Recent studies of the structure and morphology of auroral zone F region irregularities

Abstract

Recent analyses of auroral‐zone spaced‐receiver measurements have shown that the regions where sheetlike irregularities occur are confined to the equatorward portion of the nighttime scintillation zone where the westward and eastward electrojets flow. Poleward of this region, the irregularities are rodlike. For satellites in highly eccentric orbits, the spaced‐receiver technique can be used to measure ionospheric drifts. Simultaneous incoherent‐scatter radar measurements have revealed two types of F region ionization enhancements that are believed to be the source regions of persistent scintillation features on polar satellite transmissions. One type is found at the equatorward edge of the diffuse aurora and can persist for more than 10 hours. More dynamic structures often occur in pairs, which suggests an association with ‘inverted‐V’ precipitation events. Radar data have also revealed large‐scale east‐west structure in the poleward enhancements.

Structure of the poleward wall of the trough and the inclination of the geomagnetic field above the EISCAT radar

“…The boundary blob in this instance is a long-lived, though dynamic feature, constantly being reformed as a result of latitudinally restricted, transient bursts of soft-particle precipitation, though it must be noted that the maximum densities of $10 11 m A3 are generally low in this case. The classic example of a boundary blob, observed by Rino et al (1983) and subsequently modelled by Robinson et al (1985), was of cold plasma being recon®gured into a stable structure after convection across the polar cap. This is certainly not the case in the situation discussed here where the plasma blob at the polewards edge of the trough, although present for many hours, is a dynamic feature constantly undergoing a process of reformation in response to localised in situ energy input.…”

“…A key point was that the poleward edge of the trough was at the equatorial limit of auroral electron precipitation. Earlier work by Rino et al (1983) had identi®ed a boundary blob of enhanced plasma densities poleward of the trough, linked to latitudinally localised precipitation of lowenergy electrons. However, modelling work by Robinson et al (1985) indicated that the boundary blob may result from a recon®guration of structures produced well upstream in the convective¯ow.…”

Reconfiguration of polar-cap plasma in the magnetic midnight sector

“…Further restructuring of the feature in the zonal convective flow might result in a longitudinally extended, but latitudinally-confined boundary blob in the return sunward convection flow of the auroral region. Experimental observations of the boundary blobs in the evening auroral ionosphere have been reported by Rino et al (1983) and Jones et al (1997). It was suggested that the feature in the former study was produced by localised precipitation, while the blob in the latter study was observed by the EIS-CAT radar to be produced in situ by ongoing soft-particle precipitation.…”

“…A less-likely alternative interpretation might be of a uniform TOI moving spatially in-and-out of the radar field-of-view, although knowledge of the spatial and temporal structure of the convective flow on such fine scales is well beyond the capabilities of the measurement techniques available. Rino et al (1983) were the first to present the idea of a boundary blob in the evening auroral ionosphere. Using observations by the Chatanika radar they suggested that the structure may have been produced by precipitation of low-energy electrons from the inner edge of the central plasma sheet.…”

Validation of Ionospheric Models

“…The two Chatanika cases were for 27 February 1980 [Vickrey et al, 1980] and 11 November 1981 [Rino et al, 1983]. In both cases blob like structures were seen through much of the night at latitudes near Chatanika.…”