Have you ever heard of a space tsunami? Neither had we, but apparently they are common and are the forces responsible for making the Earth's aurorae dance.

These magnetic substorms, as they are known, are complex events that take place at altitudes from 100km to 150,000km. ESA says trying to understand such phenomena with a single spacecraft is like trying to predict the behaviour of a tsunami with one buoy in an ocean.

Now data* from the Cluster constellation of four satellites is shedding new light on how these so-called substorms form and how they interact with the Earth's magnetic field.

The image above shows how a substorm can affect an otherwise calm aurora, as seen in the left-hand picture. The centre and right pictures show two different kinds of disruption.

There are two competing explanations for the behaviour of substorms: the "Current-Disruption" model, and the "Near Earth Neutral Line Model". The data from Cluster seem to support the former, ESA says, but the researchers are not in a hurry to throw out the alternative explanations, saying it is unclear how generally the findings should apply.

You can read up on the phenomenon in more detail here. We really just wanted to show you the pretty pictures. ®

*In a paper called Cluster observation of plasma flow reversal in the magnetotail during a substorm, published in Annales Geophysicae, 9 August, 2006.

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