(ORDO NEWS) — (a) A wide, stable red auroral arc (SAR) is observed at 09:27 UT (left), a strong thermal emission velocity enhancement (STEVE) at 09:51 UT (center), and a partial arc with green picket fence structures at 10 :02 UT (right).
Auroral activity is observed in the lower (southern) part of each image. West – right; (b, c, and d) show decomposed images in red, green, and blue, respectively, at the same time as in panel (a). At 09:27 UT (left), no arc is observed in the green and blue channels.
At 09:51 UT (center), all three channels detect an arc. At 10:02 UT (on the right), the main emission comes from the green channel, where picket fence structures are observed. A partial arc is also observed in the red channel on the right side of the image.
A team of researchers from New Zealand, the US and Canada have confirmed the first observation of a SAR arc evolving into STEVE.
In their paper, published in the journal Geophysical Research Letters, the group details their analysis of the numerous datasets used to describe the rare and unique atmospheric phenomenon and what it showed them about similar phenomena in general.
In 2015, citizen scientist E. Griffith noticed something unusual in the night sky over Dunedin, New Zealand. An arc of light that he described as blood red moved across the dark sky. Intrigued, he took his camera and started filming the action.
Over the next 30 minutes, the arc slowly transformed into what became known as the STEVE. Intrigued by what he saw, he sent the footage to professional stargazers at the University of Otago at Dunedin.
Also intrigued group sent pictures to other colleagues. Eventually a team of researchers was formed to study the event.
The researchers collected data from other sources, including satellites and a sky imager at a local observatory.
In examining all the data, it became clear that the first images taken by Griffith showed a stable auroral red arc (SAR), which is a subauroral structure and an event that is not actually an aurora, but occurs in the atmosphere for as yet unknown reasons.
The researchers noted that its appearance coincided with a geomagnetic storm – a disturbance in Earth‘s magnetosphere – suggesting that the light was caused by charged particles in the upper atmosphere. Then, as the arc began to subside, the red color turned to lilac, a sign of STEVE that is also not well understood.
And then, as STEVE began to fade, another light appeared nearby, which was described as a green picket fence, which has been previously reported, but which is even less known.
The researchers note that all three light events were not aurors, as all three appear to have been the result of locally generated energy rather than particles coming from elsewhere. They also noted that this event was the first known when all three phenomena were observed together.
The film shows the entire sequence of observations, as well as unblurred films in the red, green, and blue channels, respectively. Credit: C. Martinis et al, Rainbow of the Night: First direct observation of the SAR arc evolving into STEVE,
Further information: C. Martinis et al, Rainbow of the Night: First direct observation of the SAR arc evolving into STEVE, Geophysical Research Letters
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