Subauroral ion drift

A subauroral ion drift (SAID), also known as a polarisation jet, is an atmospheric phenomenon driven by substorms in the Earth’s magnetosphere.[1] First discovered in 1971,[2] a SAID is a latitudinally narrow (1-2° MLAT) layer of rapid, westward flowing ions in the Earth’s ionosphere. Though not traditionally associated with an optical emission, the STEVE discovery paper[3] suggested the first link between this optical emission’s occurrence and that of an extremely fast and hot SAID event.[4]

SAIDs are observed equatorward of the auroral zone, at subauroral latitudes, typically in the local time sector between 18:00 hours and 22:00 hours.[1] They can occur individually, or as multiple events. SAIDs are characterised by a reduced density of ions, a strong westward flow, and an increased temperature. They can last between 30 minutes and 3 hours.[5] The exact characteristics of SAID events appear to have solar cycle, seasonal, and diurnal dependences.[6]

Although studied for decades, prior to the formal discovery of STEVE, SAIDs had never been associated with an optical emission.[7] STEVE was associated with a particularly extreme SAID, with a velocity over twice the norm and 100 K hotter.[7] STEVE has presented a new way for scientists, including citizen scientists, to study SAIDs.[8]

References
  1. Spiro, R. W.; Heelis, R. A.; Hanson, W. B. (August 1979). "Rapid subauroral ion drifts observed by Atmosphere Explorer C". Geophysical Research Letters. 6 (8): 657–660. doi:10.1029/GL006i008p00657.
  2. Galperin, Y. I.; Ponomarov, Y. N.; Zosinova, A. G. (1973). "Direct measurements of ion drift velocity in the upper ionosphere during a magnetic storm". Cosmicheskie Issled. 11: 273.
  3. MacDonald, Elizabeth A.; Donovan, Eric; Nishimura, Yukitoshi; Case, Nathan A.; Gillies, D. Megan; Gallardo-Lacourt, Bea; Archer, William E.; Spanswick, Emma L.; Bourassa, Notanee; Connors, Martin; Heavner, Matthew; Jackel, Brian; Kosar, Burcu; Knudsen, David J.; Ratzlaff, Chris; Schofield, Ian (14 March 2018). "New science in plain sight: Citizen scientists lead to the discovery of optical structure in the upper atmosphere". Science Advances. 4 (3): eaaq0030. doi:10.1126/sciadv.aaq0030. PMC 5851661. PMID 29546244.
  4. Patel, Kasha (14 March 2018). "Mystery of Purple Lights in Sky Solved With Citizen Scientists' Help". NASA. Retrieved 17 May 2019.
  5. Anderson, P. C.; Heelis, R. A.; Hanson, W. B. (1991). "The ionospheric signatures of rapid subauroral ion drifts". Journal of Geophysical Research. 96 (A4): 5785. doi:10.1029/90JA02651.
  6. He, Fei; Zhang, Xiao-Xin; Chen, Bo (June 2014). "Solar cycle, seasonal, and diurnal variations of subauroral ion drifts: Statistical results". Journal of Geophysical Research: Space Physics. 119 (6): 5076–5086. doi:10.1002/2014JA019807.
  7. McRae, Mike. "Everyone, Meet 'Steve' – A Weird Type of Aurora We've Never Seen Before". ScienceAlert. Retrieved 17 May 2019.
  8. Skibba, Ramin (15 March 2018). "Meet 'Steve,' a Totally New Kind of Aurora". National Geographic. National Geographic. Retrieved 17 May 2019.
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