Pulsating aurorae

A pulsating aurora is a type of aurora that occurs in patches that blink on and off in an almost periodic fashion. They usually arise in the closing phase of an auroral display, at dawn, and cover up to several hundred kilometres of the sky, at an altitude of about 100 kilometres. Many such patches sometimes cover the entire sky. The pulsations arise from intermittent injections of energetic electrons into the upper atmosphere, but just how the injections happen has been unclear because of instrumental limitations on the observations. Satoshi Kasahara and colleagues report observations that show that the energetic electrons are quasiperiodically scattered by 'chorus waves'—intense electromagnetic plasma waves that arise at the magnetic equator and move towards the poles—at the same time as pulsating aurorae are seen from the ground.