Aurora borealis or northern lights

A team of scientists has resolved one of the final mysteries surrounding the origin of the spectacularly colourful aurora borealis or northern lights that fills the sky in high-latitude regions. The study is published in the journal Nature Communications. A popular theory has been that electrons hitch a ride on Alfvén waves, a type of electromagnetic wave that spacecraft have frequently identified traveling Earthward along magnetic field lines above auroras. While space-based research has provided strong support for the theory, limitations inherent to spacecraft measurements have prevented a definitive test. To overcome these limitations, the physicists conducted laboratory experiments on the Large Plasma Device at UCLA’s Basic Plasma Science Facility. After reproducing conditions that mimicked those in Earth’s auroral magnetosphere, the team used specially designed instruments to launch Alfvén waves down the plasma device’s 20-meter–long chamber. Because Alfvén waves are thought to collect only a small portion of electrons in the plasma of space, the physicists focused on determining whether there were electrons that appeared to be traveling at a rate comparable to the electric field of the waves.

“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 Troy Carter, a professor of physics and director of the UCLA Plasma Science and Technology Institute.

Howes noted that these Alfvén waves appear following geomagnetic storms, space-based phenomena triggered by violent events on the sun, such as solar flares and coronal mass ejections. These storms can cause what is known as “magnetic reconnection” in the Earth’s magnetic field, in which magnetic field lines are stretched like rubber bands, snap and then reconnect. These shifts launch Alfvén waves along the lines toward Earth. Because regions of magnetic reconnection shift during a storm, the Alfvén waves and their accompanying surfing electrons travel along different field lines over that time period, ultimately leading to the shimmering glow of the aurora’s curtains of light, Carter said. The agreement of experiment, simulation and modeling provides the first direct test showing that Alfvén waves can produce accelerated electrons that cause the aurora.

Source: UCLA news release