The aurora borealis, or more widely known as Northern Lights, paint pitch-black skies in high-latitude regions with vibrant colours, enchanting people for millennia.
It might be surprising to know that while the phenomenon has been observed since ancient times, we actually don’t have conclusive proof as to how they form.
But a team of brilliant physicists from UCLA, Wheaton College, the University of Iowa and the Space Science Institute have finally identified the definitive formation mechanisms to demystify Mother Nature’s spectacular light show.
Last piece of the aurora puzzle.
Their study, published in the journal Nature Communications, underpins the formation of aurorae by powerful electromagnetic waves during geomagnetic storms.
Scientists know that energised particles emanating from the sun—part of the “solar wind”—travel down the Earth’s magnetic field and collide with atmospheric nitrogen and oxygen molecules, releasing lots of energy to form a colourful glowing halo around the poles.
But what’s not clear is how these groups of electrons accelerate through the magnetic field on the last leg of their journey downwards, reaching speeds of up to 72 million kilometres per hour.
Here’s where the physicists come in to solve the final piece of the puzzle. They found out that the electrons hitch a ride on Alfvén Waves—a type of electromagnetic wave that spacecraft have regularly pin-pointed travelling Earthward along with magnetic fields above aurorae.
More importantly, they’ve proven this theory experimentally, which was extremely difficult to achieve previously due to many limitations inherent to spacecraft measurements.
‘Surfing’ particles.
To conduct this groundbreaking experiment, the physicists turned to the Large Plasma Device at UCLA’s Basic Plasma Science Facility.
They reproduced conditions that imitated those in Earth’s auroral magnetosphere and then launched Alfvén Waves down the plasma device’s chamber.
“This challenging experiment required a measurement of the very small population of electrons moving down the chamber at nearly the same speed as the Alfvén waves, numbering less than one in a thousand of the electrons in the plasma,” said Dr Troy Carter, the director of the UCLA Plasma Science and Technology Institute.
“Measurements revealed this small population of electrons undergoes ‘resonant acceleration’ by the Alfvén Waves’ electric field, similar to a surfer catching a wave and being continually accelerated as the surfer moves along with the wave,” explained Dr Greg Howes, one of the study’s co-authors and an associate professor at the Department of Physics and Astronomy at Iowa.
Electrons (yellow) stream towards Earth surfing on Alfvén waves (blue lines), colliding with nitrogen and oxygen molecules (white). In upper altitudes, the collisions emit red light, while those in lower altitudes emit green light. Photo credit: Austin Montelius/University of Iowa
Alfvén waves appear following geomagnetic storms, causing “magnetic reconnection”, which stretches, snaps and reconnects Earth’s magnetic field lines. These shifts then launch the waves along the lines towards Earth. During the storm, electrons go ‘surfing’ on the waves along different field lines, eventually leading to the shimmering glow of the aurora’s curtains of light.
In physics, the phenomenon of electrons ‘surfing’ on an electrical field is known as Landau damping, where the energy of the wave is transferred to the accelerated particles. Through an innovative combination of numerical simulations and mathematical modelling, the researchers were able to demonstrate that their experimental results agreed with the predicted signature for Landau damping.
“The agreement of experiment, simulation and modelling provides the first direct test showing that Alfvén waves can produce accelerated electrons that cause the aurora,” said Dr Carter.
While the science behind this study is captivating, it also closes the curtain on a painstaking quest that lasted decades to fully understand these mesmerising lights.
It truly is humbling to think that deciphering these dancing lights took scientists much time and effort, highlighting the vastness of knowledge that we have yet to uncover.
