Aurora borealis and australis, also known as the northern and southern lights, have mesmerized humans for millennia. Ancient people could only speculate about their source, often attributing the colorful displays to departed souls or other celestial spirits. Scientists have only recently revealed the basics of how auroras work, but they hadn't been able to directly observe a key part of that process — until now.
In a new study, published in the journal Nature, an international team of researchers describe the first direct observation of the mechanism behind pulsating auroras. And while they didn't exactly find spirits dancing in the sky, their report of whistling chorus waves and "frolicking" electrons are still pretty amazing.
Auroras begin with charged particles from the sun, which can be released both in a steady stream called solar wind and in huge eruptions known as coronal mass ejections (CMEs). Some of this solar material may reach Earth after a couple days, where the charged particles and magnetic fields trigger the release of other particles already trapped in Earth's magnetosphere. As these particles rain into the upper atmosphere, they spark reactions with certain gases, causing them to emit light.
The different colors of auroras depend on the gases involved and how high they are in the atmosphere. Oxygen glows greenish-yellow at about 60 miles high and red at higher altitudes, for example, while nitrogen emits blue or reddish-purple light.
Auroras come in a variety of styles, from faint sheets of light to vibrant, undulating ribbons. The new study focuses on pulsating auroras, blinking patches of light that appear roughly 100 kilometers (about 60 miles) above the Earth's surface at high latitudes in both hemispheres. "These storms are characterized by auroral brightening from dusk to midnight," the study's authors write, "followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn."
This process is driven by a "global reconfiguration in the magnetosphere," they explain. Electrons in the magnetosphere normally bounce along the geomagnetic field, but a specific kind of plasma waves — spooky-sounding "chorus waves" — seem to make them rain into the upper atmosphere. These falling electrons then spark the light displays we call auroras, although some researchers have questioned whether chorus waves are powerful enough to coax this reaction from electrons.
The new observations suggest they are, according to Satoshi Kasahara, a planetary scientist at the University of Tokyo and lead author of the study. "We, for the first time, directly observed scattering of electrons by chorus waves generating particle precipitation into the Earth's atmosphere," Kasahara says in a statement. "The precipitating electron flux was sufficiently intense to generate pulsating aurora."
Scientists hadn't been able to directly observe this electron scattering (or "electron frolic," as it's described in the press release) because conventional sensors can't identify the precipitating electrons in a crowd. So Kasahara and his colleagues made their own specialized electron sensor, designed to detect the precise interactions of auroral electrons driven by chorus waves. That sensor is aboard the Arase spacecraft, which was launched by the Japan Aerospace Exploration Agency (JAXA) in 2016.
The researchers also released the animation below to illustrate the process:
The process described in this study probably isn't limited to our planet, the researchers add. It may also apply to the aurora of Jupiter and Saturn, where chorus waves have also been detected, as well as other magnetized objects in space.
There are practical reasons for scientists to investigate auroras, since the geomagnetic storms that spark them can also interfere with communications, navigation and other electrical systems on Earth. But even if there weren't, we'd still share our ancestors' instinctive curiosity about these seemingly magical lights.