In August and September 1859, there was a major geomagnetic storm–aka, the Carrington Event, the largest ever recorded–that generated dazzling auroras visible throughout the US, Europe, Japan, and Australia. Scientists have long been fascinated with the inherent physical processes contributing to such displays, although the simple mechanism is understood, our understanding remains incomplete. Based on a new paper published in the journal Nature Communications, electrons in the Earth’s ionosphere catch a plasma wave in order to accelerate toward Earth with sufficient energy to produce the brightest types of auroras.
The spectacular kaleidoscopic effects of the so-called northern lights (or southern lights if they’re in the Southern Hemisphere) are the result of charged particles from the Sun being dumped in the Earth’s magnetosphere, in which they collide with oxygen and nitrogen molecules–an interaction that arouses those molecules and makes them glow. Auroras generally present as shimmering ribbons at the skies, with green, purple, blue, black, and yellow hues. The lights tend to only be visible in polar areas because the particles follow the Earth’s magnetic field lines, which fan out of the vicinity of the poles.
There are different sorts of auroral displays, such as”diffuse” auroras (a faint glow near the horizon), rarer”picket fence” and”dune” displays, along with”discrete aurora arcs”–the very intense variety, which appear in the sky as shimmering, undulating curtains of light. Discrete aurora arcs may be so bright, it’s possible to read a newspaper by their own light. (Astronomers have concluded that the occurrence that earned the moniker STEVE (Strong Thermal Emission Velocity Enhancement) many decades back isn’t a genuine aurora after all, as it is due to charged particles warming up high in the ionosphere.) Scientists believe there are distinct mechanisms where precipitating particles are accelerated to create each kind.
One of the unanswered questions is exactly how electrons become accelerated before colliding with the ionosphere. Physicists from the University of Iowa, Wheaton College, University of California, Los Angeles (UCLA), and the Space Science Institute in Los Angeles were eager to explore the mechanism supporting discrete auroral arcs in particular. One of the suggested concepts is that the electrons become hastened due to so-called Alfvén waves travel Earthward.
Alfvén waves appear in plasma, a fourth state of matter that has similar properties to gases and fluids, but also contains magnetic (and occasionally electrical ) fields. They were first hypothesized in 1942 from the Swedish plasma physicist Hannes Alfvén and have since been observed in both space-based and terrestrial plasmas. Under certain circumstances, Alfvén waves may exchange energy with particles in the plasma, sometimes trapping them in the troughs of the waves. It’s been suggested that Alfvén waves are responsible for the acceleration of precipitating particles which ultimately contribute to different aurora arcs.
According into the authors, the theory goes something like this. Solar flares and coronal mass ejections can trigger powerful geomagnetic storms. These storms in turn can cause the magnetic field lines from the Southern and Northern Hemispheres to split and reform (magnetic reconnection), before snapping back toward Earth like a stretched rubber band. That rebounding launches Alfvén waves, which travel toward Earth along the magnetic field lines, accelerating along the way to up to 35,000 km/s (nearly 80 million miles ), thanks to the increasing power of Earth’s magnetic field.
Meanwhile, the electrons trapped in Earth’s magnetosphere are decreasing in thermal speed. At an altitude below 20,000 km (or 12,000 kilometers ), the Alfvén waves will be moving just a bit faster than the electrons’ thermal speed. This enables electrons traveling in the same direction to “surf” the Alfvén waves. Any surfer can inform you the secret to catching a wave would be to paddle until your board’s rate matches that of an incoming tide; otherwise the wave will only shoot right past, leaving you bobbing forlornly behind on your surfboard, watching everyone else have all the fun. The electrons do essentially the exact same thing.
As energy is transferred from waves into electrons, those electrons speed to 20,000 km/s (or 45 million miles ) before colliding with molecules in the thin air of the upper atmosphere, making a different aurora arc. It is a phenomenon called Landau damping, following the Soviet physicist Lev Landau who described it in 1946. The result is also vital for equilibrium in particle accelerators, because it suppresses any undesirable motions from particle beams interacting with their surroundings via electromagnetic wakefields.
There is already some evidence in support of the theory from observations of Alfvén waves moving Earthward over auroras made throughout the flight of sounding rockets and certain spacecraft missions. However a definitive measurement for both Alfvén waves along with the accelerated electrons was still lacking. So the team decided to run a series of experiments in the Large Plasma Device (LPD) at UCLA’s Basic Plasma Science Facility, which creates plasmas effective at supportin