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
^
abSpiro, 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.
Bibcode:
1979GeoRL...6..657S.
doi:
10.1029/GL006i008p00657.
^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.
^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.
Bibcode:
1991JGR....96.5785A.
doi:
10.1029/90JA02651.