For the first time the acceleration of electrons in Earth’s magnetosphere, which generate the colouful aurorae that glisten in colourful curtains over the polar regions, has been witnessed in action. Dr Colin Forsyth of University College London’s Mullard Space Science Laboratory (MSSL) today presented new results from the ESA/NASA Cluster spacecraft at the Royal Astronomical Society’s National Astronomy Meeting at the University of Glasgow, which promise to provide a new means by which to monitor aurorae.

The four identical Cluster spacecraft, which provide different viewpoints of the magnetosphere to allow scientists to build up three-dimensional representations of the region 50,000 kilometres above our heads, detected the electrons using an instrument they all share called PEACE, or the Plasma Electron And Currents Experiment, that was built at MSSL. PEACE has the ability to measure the velocity of electrons with energies up to 30 kiloelectronvolts. As the electrons, which originate on the solar wind, build up speed, looping around on Earth’s magnetic field lines towards the poles, they strike atmospheric molecules, causing them to glow as aurorae.

When two of the Cluster spacecraft – Cluster 1 and Cluster 3 – found themselves crossing the same magnetic field line, but with one of the spacecraft a 1,000 kilometres higher, they directly measured electrons being accelerated along the field line towards Earth for the first time.

“The Cluster spacecraft had been manoeuvred such that one of them was at a higher altitude than the others when they passed over the auroral regions,” says Forsyth, who led the research. “We were able to measure the particle energies at different heights and thus their acceleration. These exciting new results will give us new insight into the accelerating processes and the transfer of energy from the magnetosphere into the atmosphere.”