Is "Steve" not an aurora?

Novel luminous phenomenon in the sky remains riddled

Reddish-white light streaks of the Steve phenomenon over Canada, to the right of it the distant glow of a normal polar light. © Ryan Sault
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Mysterious glow: The "Steve" baptized sky phenomenon is obviously not a polar light variant. Because new satellite data reveal that the illumination of these reddish light stripes is not accompanied by the usual particle flows in the upper atmosphere. What causes this atmospheric glow, however, remains unknown. Researchers are now speculating about whether it could be a variant of the "Airglow" - or something completely unknown.

} A few years ago, images of aurora photographers caught an unusual sky phenomenon: a narrow, but thousands of kilometers long strip of reddish light that stretched across the sky from the horizon, slowly moving westward. Strange also: While auroras usually occur only poleward of the 65th parallel, these arcs occur outside the known aurora oval.

At the beginning of 2018, researchers believed that they had solved the riddle of Steve at least partially. Data from the SWARM satellites during a Steve event revealed that this was associated with an abrupt rise in temperature and a strong flow of fast ions in the upper atmosphere. Steve showed features of an aurora phenomenon called subauroral ion drift.

But now, Bea Gallardo-Lacourt of the University of Calgary and her team have analyzed more data about Steve - and come to a very different conclusion. The impetus for this was another happy coincidence of satellite overflight and Steve arc. On March 28, 2008, the weather satellite NOAA-17 crossed a currently active Steve event, measuring proton and electron density in the upper atmosphere.

No particle flow detectable

The surprising result: Contrary to expectations, the satellite data showed no increased particle flows at around 800 kilometers altitude. But that contradicts an interpretation of this phenomenon as a polar light variant, as the researchers explain. For in an aurora, protons and electrons from the solar wind and the magnetosphere of the earth "rain" into the ionosphere and, in the event of a collision with the air particles, produce the glow. display

"Steve" in action, shot in British Columbia. Andy Witteman - @CNLastro

But Steve was missing these high-energy electrons and protons. "Although we observe a rise in low-energy electrons of 50 to 1, 000 electron volts, even the accumulated energy flux of these particles was too weak to produce an optical phenomenon, " explain Gallardo-Lacourt and her colleagues. Even with the protons there was only a slight increase in the very low-energy range.

Heat and radiation instead of particles?

"Our conclusion is that Steve is not an Aurora, " says Gallardo-Lacourt. For there is no evidence that this phenomenon is accompanied by a particle rainfall into the ionosphere. "At the moment, we still know little about Steve which is intriguing given the fact that photographers have known this luminary for decades."

The scientists can still only speculate about how Steve's lightbeams are created. One possibility would be that it is a still unknown variant of the "Airglow", also known as night sky light in German. "This is an emission of light that results from chemical reactions of solar UV radiation with the atoms and molecules of the upper atmosphere, " explain the researchers. Steve could be a variant of this phenomenon, where local heating of the ionosphere by radiation or particles leads to light emission.

"A second possibility would be an as yet unknown process that can generate an influx of electrons below 800 kilometers altitude, " says Gallardo-Lacourt. But it is not even clear whether Steve originated in the ionosphere or the magnetosphere of the earth. That means: A polar light does not seem to be Steve. But what creates these gleams instead is still puzzling. (Geophysical Research Letters, 2018; doi: 10.1029 / 2018GL078509)

(American Geophysical Union, 21.08.2018 - NPO)