For decades, the exact science of how the northern lights form in our night sky has
eluded experts. And we have finally proved just how these dazzling light shows happen.
Auroras appear in the upper atmosphere near the poles and they first were mentioned in
texts thousands of years ago. But it wasn’t until the late 1800’s that
a Norweigian physicist first made the connection between electric currents in Earth’s magnetic
field and auroras in the sky. And it’s only been since the beginning of the 20th century
that scientists have known the basics of how auroras work.
Today, we know that all auroras begin with solar activity. The sun puts out a continuous
stream of charged particles called the solar wind. These energetic particles strike oxygen
and nitrogen molecules in the atmosphere, bumping them up to an excited state.
When they relax, the molecules emit photons that light up the night skies,
producing beautiful auroras.
Occasionally, the sun will burp out a coronal mass ejection or solar flare, sending
electrons slamming into Earth’s magnetic field in what’s called a geomagnetic storm.
They’re responsible for producing the most intense auroras. They disrupt the magnetic
field lines, creating ripples that rebound back toward Earth known as Alfvén waves.
And this is where the mystery starts. For the last 40 years, scientists have hypothesized
that electrons can accelerate from space to Earth... and they do it by “surfing” on
Alfvén waves. Any surfer will tell you that the key to catching a wave is paddling along
with it. If you paddle close to the speed of the wave, you’ll be picked up and accelerated.
The energy transfer from the wave to the electron happens through a phenomenon known as Landau
damping. But up until this point, scientists have struggled
to prove this theory, so they did what scientists do and decided to recreate it in a lab.
To see if electrons were accelerated by the electric field of an Alfvén wave, the team had to scale
the vast distances of space into the confines of a lab.
For this, they turned to UCLA’s Large Plasma Device. This nearly 20-meter-long cylindrical
chamber creates a field of highly charged particles called plasma.
The team also had to develop new instruments and techniques to detect
a very small population of electrons moving within a narrow range of velocities.
The experiment went down like this. The plasma in the chamber forced electrons up and down
the magnetic field. A specially designed antenna sent Alfvén waves
down the chamber. Further down the chamber, the scientists used the two new sensors they
created to measure variations in the electric and magnetic field, and the electrons in the
plasma. And they were right. The data showed a small population of electrons, like we’re
talking less than one in a thousand, surfing on epic Alfvén waves.
So now that this hypothesis has been proven correct, what’s next? The techniques developed
in this study can help scientists better understand other phenomena in space where particles are
energized. Like how the sun's corona is heated to a million degrees, how cosmic rays get
close to the speed of light, or how radiation belts, like the one near Earth, affect satellites
that we depend on for communication and navigation. So next time you take a look at those dancing
lights in the sky, remember the four-decade-long mystery, and how solving it will help us better
understand the vast universe that surrounds us.
If you find the magnetic field that helps produce Earth’s auroras interesting, you
should check out Amanda’s video on Mars’ magnetic field. If there’s a space breakthrough
you’d like to see us cover, let us know in the comments below and as always, thanks
for watching Seeker.