Quirks & Quarks

The secret of STEVE's glow — understanding the purple pal of the aurora borealis

STEVE’s mauve streaks caused by heating of charged particles high up in the atmosphere

STEVE’s mauve streaks caused by heating of charged particles high up in the atmosphere

Amateur astronomer’s photograph used in the new research. The major structures are two bands of upper atmospheric emissions 160 kilometers above the ground, a mauve arc (STEVE) and green picket fence. (Rocky Raybell)
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Researchers think they have the start of an explanation for the odd celestial phenomenon whimsically named STEVE, that produces a mauve streak in the night sky. They've found that STEVE and its occasional 'picket fence' green sidekick are produced by two different processes.

For the first time, scientists used a combination of satellite data and ground-based photography to come up with a global picture of STEVE events that helps explain what drives the phenomena. The results from the study were published in the journal Geophysical Research Letters

STEVE appears rarely in the sky, but has been observed in Canada's west by sky-watchers. It's a purple streak that runs east-west across they sky, considerably south of the normal aurora borealis.  It was first named after a mysterious phenomenon in the animated movie Over the Hedge but scientists later created a 'backronym' for it: Strong Thermal Emission Velocity Enhancement.

"Whenever the aurora is very active in a process that is called a 'substorm' — when the aurora is dancing in the night sky — about an hour later STEVE is observed," said Bea Gallardo-Lacourt, in conversation with Quirks & Quarks host Bob McDonald. She studied the phenomenon as a postdoctoral scholar at the University of Calgary.

Understanding what is happening in the ionosphere, and the connection that the ionosphere has with the magnetosphere, is crucial for us, but also will have a very high economic and social costs if we don't understand how this connection happens.- Bea Gallardo-Lacourt, University of Calgary

"If STEVE was caused by the aurora, a satellite crossing by the structure would be seeing a rain of charged particles — ion and electrons — producing the structure. We didn't see that when STEVE was present," explained Gallardo-Lacourt. "So that means it's locally created in the ionosphere."

The ionosphere is the very upper part of our atmosphere — the layer where atoms are stripped of their electrons due to solar and cosmic radiation  and become electrically charge ions. And right outside of it is the magnetosphere generated by the Earth's magnetic field. It's the interplay between the two that creates an opportunity for STEVE to shine.

Alberta Aurora Chasers capture STEVE, the new-to-science upper atmospheric phenomenon, on the evening of April 10, 2018 in Prince George, BC. (Ryan Sault)

Gallardo-Lacourt said that figuring out what happens in the ionosphere is important because disturbances in it can disrupt satellite communications.

"Understanding what is happening in the ionosphere, and the connection that the ionosphere has with the magnetosphere, is crucial for us, but also will have a very high economic and social costs if we don't understand how this connection happens."

We can thank the sun for STEVE

"It's plasma and magnetic field from the sun that powers the whole magnetosphere-ionosphere system," said Gallardo-Lacourt.

She described what happens to the system as being like a "pressure shock" that modulates Earth's magnetic field and charges the ionosphere with energy — creating "fast flows" of charged ions in the ionosphere — to locally create STEVE.

"We know that the ions are moving really fast," said Gallardo-Lacourt. "We know they're there, we know they exist — we have never seen them as strong as we see them for STEVE. So we think these ions could collide with neutral [molecules] that are already in the ionosphere producing heating and making the [neutral molecules] glow."

STEVE (Strong Thermal Emission Velocity Enhancement) and the Milky Way at Childs Lake, Manitoba, Canada. The picture is a composite of 11 images stitched together. (Krista Trinder)

This kind of heating that creates STEVE is similar to how electricity in an incandescent light bulb heats the filament until it glows.

Scientists still don't know why the streams are purple in the first place, but that's something they hope to learn in future studies.

A different explanation for the 'picket fence'

As for STEVE's "picket fence," the green sidekick that occasionally shows up along with the mauve streak, data from this study found that it's a unique type of aurora that occurs farther south of the typical dancing aurora we see in northern latitudes.

"The interesting thing is that whenever that picket fence was there, for the first time we could see a conjugate event where you see the same thing happening in the north and in the southern hemisphere," said Gallardo-Lacourt. "That means you have one magnetic field line connecting the two hemispheres, so this picket fence will have a magnetospheric origin."

They think what's happening is that disturbances to the magnetosphere, which envelopes the planet, are causing plasma waves to knock electrons out of the magnetosphere into the ionosphere. It's these electrons scientists think create the striped patterns that look like a picket fence.

"Whenever STEVE was observed together with a picket fence, we saw particles raining down like aurora," said Gallard-Lacourt. "So we did see high energy electrons raining down into the ionosphere."

The THEMIS and SWARM satellites (left and top) observed waves (red squiggles) that power the STEVE atmospheric glow and picket fence (inset), while the DMSP satellite (bottom) detected electron precipitation and a conjugate glowing arc in the southern hemisphere. (Emmanuel Masongsong, UCLA, and Yukitoshi Nishimura, BU/UCLA)

Global view of STEVE

This paper is the first time, scientists have a global image of the aurora oval — that from space  look like rings around the poles — with STEVE.

"What you will see is an extremely narrow [mauve] ribbon in the sky right below the aurora oval where all the bright aurora is produced."

The scientists behind the paper say they hope these and future findings about STEVE will help develop a better understanding of how particles flow in the ionosphere.

"It's like someone reached in from space and drew a line with a purple magic marker across the Earth." 0:30

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