The auroras that are the northern lights have captured the imagination of people for thousands of years. While theories explaining how they came about exist, until now, no one has been able to provide conclusive evidence for these theories.
For some time, scientists have thought that auroras are produced by powerful electromagnetic waves during geomagnetic storms known as Alfven waves. These waves accelerate electrons towards the Earth’s magnetic field lines in the upper atmosphere where they collide with oxygen and nitrogen molecules. This causes the electrons to enter an excitatory state, after which they relax and emit the lights we know today as auroras.
This theory has been supported by spacecraft missions that have frequently found Alfven waves travelling towards Earth above auroras. But while space-based measurements confirmed this theory, limitations in spacecraft and rocket measurements meant that until now, no definitive test could be conducted to prove that this is how auroras exist.
Now, however, in their new study, researchers were able to conduct a series of experiments at the Large Plasma Device (LPD) in UCLA's Basic Plasma Science Facility. Via their numerical simulations and mathematical models, they were able to show that the results of their experiment agreed with the predicted signature for Landau damping, the phenomenon of electrons ‘surfing’ on the electric field of a wave.
"This challenging experiment required a measurement of the very small population of electrons moving down the LPD chamber at nearly the same speed as the Alfven waves, numbering less than one in a thousand of the electrons in the plasma," says Troy Carter, one of the authors behind the study.
"The idea that these waves can energize 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," says Craig Kletzing, co-author of the study.
"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."
Sources: EurekAlert, Nature