Settling decades of scientific debate, researchers from UCLA and the British Antarctic Survey have discovered the final link between electrons trapped in space and the glow of light from the upper atmosphere known as the diffuse aurora.
The finding will help scientists better understand space weather and how space storms affect the Earth's atmosphere from the top down, with potential benefits for the satellite, power-grid and aviation industries, the researchers said.
Their research appears Oct. 21 in the journal Nature.
Scientists have long known that the diffuse aurora is caused by electrons striking the Earth's upper atmosphere. The question has long been how these electrons reach the atmosphere, since electrons are normally trapped much higher up in the Earth's magnetic field through a long chain of events starting with the sun.
Since the 1970s, scientists have debated whether very low-frequency (VLF) radio waves could be responsible for scattering the trapped electrons into the atmosphere. Two types of VLF waves were identified in space as the possible cause of the diffuse aurora, but despite years of argument and research, no conclusive result had been reached.
The new research shows, "without doubt, that VLF waves known as 'chorus' are responsible, so-called since the signals detected by ground-based recording equipment sound like the bird's dawn chorus when played back through a loud speaker," said the Nature paper's lead author, Richard Thorne, a UCLA professor of atmospheric and oceanic sciences.
Chorus waves are very low-frequency radio waves that come from space and are first detected on the ground.
Through detailed analysis of satellite data, Thorne and his colleagues were able to calculate the effects on the trapped electrons and identify which radio waves were causing the scattering.
"The breakthrough came when we realized 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," Thorne said. "We could then analyze our satellite data on the two types of VLF waves, and by running calculations on them — including the rate at which the electrons were being lost to the Earth's atmosphere — we could clearly see that chorus waves were the cause of 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 co-author Professor Richard Horne of the British Antarctic Survey. "We are also including the VLF waves in computer models to help predict 'space weather,' which affects not only satellites and power grids, but also the accuracy of GPS navigation and high-frequency radio communications with aircraft on polar routes."
The diffuse aurora is not the same as the discrete aurora, also known as the northern and southern lights. The discrete aurora looks like fiery moving curtains of colorful light and can be seen by the unaided eye, while the diffuse aurora is much fainter but more extensive. The diffuse aurora, which typically accounts for three-quarters of the energy input into the upper atmosphere at night, varies according to the season and the 11-year solar cycle.
The research was funded by the National Science Foundation and by the United Kingdom's Natural Environment Research Council.
Co-authors are Binbin Ni, a researcher at UCLA; Xin Tao, a postdoctoral scholar in Thorne's laboratory of UCLA; and Nigel Meredith of the British Antarctic Survey.
Images are available at ftp://ftp.nerc-bas.ac.uk/pub/photo/Nature-diffuse-aurora/ and should be credited to British Antarctic Survey.
The British Antarctic Survey (BAS), a component of the Natural Environment Research Council, delivers world-leading interdisciplinary research in the polar regions. The BAS has more than 450 staff and operates five research stations, two Royal research ships and five aircraft in and around Antarctica.
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