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.
SAIDs are characterised by a reduced density of ions, a strong westward flow, and an increased temperature.
[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.