Over the next half hour, an amateur skywatcher observed that arc as it transformed before his eyes into one of Earth's most puzzling atmospheric mysteries — the eerie ribbon of light known as STEVE — newly released images reveal.

Scientists who studied the phenomenon soon confirmed that STEVE was not an aurora — the multi-colored glow that appears at high latitudes when solar particles collide with atoms high in Earth's atmosphere.

Both STEVE and its picket fence friend appear much lower in the sky than a typical aurora does, in a part of the atmosphere known as the subauroral region, where charged solar particles are unlikely to trespass.

Now, new research published in the journal Geophysical Research Letters has linked STEVE to another subauroral structure, known as stable auroral red (SAR) arcs, for the first time.

Satellite data showed that the arc's appearance coincided with a strong geomagnetic storm — a showering of charged solar particles into Earth's upper atmosphere — that lasted for approximately half an hour.

Just before STEVE faded, the green picket fence structure shimmered into view.

Known as subauroral ion drift (SAID), these streams of hot, fast particles typically appear in the sky's subauroral zone during geomagnetic storms.

The satellite observations also showed that the stream's heat and speed 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 streams and nitrogen molecules in the subauroral zone; when the charged, hot particles bash against nitrogen molecules, the molecules become excited, emitting mauve light to burn off their extra energy.

The new study illuminates parts of the mysterious phenomenon, but more observations of STEVE — from citizen scientists and professional researchers alike — are needed to further pin down this theory.