break and reform, just as waves on the ocean do.

Now, researchers using the European Space Agency's (ESA) Cluster constellation think they have seen the phenomenon in action.

The Earth's bow shock wave is analogous to the water pushed out in front of a boat as it moves. The earth is the boat, and the water is the solar wind. When the charged particles of the solar wind hit the magnetosphere, they slow down, and a charged barrier builds up at around a quarter of the distance to the moon; the shock wave.

In January 2001, the Cluster spacecraft were flying near the region of the shock wave in a tetrahedron formation, separated by roughly 600km. The researchers expected that since the craft were so close to one another, their readings would be almost identical as they approached the bow shock.

Instead, the readings were very different, showing large fluctuations in the magnetic and electric field surrounding each satellite. The data also showed "marked variations in the number of solar wind protons that were reflected by the shock and streaming back to Sun", ESA says.

According to Vasili Lobzin of the Centre National de la Recherche Scientifique in France, who headed this study, the readings are "the first convincing evidence in favour of the shock reformation model".

The discovery will be useful for researchers studying bow shock around distant celestial objects, where the phenomenon is also caused by exploding stars and the strong hot stellar winds from very young stars. Reforming bow shocks can also accelerate particles to extremely high energies and throw them across space.

Krasnoselskikh says the discovery of bow shock reformation here in our own solar system "is a unique opportunity to study distant astrophysical objects in the kind of detail not available in any laboratory". ®

For those who like to keep track of these things, the work was published in the paper, Nonstationarity and reformation of high-Mach-number quasiperpendicular shocks: Cluster observations, by V V Lobzin et al. published on 9 March 2007 in the Geophysical Research Letters. ®