Subauroral ion drift explained

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. 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. STEVE has presented a new way for scientists, including citizen scientists, to study SAIDs.[8]

Notes and References

  1. Spiro . R. W. . Heelis . R. A. . Hanson . W. B. . Rapid subauroral ion drifts observed by Atmosphere Explorer C . Geophysical Research Letters . August 1979 . 6 . 8 . 657–660 . 10.1029/GL006i008p00657. 1979GeoRL...6..657S .
  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 . 1973KosIs..11..273G.
  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 . New science in plain sight: Citizen scientists lead to the discovery of optical structure in the upper atmosphere . Science Advances . 14 March 2018 . 4 . 3 . eaaq0030 . 10.1126/sciadv.aaq0030. 5851661 . 29546244. 2018SciA....4...30M .
  4. Web site: Patel . Kasha . Mystery of Purple Lights in Sky Solved With Citizen Scientists' Help . NASA . 17 May 2019 . 14 March 2018.
  5. Anderson . P. C. . Heelis . R. A. . Hanson . W. B. . The ionospheric signatures of rapid subauroral ion drifts . Journal of Geophysical Research . 1991 . 96 . A4 . 5785 . 10.1029/90JA02651. 1991JGR....96.5785A .
  6. He . Fei . Zhang . Xiao-Xin . Chen . Bo . Solar cycle, seasonal, and diurnal variations of subauroral ion drifts: Statistical results . Journal of Geophysical Research: Space Physics . June 2014 . 119 . 6 . 5076–5086 . 10.1002/2014JA019807. 2014JGRA..119.5076H . free .
  7. Web site: McRae . Mike . Everyone, Meet 'Steve' – A Weird Type of Aurora We've Never Seen Before . ScienceAlert . 17 May 2019 . en-gb.
  8. Web site: Skibba . Ramin . Meet 'Steve,' a Totally New Kind of Aurora . National Geographic . National Geographic . 17 May 2019 . en-gb . 15 March 2018.

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