Little Green Cannonballs of Light

Nov. 22, 2020: Just when you thought STEVE couldn’t get any weirder. A new paper published in the journal AGU Advances reveals that the luminous purple ribbon we call “STEVE” is often accompanied by green cannonballs of light that streak through the atmosphere at 1000 mph.

“Citizen scientists have been photographing these green streaks for years,” says Joshua Semeter of Boston University, lead author of the study. “Now we’re beginning to understand what they are.”

STEVE is a recent discovery. It looks like an aurora, but it is not. The purple glow is caused by hot (3000 °C) rivers of gas flowing through Earth’s magnetosphere faster than 13,000 mph. This distinguishes it from auroras, which are ignited by energetic particles raining down from space. Canadian aurora watchers first called attention to the phenomenon about 10 years ago, whimsically naming it STEVE; researchers have been studying it ever since.

There’s a dawning realization that STEVE is more than just a purple ribbon. Photographers often catch it flowing over a sequence of vertical pillars known as the “picket fence.” They’re not auroras either. And, now, Semeter’s team has identified yet another curiosity in their paper, entitled “The Mysterious Green Streaks Below STEVE.”

“Beneath the picket fence, photographers often catch little horizontal streaks of green light,” explains Semeter. “This is what we studied in our paper.”

Semeter’s team gathered pictures of the streaks taken by citizen scientists in Canada, the United States and New Zealand. In some cases, the same streaks were photographed by widely-separated photographers, allowing a triangulation of their position. Analyzing dozens of high-quality images, the researchers came to these conclusions:

1. The streaks are not streaks. They are actually point-like balls of gas moving horizontally through the sky. In photos, the ‘green cannonballs’ are smeared into streaks by the exposure time of the cameras.

2. The cannonballs are typically 350 meters wide, and located about 105 km above Earth’s surface.

3. The color of the cannonballs is pure green–much moreso than ordinary green auroras, reinforcing the conclusion that they are different phenomena.

Above: The pure green of STEVE’s cannonballs (upper left) is compared to the blue-green and other mixed colors of auroras. Credit: Joshua Semeter, Boston University

So, what are the cannonballs? Semeter believes they are a sign of turbulence. “During strong geomagnetic storms, the plasma river that gives rise to STEVE flows at extreme supersonic velocities. Turbulent eddies and whirls dump some of their energy into the green cannonballs.”

This idea may explain their pure color. Auroras tend to be a mixture of hues caused by energetic particles raining down through the upper atmosphere. The ‘rain’ strikes atoms, ions, and molecules of oxygen and nitrogen over a wide range of altitudes. A hodge-podge of color naturally results from this chaotic process. STEVE’s cannonballs, on the other hand, are monochromatic. Local turbulence excites only oxygen atoms in a relatively small volume of space, producing a pure green at 557.7 nm; there is no mixture.

“It all seems to fit together, but we still have a lot to learn,” says Semeter. “Advancing this physics will benefit greatly from the continued involvement of citizen scientists.”

If you’re an aurora photographer looking to contribute, be sure to read Semeter et al’s original research at https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020AV000183.

One thought on “Little Green Cannonballs of Light

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s