In August and September 1859, there was a major geomagnetic storm – the Carrington event, the largest ever recorded – that produced dazzling aurora borealis visible throughout the United States, Europe, Japan, and Australia. Scientists have long been fascinated by the underlying physical processes that give rise to such screens, but while the basic mechanism is understood, our understanding is still incomplete. According to a new paper published in the journal Nature Communications, electrons in the Earth’s ionosphere capture a plasma wave to accelerate toward Earth with enough energy to produce the brightest types of auroras.
The spectacular kaleidoscopic effects of the so-called northern lights (or southern lights, if they are in the southern hemisphere) are the result of charged particles from the sun dumped into the earth’s magnetosphere, where they collide with oxygen and nitrogen molecules – an interaction that excites these molecules and makes them shine. Auroras are typically found as shiny ribbons in the sky with shades of green, purple, blue and yellow. The lights tend to be visible only in polar regions because the particles follow the earth’s magnetic field lines, which blow out from near the poles.
There are different kinds of auroral exhibits, such as “diffuse” auroras (a faint glow near the horizon), less frequent “fence” and “dune” displays, and “discrete aurora arches” – the most intense variety displayed in the sky as shiny, undulating light curtains . Discrete aurora arcs can be so bright that it is possible to read a newspaper by their light. (Astronomers have concluded that the phenomenon that served the moniker STEVE (Strong Thermal Emission Velocity Enhancement) several years ago is not a true aurora, as it is caused by charged particles that heat up high in the ionosphere.) Scientists believe that are different mechanisms by which precipitating particles are accelerated to produce each type.
One of the unanswered questions is exactly how electrons are accelerated before colliding with the ionosphere. Physicists from the University of Iowa, Wheaton College, the University of California, Los Angeles (UCLA) and the Space Science Institute in Los Angeles were eager to explore the mechanism behind especially auroral arcs. Among the proposed theories is that the electrons are accelerated due to so-called Alfvén waves driving the earth.
Alfvén waves occur in plasma, a fourth state of matter that has similar properties to liquids and gases, but which also contains magnetic (and sometimes electric) fields. They were first assumed in 1942 by the Swedish plasma physicist Hannes Alfvén and have since been observed in both space-based and terrestrial plasmas. Under certain conditions, Alfvén waves can exchange energy with particles in the plasma and sometimes trap them in the troughs of the waves. It has been suggested that Alfvén waves are responsible for the acceleration of precipitating particles, which ultimately give rise to discrete aurora arcs.
According to the authors, the theory goes like this. Sunbeams and coronal mass emissions can trigger strong geomagnetic storms. These storms can again cause the magnetic field lines from the southern and northern hemispheres to break and reform (magnetic reconnection) before slamming back against the ground like a stretched elastic band. This return launches Alfvén waves that move toward the Earth along the magnetic field lines and accelerate along the way to as much as 35,000 km / s (almost 80 million mph) thanks to the increasing strength of the Earth’s magnetic field.
Meanwhile, the electrons trapped in the Earth’s magnetosphere fall at thermal speed. At an altitude below 20,000 km (or 12,000 miles), the Alfvén waves will travel slightly faster than the thermal speed of the electrons. This allows electrons traveling in the same direction to “surf” the Alfvén waves. Any surfer can tell you that the trick to catching a wave is to paddle until the speed of your table equals the speed of an incoming wave; Otherwise, the wave just shoots right past, leaving you bubbling abandoned at the back of your surfboard and watching everyone else have fun. The electrons do pretty much the same thing.
When energy is transferred from waves to electrons, these electrons accelerate up to 20,000 km / s (or 45 million km / h) before colliding with atoms in the thin air of the upper atmosphere and producing a discrete aurora arc. It is a phenomenon known as Landau attenuation after the Soviet physicist Lev Landau, who first described it theoretically in 1946. The effect is also essential for stability in particle accelerators, as it suppresses unwanted movements from particle beams that interact with their surroundings via electromagnetic wakefields.
There is already some evidence to support this theory from observations of Alfvén waves moving the earth over auroras made during the flight of sounding rockets and certain spacecraft missions. But there was still a lack of a final measurement for both Alfvén waves and the accelerated electrons. So the team decided to conduct a series of experiments on the Large Plasma Device (LPD) at UCLA’s Basic Plasma Science Facility, which creates plasmas capable of supporting Alfvén waves – similar to plasmas in space, albeit on a smaller, terrestrial scale.
It was a deterrent challenge as they needed to measure a very small population of electrons as they descended the LPD chamber at close to the same speed as the Alfvén waves. So physicists had to develop a number of new instruments and techniques – not only a device that was sensitive enough to measure a few electrons, but also a powerful antenna to start Alfvén waves with the right properties to be able to accelerate these electrons. They also had to figure out how to combine measurements of electrons and electric fields to get a unique signature for this acceleration.
All electrons in the plasma created within the experimental chamber moved at a series of speeds, but less than one in a thousand moved down the chamber at almost the same speed as the Alfvén waves. And as predicted, “Measurements revealed that this small population of electrons undergoes ‘resonant acceleration’ of the Alfvén wave’s electric field, similar to a surfer who catches a wave and is constantly accelerated as the surfer moves along with the wave,” said co-author Greg Howes, a physicist at the University of Iowa. The experimental results matched their predicted signature for the damping effect.
“The idea that these waves can provide energy to the electrons that create the aurora goes back more than four decades, but this is the first time we have been able to finally confirm that it works,” said co-author Craig Kletzing, also a physicist. . at the University of Iowa. “These experiments allow us to make the key measurements that show that the space measurements and the theory actually explain an important way in which the aurora is created.”
DOI: Nature Communications, 2021. 10.1038 / s41467-021-23377-5 (About DOIs).