Chirping Chorus Waves Make Auroras Flash And Flicker

Pulsating auroras don’t look like much in still photos — they turn to mush —  but they’re wonderful to see live. Bob King

Viewers in the northern states may have a brush with the aurora tomorrow night (Feb. 25). A minor G1 storm is expected from around 9 p.m. to midnight Central time. Hopefully, the half-moon won’t be shiny enough to erase the arcs or rays we might see.

The aurora takes many forms.  Quiet arcs, bands that break into minions of bright rays, the coronal crown and pulsating patches. The last will be familiar to anyone who’s stuck long enough to watch a bright aurora subside into flashing or blinking patches of light. They throb in and out of view across the northern sky with the rhythm of an irregular heartbeat with periods that range from several to 10 seconds. Called pulsating aurora, they’re more common late at night or before the start of dawn. Easy to see, they’re hard to photograph because they move flash quickly and end up looking like static glows in time exposures. Experiencing them live or through video is the way to go.

The visualization above shows the magnetic field around Earth (called the magnetosphere) as it might look from space. This view is conceptual, but based on real science observations. The orange and blue lines depict the opposite north and south poles of Earth’s magnetic field lines. The magnetosphere is squeezed on the right side from the wind of particles arriving from the sun and more extended on the night side (left) opposite the sun. Greg Shirah and Tom Bridgman, NASA/Goddard Space Flight Center

Scientists have measured the flickers and they range in size from tens to hundreds of miles across and appear at altitudes of around 60 miles (100 km). As to their cause, we may now have a better idea after research done by Satoshi Kasahara (University of Tokyo) and his team and reported in the February 15 issue of Nature.

We’re all familiar with precipitation in the form of snow, rain and the like. Electrons, the tiny, energetic that whizz around the centers of atoms and power our TVs and  microwaves, can precipitate too. When they arrive in blasts of strong winds from the sun, they can “rain” into the atmosphere from Earth’s magnetic envelope, collide with the atoms and molecules in the upper air and create the aurora. That envelope is called the magnetosphere and traces the reaches of our planet’s magnetic field similar to how iron filings trace out the invisible magnetic field lines around a magnet.

Kasahara used data from JAXA’s (Japan Aerospace Exploration Agency) ERG spacecraft taken in March 2017 to show that the pulsations likely arise from the interaction between those high-flying electrons and electromagnetic waves called whistler-mode chorus. I know it sounds like gobbledygook but stay with me.

This animation shows how many auroras originate. A high speed wind of electrons and protons from the sun (at left) links into Earth’s magnetic field, gets pushed back to the nightside, folds over and reconnects. The reconnection causes energy to be rapidly released along the field lines causing the auroras to brighten.
NASA/Goddard Space Flight Center- Conceptual Image Lab

Electrons and protons, mostly from the sun, get inside Earth’s magnetosphere. As they cycle about our planet’s magnetic domain, riding up and down magnetic field lines, they can produce different kinds of radio waves that we can actually hear using a special VLF (Very Low Frequency) radio receiver like this one. Among them are whistler-mode chorus waves that sounds eerily like peeping frogs or singing birds. To hear them, click the red button on the sound file above.

Chorus waves (whorls) interact with electrons to send them in bursts into the upper atmosphere to create the auroral pulsations. ERG science team

Kasahara found that electrons can interact with chorus waves and careen out of the magnetosphere into the upper atmosphere, where they generate flashes of auroral light — the pulsating aurora. Imagine it as a series of short-lived, electron hurricanes that blast down along our planet’s magnetic field lines to make the air flash and glow with frenetic intensity. Although scientists have long suspected a connection between the chirpy chorus waves and electrons, this is the first hard evidence that it happens.

Origin of the pulsating aurora. ERG science team

Now that you know, maybe it will pique your interest to go in for some really late night aurora-watching. If you purchase a receiver and listen while you watch, you’ll experience both the rarefied vastness of the magnetosphere and its tiny inhabitants.