Strong Thermal Emission Velocity Enhancement and, according to Spaceweather.com, "(t)he phenomenon is caused by hot (3000°C) ribbons of gas flowing through Earth's magnetosphere at speeds exceeding 6 km/s (13,000 mph). These ribbons appear during some geomagnetic storms, revealing themselves by their soft purple glow." The source, it says may be "(m)agnetic explosions called 'substorms' more than 22,000 km above Earth's surface (that) hurl streams of hot plasma toward Earth. When the material reaches an altitude ~250 km above Earth's surface, it begins to emit a mauve light."
Several STEVE sightings were reported and photographed in Canada over Labor Day weekend during an auroral event linked to a geomagnetic storm.