Northern lights: How 'black' auroras actually work

About 10 years ago, Tomas Karlsson started to notice something strange in the data being sent down from the ESA Cluster project's four satellites, which have been orbiting Earth since 2000, collecting data on electric and magnetic fields and particle density.

"I noticed a weird combination of electrical and magnetic field measurements that were different from normal, and I wanted to understand the physics behind the data," says Karlsson, a researcher in Space and Plasma Physics at Sweden's KTH Royal Institute of Technology.

"On each occasion, the Cluster spacecraft were flying over the night-time auroral region."

Then in 2012, Karlsson teamed up with mathematics theorists from the UK's University of Dundee and St Andrew's University to crack the mystery. What they learned opens up a whole new understanding of how dark recesses are formed inside Earth's aurora borealis.

"For the first time we are able to reproduce the phenomenon of the black aurora and in particular what happens at its heart, where strong electric fields are present," Karlsson says. "We hope that this will lead to a better understanding of the interaction between the upper atmosphere and the space environment."

Karlsson worked with Alexander Russell, a post-doctoral Research Fellow in the Department of Mathematics, University of Dundee, UK, and Andrew Wright, a researcher at the School of Mathematics and Statistics in University of St Andrews. He also credits colleague Göran Marklund, a professor of space plasma physics at KTH who has made other important contributions in the field and has studied black auroras with Karlsson for years.

"This is a very nice example of how theorists and experimenters can make much greater progress when they work together instead of separately," said Karlsson. Their paper was published in the Journal of Geophysical Research -- Space Physics.

Auroras happen when highly energetic electrons bombard the earth's atmosphere. The currents follow the lines of magnetic force generated by the earth's core and flow through the magnetosphere -- the invisible bubble of magnetic fields surrounding the planet.

When these electrons meet with atoms of oxygen and nitrogen about 90 to 300 km above Earth, they form colourful beams that are visible mainly from the regions closest to Earth's poles.

Karlsson says the researchers found that inside these colourful displays, another process is happening that creates the black regions.

"Whereas ordinary aurora are associated with downward-flowing electrons bombarding the atmosphere, the black ones are associated with electrons being sucked out from the atmospheres into space," he says. "This leaves deep cavities in the upper, electrically conducting-atmosphere, known as the ionosphere."

According to the researchers' model, if the downward current intensifies, it can cause a large number of electrons to move upward into the magnetosphere, thus depleting the ionosphere and creating a density cavity.

Wright, who was lead author of the study, says that the currents flowing into the ionosphere are carried by waves that propagate along magnetic field lines.

"This a key feature of our theory," he says. "The depleted density and electrical conductivity in a black aurora substantially modify the wave reflected from the ionosphere, producing signatures in the magnetosphere like the unusual Cluster observations."

Karlsson says the finding is significant in a number of ways. "In general, the strong electric currents associated with the auroral electrons induce currents on the surface of Earth that can disturb power grids, and other technological systems.

"For example, changes in the electron content of the ionosphere can significantly reduce the accuracy of GPS signals. But improved modeling of the ionosphere can be used to make the necessary corrections," he says.

"Finally, there is some speculation that phenomena associated with the aurora may even affect the climate, via changing the way clouds form," he says.