Physics Department Seminar University of Alaska Fairbanks

J O U R N A L    C L U B


The STEVE formerly known as Aurora

Don Hampton
Geophysical Institute, UAF



The 2013/2104 winter aurora season (northern hemisphere) was particularly active, being near solar maximum. During this season reports began arriving from aurora photographers of “proton arcs”, structures that looked like aurora, but were well separated from the discrete electron auroral zone and had a pink or mauve color. The “proton arc” label was clearly wrong, since these structures were only 10s of km wide (N-S), compared to more typical widths of 100 to 200 km for proton aurora. These arcs were typically associated with large solar storms, and were offset south of the discrete electron aurora by 100s of km. This different color, location and morphology clearly indicated something different. With the auroral science community at a loss, the photographers decided to call the phenomenon “STEVE” after the mysterious hedge in the popular cartoon strip Over The Hedge. Subsequent studies that correlated measurements from the SWARM satellite with U. Calgary imagers found that these STEVE events were correlated quite tightly with Subauroral Ion Drift (SID) events, where the ion drift speeds were 2 to 5 km/s, and electron temperatures were in the 3000 to 5000 K range. With this knowledge, a “backronym” was coined – Strong Thermal Emissions Velocity Enhancements. Still the spectral characteristics of images from DSLR cameras were not sufficient to tease out the source of the emission. We (GI) built two spectrographs for U. Calgary as part of their TREX project, one of which was fielded in Luck Lake, Saskatchewan, in the winter of 2016/2017, and which captured spectra from several STEVE events. As we suspected the spectral characteristics are not those of aurora – discrete emissions caused by electron impact excitation of atoms and molecules – but are broadband, extending from 400 to 800 nm with little or no discrete emissions. This is consistent with the electron temperatures observed by SWARM, but the mechanism was not clear until, perhaps, this week. I will reveal our best guess as to the source of the emissions (if the numbers work out), and describe our next steps in understanding what STEVE is and what impact it may have on our upper atmosphere.


Friday, 11 Oct. 2019

Globe Room, Elvey Building