STEVE (Strong Thermal Emission Velocity Enhancement) is an atmospheric optical phenomenon that appears as a purple and green light ribbon in the sky, named in late 2016 by aurora watchers from Alberta, Canada. According to analysis of satellite data from the European Space Agency's Swarm mission, STEVE is caused by a 25 km (16 mi) wide ribbon of hot plasma at an altitude of 450 km (280 mi), with a temperature of 3,000 °C (3,270 K; 5,430 °F) and flowing at a speed of 6 km/s (3.7 mi/s) (compared to 10 m/s (33 ft/s) outside the ribbon). The phenomenon is not rare, but had not previously been investigated. In August 2018, researchers determined that the phenomenon's skyglow was not associated with particle precipitation (electrons or ions) and, as a result, could be generated in the ionosphere.
STEVE has been observed by auroral photographers for decades, with some evidence to suggest that observations may have been recorded as early as 1705. However, the first accurate determination of what STEVE is was not made until after members of a Facebook group called Alberta Aurora Chasers named it, attributed it to a proton aurora, and called it a "proton arc". When physics professor Eric Donovan from the University of Calgary saw their photographs, he suspected that was not the case because proton auroras are not visible. He correlated the time and location of the phenomenon with Swarm satellite data and one of the Alberta Aurora Chasers' photographers, Song Despins, whose photos are not shown on this page. She also included GPS coordinates from Vimy, Alberta, that helped Donovan link the data to identify the phenomenon.
One of the aurora watchers, photographer Chris Ratzlaff, suggested the name "STEVE" from Over the Hedge, an animated comedy movie from 2006, in which its characters chose that as a benign name for something unknown. Reportage of the heretofore undescribed unusual "aurora" went viral as an example of citizen science on Aurorasaurus.
Robert Lysak, during the fall meeting of the American Geophysical Union in December 2016, suggested "Strong Thermal Emission Velocity Enhancement" as a backronym of STEVE, one that has since been adopted by the team at NASA Goddard Space Flight Center studying the phenomenon.
STEVE may be spotted closer to the equator than the aurora, and as of March 2018 has been observed in the United Kingdom, Canada, Alaska, northern U.S. states, and New Zealand. STEVE appears as a very narrow arc extending for hundreds or thousands of miles, aligned east–west. STEVE generally lasts for 20 minutes to an hour. As of March 2018, STEVE has only been spotted in the presence of an aurora. STEVE was not observed from October 2016 to February 2017, or from October 2017 to February 2018, leading NASA to believe that STEVE may only appear in certain seasons.
A study published in March 2018 by Elizabeth A MacDonald and other co-authors in the peer-reviewed journal Science Advances suggested that STEVE accompanies a subauroral ion drift (SAID), a fast-moving stream of extremely hot particles. STEVE marks the first observed visual effect accompanying a SAID. In August 2018, researchers determined that the phenomenon's skyglow was not associated with particle precipitation (electrons or ions) and, as a result, could be generated in the ionosphere.
STEVE often, although not always, is observed above a green, "picket-fence" aurora. Although the picket-fence aurora is created through precipitation of electrons, they appear outside the auroral oval and so their formation is different from traditional aurora. The study also showed these phenomena appear in both hemispheres simultaneously. Sightings of picket-fence aurora have been made without observations of STEVE.
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