Chorus causes aurora

Chorus waves are responsible for the diffuse aurora – a faint glow of light in the upper atmosphere not generally visible to the naked eye.

The waves – so called because their signals detected by ground-based recording equipment sound like a bird’s dawn chorus when played back through a loud speaker – scatter trapped electrons from high up in the Earth’s magnetic field towards the upper atmosphere.

“The breakthrough came when we realised that the electrons being lost from space to the Earth’s atmosphere were leaving a signature, effectively telling a story about how they were being scattered,” said lead author Professor Richard Thorne from the University of California.

Scientists had long understood that the aurora is caused by electrons hitting neutral atoms in the upper atmosphere and have debated whether very low frequency radio waves could cause the scattering of electrons – which are normally trapped much higher in the Earth’s magnetic field – to enable them to reach the upper atmosphere.

Detailed analysis of satellite data enabled an international team – which also included scientists from the University of Sheffield and the British Antarctic Survey (BAS) – to calculate which radio waves were causing the scattering.

 “Our finding is an important one because it will help scientists to understand how the diffuse aurora leads to changes in the chemistry of the Earth’s upper atmosphere, including effects on ozone at high altitude, which may affect temperature right through the atmosphere,” said Professor Richard Horne from the BAS.

Horne says VLF waves will also be included into computer models to help predict space weather, which affects satellites and power grids, the accuracy of GPS navigation and high frequency radio communications with aircraft on polar routes.