The four Cluster spacecraft flying in the outer boundary of the magnetosphere
In this image, the four Cluster spacecraft are flying in tetrahedral formation in the vicinity of the magnetopause - the outer boundary of the magnetosphere. Here, one spacecraft (right) is still inside the magnetic field, one (centre) is at the undulating magnetopause and the other two have passed into interplanetary space.
Events such as this will take place on a regular basis over the next two years. By comparing data from each member of the Cluster quartet, scientists are gathering the first detailed, three-dimensional views of the most important regions in the magnetosphere.
See also the "Waves on the magnetopause" animation, illustrating how the four Cluster spacecraft can measure different parts of the waves as they move through the magnetopause.