The Northern Lights have fascinated people for thousands of years, but how they are created has not been proven until now, scientists say.
In a new study, physicists report evidence that the most brilliant auroras are produced by powerful electromagnetic waves during geomagnetic storms.
The phenomena, known as Alfven waves, accelerate electrons toward Earth, causing the particles to produce the light show which fills the sky in high-latitude regions.
A research team, led by the University of Iowa, say the findings conclude a decades-long quest to demonstrate experimentally the physical mechanisms for the acceleration of the electrons in this way.
Greg Howes, associate professor in the department of physics and astronomy at Iowa and study co-author, said: “Measurements revealed this small population of electrons undergoes ‘resonant acceleration’ by the Alfven wave’s electric field, similar to a surfer catching a wave and being continually accelerated as the surfer moves along with the wave.”
For some time scientists have known that energised particles which emanate from the Sun – such as electrons racing at approximately 45 million miles per hour – precipitate along the Earth’s magnetic field lines into the upper atmosphere.
Here they collide with oxygen and nitrogen molecules, kicking them into an excited state.
These excited molecules relax by emitting light, producing the colourful hues of the aurora.
The theory was supported by spacecraft missions that frequently found Alfven waves traveling towards Earth above auroras.
The physicists found confirmatory evidence in a series of experiments conducted at the Large Plasma Device (LPD) in UCLA’s Basic Plasma Science Facility, a national collaborative research facility supported jointly by the US Department of Energy and National Science Foundation.
Craig Kletzing, professor in the department of physics and astronomy at Iowa and a study co-author, said: “The idea that these waves can energise the electrons that create the aurora goes back more than four decades, but this is the first time we’ve been able to confirm definitively that it works.
“These experiments let us make the key measurements that show that the space measurements and theory do, indeed, explain a major way in which the aurora are created.”
The phenomenon of electrons surfing on the electric field of a wave is a theoretical process known as Landau damping, first proposed by Russian physicist Lev Landau in 1946.
Researchers demonstrated that the results of their experiment agreed with the predicted signature for Landau damping using numerical simulations and mathematical modelling.
– The findings are published in the Nature Communications journal.
