The ionospheric signatures of rapid subauroral ion drifts

Subauroral ion drifts (SAID) are latitudinally narrow regions of rapid westward ion drift located in the evening sector and centered on the equatorward edge of the diffuse aurora. Observations of SAID as identified by the ion drift meters on the Atmosphere Explorer C and Dynamics Explorer B spacecraft are utilized to determine their effect on the F region ion composition, their relationship to the mid‐latitude trough, and their temporal evolution. At altitudes near the F peak a deep ionization trough is formed in regions of large ion drift where the O+ concentration is considerably depleted and the NO+ concentration is enhanced, while at higher altitudes the trough signature is considerably mitigated or even absent. SAID have been observed to last longer than 30 min but less than 3 hours, and their latitudinal width often becomes narrower as time progresses. The plasma flows westward equatorward of the SAID and becomes more westward as invariant latitude increases. Poleward of the SAID, the flow is, on average, westward throughout the auroral zone in the evening, while near midnight it becomes eastward.