A rare combination of factors might have combined to make a solar storm in March 2015 the strongest seen for a decade.

Credit: ddp USA/REX

Like most such storms, this one began when the Sun spurted fast-moving plasma in an event called a coronal mass ejection. A different part of the Sun then sent out a stream of plasma as 'solar wind'. This wind could have pushed the coronal mass ejection from behind, suggests a team led by Ryuho Kataoka at the National Institute of Polar Research in Tokyo. The whole mass could have then ploughed through space, piling up dense particles from earlier blasts of solar wind ahead of it. The Sun's magnetic field lines also happened to be oriented to drive the storm powerfully towards Earth. On hitting Earth's atmosphere, it sparked aurora (pictured) around the Northern Hemisphere on 17 March.

Geophys. Res. Lett. http://doi.org/5wn (2015)