A team of physicists has just completed a decades-long quest to experimentally demonstrate the physical mechanism of auroras, the shimmering curtains of light that glow above the poles. Moreover, they were able to do it in the comfort of a university lab – rather than having to travel a long way to see an aurora.
We’ve long known that these incredible lightshows are formed by energetic particles from the Sun. These fast-moving particles (mostly electrons) are funnelled down along magnetic field lines and slam into our atmosphere at high latitudes, colliding with oxygen and nitrogen atoms and boosting them up to an excited state. As the atoms “relax” again, they release energy in the form of flashes of coloured light.
But how are these electrons accelerated to high speeds before colliding with the Earth?
This new study, published in Nature Communications, shows that the answer lies in powerful electromagnetic waves called “Alfven waves”, created during geomagnetic storms.
Co-author of the study, physicist Greg Howes from the University of Iowa, US, explains: “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.”
Previously, spacecraft have observed Alfven waves travelling towards Earth during aurora events, but the research team has now been able to confirm these observations with lab-based experiments. These were conducted at the Large Plasma Device (LPD) in the Basic Plasma Science Facility at the University of California, Los Angeles, in the US.
Specifically, the experiments looked at a small population of electrons moving down the LPD chamber at nearly the same speed as the Alfven waves, showing that they can “surf” on the electric field of a wave.
The theory behind this is known as Landau damping; the experimental results now align with the predicted signature from theory, as well as agreeing with simulations.
“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 another co-author, Craig Kletzing also from the University of Iowa.
“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.”