On March 17, 2015, a blood-red arc cut through the sky hundreds of miles across New Zealand. Over the next half hour, an amateur skywatcher observed that arc as it transformed before his eyes into one of the earth’s most confusing atmospheric mysteries – the eerie ribbon known as STEVE – revealing recently released photos.
STEVE, short for “strong thermal velocity improvement,” is an atmospheric oddity first described in 2018, after amateur pursuers of the Northern Lights saw a narrow stream of smooth purple arcs across the sky over northern Canada. Researchers who studied the phenomenon soon confirmed that STEVE was not one northern lights – the multicolored glow that appears at high latitudes when solar particles collide with atoms high Jordens atmosphere. Rather, STEVE was a separate and unique phenomenon that is “completely unknown“to science.
In contrast to the Northern Lights, which tend to shimmer in wide bands of green, blue or reddish light depending on the altitude, STEVE usually appears as a single band of purple-white light sticking straight up for hundreds of miles. Sometimes it is accompanied by a broken green line of light with the nickname “dick fence” phenomenon. Both STEVE and its fencing friend appear to be much lower in the sky than a typical northern light does, in a part of the atmosphere known as the subauroral region, where charged solar particles are unlikely to penetrate.
New research has now been published in the journal Geophysical research letters has connected STEVE to another subauroral structure, known as stable auroral red (SAR) arcs, for the first time.
In the new study, the authors compared New Zealand skywatchers’ March 2015 footage with simultaneous satellite observations and data from a full-sky image at the nearby University of Canterbury Mount John Observatory. The combination of these three sources gave the researchers a comprehensive look at STEVE’s unexpected appearance that evening.
Tonight’s sky show began with the appearance of a blood-red SAR arc sweeping at least 300 kilometers across Dunedin, New Zealand. Satellite data showed that the appearance of the arc coincided with a strong geomagnetic storm – a shower of charged solar particles into the Earth’s upper atmosphere – which lasted for about half an hour.
As the storm calmed down, the red bow gradually gave way to the characteristic purple line of STEVE, which cut through the sky in almost exactly the same place. Just before STEVE faded, the green picket fence structure glistened. According to the researchers, this is the first recorded occurrence of all three structures that appear in the sky together, one after the other – possibly revealing new clues about the formation and development of STEVE.
“These phenomena are different from the Northern Lights, as their optical signatures appear to be triggered by extreme thermal and kinetic energy in the Earth’s atmosphere, rather than produced by energetic particles that rain down into our atmosphere,” the researchers wrote in the new study.
Satellite observations of the event suggest that the night’s geomagnetic storm may have played a key role in this parade of skylights.
During the storm, a fast-moving jet of charged particles appeared along the red SAR arc, the researchers wrote. Known as subauroral ion drift (SAID), these currents of hot, fast particles usually appear in the subauroral zone of the sky during geomagnetic storms. The satellite observations also showed that the heat and velocity of the creek were intensified when STEVE appeared about 30 minutes later.
According to the researchers, a “plausible generation mechanism” for STEVE could be the interaction between these fast-moving ion currents and nitrogen molecules in the subauroral zone; when the charged, hot particles strike nitrogen molecules, the molecules are excited, emitting purple light to burn off their extra energy.
The new study sheds light on parts of the mysterious phenomenon, but more observations of STEVE – both from citizen researchers and professional researchers – are needed to find this theory further.
Originally published on Live Science.