Making heaven in a lab: Scientists solve the mystery of the aurora
Testing miniature versions of the mechanisms that fuel the aurora sheds light on the phenomenon. By
Think of the aurora — those lustrous shows of light pouring across polar night skies — as nature’s answer to preposterous party streamers. Displayed as sweeping greens, reds and yellows, these phenomena shoot through the heavens when air molecules and the Sun’s charged particles collide above our extreme northern and southern latitudes. Especially to Scandinavia’s first settlers – and to humanity since – the aurora has persisted as the stuff of untrammelled mystery.
“The lights flare up again, tongues of flame that writhe and lick the heights of the sky,” muses Georgina Harding’s hero in her 2007 novel, The Solitude of Thomas Cave, as the whaler-turned-environmentalist tries to outlast a polar winter, alone, in 17th-century Svalbard. Those lights “melt away, and in the moonlight she is no longer there”, Thomas Cave observes, as he hallucinates his dead lover into being.
Now, by reproducing scaled, miniature versions of the mechanisms said to fuel the aurora, scientists say they have demonstrated the fundamentals underpinning this thing of atmospheric wonder.
A chink in the matrix
The new results, published on Monday 7 June in Nature Communications and replicated by a team of US scientists in a lab, were first predicted back in the 1940s by the work of Soviet physicist Lev Landau, winner of the 1962 Nobel Prize in Physics. Although not shown in action, his theory suggested that geomagnetic storm waves would speed up solar electrons to such an extent that their impact with atmospheric molecules would explode into the distinct curtains of light we call the aurora.
But this rippling kaleidoscope is just the encore — a chink in the matrix of what actually happens thousands of kilometres above Earth’s surface in the magnetosphere, that region of space forged by the dance between our magnetic field and the solar wind.
Up there, according to established theories, the aurora’s makings lie along the trajectory of a cavalry of electrons, charging towards Earth on an electric field associated with “Alfvén waves”. Named after 1970 Nobel prizewinner and Swedish plasma physicist Hannes Alfvén, these waves are triggered by rebounding magnetic field lines — stirred up, in turn, by geomagnetic storms raging over Earth. This violent weather, also called solar storms, is the product of solar reactions such as flares and coronal mass ejections disturbing the flow of the Sun’s wind as it powers through space.
When electrons are surfing in the same direction as Alfvén waves, these waves’ energy is transferred to the speeding electrons through a process known as “Landau Damping”. Now, picture these intrepid little electrons living their best lives as they barrel through the magnetosphere at up to 20,000km/s.
As Alfvén waves hasten towards Earth, the growing strength of our planet’s magnetic field accelerates the waves from typical speeds of 5,000km/s up to almost 35,000km/s. The Alfvén wave momentum sends the electrons crashing into nitrogen and oxygen molecules in the upper atmosphere, where, during the orgiastic encore, the molecules emit a light show so tantalising they manifest as a burst of distinct curtain calls. Or so the theory goes.
Sounding rocket flights and spacecraft missions have provided evidence that Alfvéns speed up auroral electrons – even so, such measurements have been limited, the new paper’s authors note, thwarting confirmation of this theory up to now. The real decades-long, if slightly bizarre, questions were thus: Just how are these particles accelerated from space towards Earth? And how does one replicate the magnetosphere in a lab?
Someone please call the LAPD
The researchers’ answer to the frustrations of the past? Doing scaled laboratory experiments in a giant cylinder vacuum called the Large Plasma Device (LAPD), based at the Basic Plasma Science Facility at the University of California, Los Angeles. The University of Iowa, Wheaton College and the Space Science Institute also collaborated on this research.
Here, their idea was to recreate conditions mirroring those of Earth’s auroral magnetosphere above the poles: researchers needed to measure just a small population of electrons charging down the chamber at almost the same speed as the Alfvén waves (a bit like measuring only the fastest surfers paddling to catch an ocean wave).
To calculate their measurements, the team invented, tested, developed and refined precision instruments, such as a new type of electromagnetic probe, and a high-power antenna for launching Alfvén waves. They also exploited a recently developed field-particle technique.
Next, the team launched the Alfvéns through the LAPD, a one-metre-diameter cylinder vacuum spanning about 20m (more than double the length of an old London Routemaster bus). Wrapped in water-cooled electrical coils that can generate a force about 3,500 times stronger than Earth’s magnetic field in Los Angeles, the chamber was fired up with a plasma heated to an electron temperature of some 45,000°C.
Finally, by combining measurements of the Alfvéns’ electric field as well as the electrons, researchers say this challenging exercise reproduced the holy grail of aurora results — that is, a “unique signature” of electron acceleration by Landau Damping. The signature was further supported by numerical simulations and analytical modelling. This, they announced during a press conference this week, provided the first direct test that Alfvén waves do unleash fast, aurora-creating electrons.
Appealing to our sense of awe
The findings supplied “an important piece of the puzzle”, argued Wheaton College’s Jim Schroeder, an author on the study.
“Alfvén waves are present above a large fraction of auroras, especially the bright and active auroras that occur during geomagnetic storms. Being able to say definitively that electrons are accelerated towards Earth in these conditions by surfing Alfvén waves helps us understand these brilliant auroral displays,” Schroeder said. “It’s a result that appeals to our sense of awe and wonder; our eyes have been drawn upwards by northern and southern lights for millennia.”
Schroeder said that “understanding the physics of near-Earth space is practical too” as geomagnetic storms and the aurora could “adversely impact” this “region of space, heavily populated with satellites” for communication and navigation.
According to principal investigator Gregory Howes, of the University of Iowa, “the project required the development of specialised equipment and techniques over a number of years to show finally that Alfvén waves can accelerate electrons above the aurora.
“After reproducing conditions in space above the aurora, our collaboration launched a large Alfvén wave through the machine and, after a tense wait while processing our measurements, we were thrilled to see we had finally succeeded in measuring the acceleration of the electrons as they surf on Alfvén waves.”
Reacting to the findings, Vyacheslav Lukin, US National Science Foundation programme director for plasma physics, noted the “experimental confirmation” of the aurora physics was due to the researchers’ “persistent ingenuity”.
The results were “exciting”, added Michael Hahn, a research scientist at Columbia University’s astrophysics laboratory.
“Showing how Alfvén waves accelerate electrons to form the aurora are a great example of how the interplay between observational and laboratory astrophysics can advance astronomy,” Hahn said. “Making measurements directly in the magnetosphere is difficult. By reproducing similar conditions in the laboratory we can put the physics under a microscope and understand in detail what is going on.”
After a tense wait while processing our measurements, we were thrilled
to see we had finally succeeded in measuring the acceleration of the electrons as they surf
on Alfvén waves