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]