Two belts of trapped high-energy particles, held in place by Earth’s magnetic field, surround Earth: the Van Allen radiation belts. Killer electrons in the outermost belt move close to the speed of light and carry a lot of energy. These can penetrate satellite shielding and cause microscopic lightning strikes that damage and sometimes destroy vital components.
Even before the Space Age, the behaviour of Earth’s magnetic field during solar storms suggested that a variable ring current generated by electrically charged particles from the Sun surrounds Earth. Such particles were indeed found in the Van Allen belts, but it was only some 40 years after their discovery that Cluster was able to make an accurate determination of the permanent ring current around Earth, through its detailed measurement of the magnetic field in this region.
Intense solar activity can disrupt the Van Allen belts, and Cluster made another vital discovery when it was on hand to observe directly the effects of a particularly strong solar shock wave hitting the magnetosphere in 2004.
It saw the creation of killer electrons through what turns out to be a two-step process. The electrons are initially accelerated by the shock wave compressing Earth’s magnetic field. Then Earth’s magnetic lines wobble, creating something like a very large-scale, low frequency laser, which accelerates the electrons even more, up to ‘killer’ energies.
And Cluster showed that this transition, from normal to killer intensities, can happen in just 15 minutes.