Multi-wavelength imaging observations of STEVE at Athabasca, Canada

Abstract:

The space physics community has recently stumbled upon a new optical phenomenon known as STEVE (strong thermal emission velocity enhancement). In optical data, STEVE is identified as a latitudinally narrow band of luminosity equatorward of the auroral oval. Imaging observations of STEVE in earlier studies were mostly based on the panchromatic and red-line cameras. In the present study, we use monochromatic images at six wavelengths to study the evolution of STEVE by using a highly-sensitive Optical Mesosphere Thermosphere Imager (OMTI) at Athabasca (magnetic latitude = 61.7°N), Canada. The study is based on three STEVE events which were accompanied by picket fence structures in the green-line. Although the STEVE arc was dominant in 630-nm and 557.7-nm, weak emissions were also found in other wavelengths including OI at 844.6-nm, Hβ, Na, and the nominal background filter at 572.5-nm. Similar to the arc in 630-nm, the arc in the 557.7-nm also detached from the auroral oval. In addition, the 557.7-nm arc exhibited picket fence structure at later times after it moved equatorward. The picket fence was sometimes found to persist even after the 630-nm arc had disappeared. The STEVE arcs in both the 630-nm and 557.7-nm displays an interesting ribbon-like motion. One of the distinctive features that emerged from the monochromatic images is that the STEVE arc is located in a region of sharp boundaries and these boundaries are different in red- and green-line. While green-arc is located in a region of broad minimum, red-arc appeared at the equatorward edge of the diffuse emissions. The sharp decrease in the intensity at the immediate poleward edge of the green-arc is reflected in terms of a ‘black-band’ in the green-line images. Based on the horizontal component of the geomagnetic field at Fort Smith (magnetic latitude 67.28°N), we find that the STEVE arc detachment, its equatorward motion, and its brightness coincided with additional magnetic activity in the recovery phase of a substorm. Our results provide new insights about STEVE and raise new questions about the mechanisms underlying its generation.