A mysterious "non-aurora" called STEVE illuminates the purple sky and we finally know how

The mysterious purple skyglow named STEVE was unmasked last year. Physicists have proved that STEVE was not a new type of dawn – in fact, it's not at all a dawn. But that still left a big gaping question: what exactly is it?

Now we have the answer. Unlike the charged particles of the solar wind that rain in the ionosphere and stimulate the atoms to produce auroras, STEVE is created by a river of charged particles crossing the terrestrial ionosphere.

This phenomenon has been known for decades by photographers and aurora hunters who gave it the nickname "steve"; but scientists only knew it in 2016. They officially dubbed it STEVE (Strong Thermal Emission Velocity Enhancement), and initially determined that it was all over the world. 39, a type of aurora.

It appears as long incandescent purple ribbons that run down into the sky, sometimes accompanied by a ribbon of green striped glow, called "palisade".

Now, using satellite data and ground-based photographs of the phenomenon, space physicists have located the source region of STEVE in the space and identified the mechanisms of both STEVE and the fence.

The palisade is caused by a mechanism similar to the one that causes auroras, although much closer to the equator than auroras.

STEVE and the palisade. (Rocky Raybell)

For STEVE, however, the flow of charged particles into the ionosphere creates friction; in turn, it generates heat that emerges in the form of a beautiful purple glow. This occurs much higher in the atmosphere.

"Aurora is defined by the precipitation of particles, electrons and protons falling into our atmosphere while the atmospheric glow of STEVE comes from heating without precipitation of particles," said physicist Bea Gallardo-Lacourt of the University of Calgary.

"The electrons that cause the green fence are therefore auroras, although this happens outside the auroral zone, so it's really unique."

The palisade is actually pretty incredible. The team discovered that it was caused by energetic electrons flowing into the Earth's atmosphere thousands of kilometers away, a distance large enough to occur simultaneously in the northern and southern hemispheres.

According to satellite data, high-frequency waves that move from the magnetosphere to the ionosphere can excite the electrons and eject them from the magnetosphere, creating a linear STEVE equipment.

We still do not know much about STEVE. We know that the skyglow was not seen between October 2016 and February 2017, and then between October 2017 and February 2018, suggesting that there might be a seasonal component.

Moreover, it appears only in the presence of auroras (although auroras can appear without STEVE, so it is very sticky).

We also know that STEVE appears in both hemispheres and is aligned east-west. These are all clues that help scientists understand why this river of charged particles appears in the sky.

It could also help us understand how charged particles circulate in the magnetosphere and the ionosphere, particularly because they can affect satellite communications.

But this knowledge would also most likely be welcome in the community of dawn hunters and photographers – seekers of the beauty of our planet – around the world.

The search was published in Geophysical Research Letters.