The ionospheric signatures of rapid subauroral ion drifts

Abstract

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.

Investigation of subauroral ion drifts and related field-aligned currents and ionospheric Pedersen conductivity distribution

“…The quantitative analysis of the conductivity gradient measured at both edges of the reduced conductivity regions revealed that the less sharply reduced conductivity regions were detected in the early evening sector, whereas the sharpest reduced conductivity regions were de- tected on the 20:00 to 23:00 MLT sector, where the most intense SAID electric field peaks are also found. In some previous studies, the relation between the SAID occurrence and the ionospheric region of low plasma density, known as the mid-latitude trough, has been suggested (Anderson et al, 1991;Rodger et al, 1992;Karlsson et al, 1998;Anderson et al, 2001). Aiming to analyze the relation between both the SAID peak and the corresponding Pedersen conductivity minimum positions, and the position of the maximum depletion in the ionospheric trough, the empirical model for the position of the ionospheric trough minimum proposed by Werner and Prölss (1997) was used.…”

“…They concluded that the imposed electric fields in combination with thermal plasma escape, can produce deep ionization troughs within this region. In a more recent study, Anderson et al (1991) showed that in most cases a mid-latitude trough was present prior to SAID formation. Very deep troughs were observed associated with SAID events, resulting from a deepening caused by the SAID of the poleward most extent of the preexisting trough.…”

“…Although different measurement techniques and different definitions were used, there are several features of this phenomenon generally agreed upon. The SAID phenomenon is a manifestation of magnetosphere-ionosphere coupling outside of the auroral region, characterized by westward ion drifts with velocities greater than 1000 m/s by many of the authors (Anderson et al, 1991;Galperin et al, 1997;Burke et al, 2000). Some authors have focused on the corresponding intense poleward-directed electric fields, reporting events with intensities greater than approximately 30 mV/m (Smiddy et al, 1977;Maynard, 1978;Maynard et al, 1980;Rich et al, 1980;Karlsson et al, 1998).…”

Electromagnetic Wave Structures Within Subauroral Polarization Streams

“…~ [E x BQ]". Anderson et al [1991] have argued that the plasma outflow related to fiictional ion heating is the main cause of the formation of the deep troughs. Moffett et al [1998] reported enhancements of the ion and electron temperatures ~4000 and 6000 K within the SAPS-related troughs.…”

Seasonal and magnetic activity variations of ionospheric electric fields above the southern mid-latitude station, Bundoora, Australia

“…The agreement with the westward perturbation drifts, especially during the pre-midnight hours, is excellent. Subauroral ion drift events (SAIDs; see Anderson et al, 1991) are also known to have peak westward flows in the pre-midnight hours. Hence, the relationship between SAIDs and the mid-latitude westward drifts, observed here, requires further investigation.…”