Heavenly chorus is cause of diffuse aurora
21 October 2010, by Tamera Jones
Scientists have discovered how low-energy electrons get from space into the Earth's atmosphere to create a type of aurora similar to the northern and southern lights, settling decades of debate.
The aurora borealis or northern lights.
The findings bring researchers closer to forecasting the sort of space weather that could fry satellites, knock out power grids and throw navigation systems into chaos.
As well as the aurora borealis (the northern lights) and the aurora australis (southern lights) there's another type which scientists call diffuse auroras. Like the northern and southern lights, diffuse auroras are caused by electrons from space raining down on atoms high up in the Earth's atmosphere, in what's known as auroral precipitation.
'Diffuse auroras are like drizzle, whereas the discrete aurora [the northern and southern lights] are more like a tropical storm,' Dr Nigel Meredith from the British Antarctic Survey explains. Meredith is an expert in space physics and co-authored this latest study, published in Nature.
Discrete auroras tend to be bright and occur, as their name suggests, in a distinct region of the sky. Diffuse auroras are much fainter and can cover the whole sky.
'When they hit satellites, they can cause massive damage: they can embed into satellites' electronics, disrupt the circuitry, and in some cases cause the complete loss of a satellite.'
Professor Richard Horne, British Antarctic Survey
Although scientists have long known that diffuse auroras are caused by low-energy electrons hitting atoms anywhere from 100 to 300 km up in the Earth's atmosphere, they weren't sure how these particles get there. This is because electrons normally get trapped high above the atmosphere, in the Earth's magnetic field.
Previous scientists had got as far as figuring out that two types of very low frequency (VLF) radio waves could scatter the electrons into our atmosphere. But decades of research hadn't resolved which wave was the most dominant. That is, until now.
'We knew the radio waves responsible would be either electron cyclotron harmonic (ECH) waves or chorus waves,' says Meredith. Chorus waves are so-called because when the signals recorded by ground-based equipment are played back through a loudspeaker, they sound like birds' dawn chorus.
Researchers from the University of California and the British Antarctic Survey decided that if they joined forces they might be able to figure out which type of radio waves scatter the electrons.
This is because BAS scientists had a developed a large database of satellite data showing the intensities of the waves and where they occur, while researchers from California and BAS had jointly developed computer models to calculate the effects of the waves on the electrons.
It turns out that when the electrons leave space and get into the Earth's atmosphere, they leave behind a telltale signature, which shows how they were scattered.
'We found that only chorus waves could cause particles to come down into the atmosphere at the rate and signature we saw. The other waves can contribute, but they're a lot slower than the chorus waves,' explains Meredith.
Chorus waves also accelerate electrons in space, giving them the potential to do serious physical damage.
'When they hit satellites, they can cause massive damage: they can embed into satellites' electronics, disrupt the circuitry, and in some cases cause the complete loss of a satellite,' explains Professor Richard Horne from the British Antarctic Survey and co-author of the Nature paper.
Satellite damage is just one aspect of space weather. It can disrupt electrical power networks on the ground, communications with aircraft on polar routes, and GPS navigation signals.
'As these systems have developed they have become more vulnerable to space weather,' explains Horne.
'If we can predict space weather, as we do the normal weather, we can give operators the information they need to protect their systems and avoid them being damaged,' he adds.
The ultimate aim is to forecast space weather more accurately. Indeed in 2012, the US will launch two satellites into the so-called outer radiation belt above the Earth to help answer questions about space weather.
Richard M. Thorne, Binbin Ni, Xin Tao, Richard B. Horne & Nigel P. Meredith, Scattering by chorus waves as the dominant cause of diffuse auroral precipitation, Nature, published 21 October 2010, doi:10.1038/nature09467
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