F region electron density irregularity spectra near auroral acceleration and shear regions

Abstract

Spectral characteristics of auroral F region irregularities were studied by the use of high‐resolution (∼35 m) density measurements made by the retarding potential analyzer (RPA) on board the Atmosphere Explorer D (AE‐D) satellite during two orbits when the satellite was traversing the high‐latitude ionosphere in the evening sector. Coordinated DMSP passes provided synoptic coverage of auroral activity. The auroral energy input was estimated by intergrating the low‐energy electron (LEE) data on AE‐D. It was found that the one‐dimensional in situ spectral index (p1) of the irregularities at scale lengths of <1 km showed considerable steepening in regions of large auroral acceleration events with p1 values of ∼ −3. This is interpreted as resulting from the effects of E region conductivity on the F region irregularity structure. The regions in between the precipitation structures, where presumably the E region conductivity was small, were generally associated with large shears in the horizontal E‐W drifts and large velocities, as measured by the ion drift meter on board AE‐D. The maximum drifts measured were ∼2 km s−1, corresponding to an electric field of 100 mV m−1. The large‐velocity regions were also associated with substantial ion heating and electron density depletions. The largest shear magnitudes observed were ∼80 m s−1 km−1, and the shear gradient scale lengths were ∼10 km, which was approximately the resolution of the ion drift meter data set used. The spectral characteristics of irregularities in the large, variable flow regions were very different, with p1 being ∼−1. Since these regions were also associated with the largest irregularity amplitudes, it seemed probable that either the velocity shears or, alternatively, the large velocities provided a source of irregularities in the auroral F region ionosphere. Current work on plasma instabilities related to velocity shears and two‐dimensional fluid turbulence is briefly summarized and critically compared to the findings of the present study.

Rocket measurements within a polar cap arc: Plasma, particle, and electric circuit parameters

“…In the presence of large electric fields the ion temperature is increased, which results in an increase in the reaction 0 + + N2 --• NO + + N. This effect was further described by Schunk [1987, 1988], who calculated a decrease in the F layer peak density by 25% accompanied by a much larger and rapid increase (x 10) in NO +. Direct experimental evidence of this effect was shown by Basu et al [1984]. In an analysis of velocity shears in the auroral oval, regions of decreased ion density and regions of greatly increased ion temperature were located at the velocity shear region.…”

Collisional shear instability in the equatorial F region ionosphere

“…The issue of shear flow stabilization remains unresolved, and a number of recent theoretical and computational studies continue to support the premise [ Hassam , 1992; Sekar and Kelley , 1998; Chakrabarti and Lakhina , 2003]. Studies have also investigated the effects of shear on parallel electron dynamics important at high latitudes [e.g., Satyanarayana et al , 1987b; Shukla and Rahman , 1998] as well as the generation of kilometric plasma irregularities by parallel shear flow near auroral arcs [e.g., Basu et al , 1984; Basu and Coppi , 1988, 1989; Willig et al , 1997].…”

Nonlinear evolution of high‐latitude ionospheric interchange instabilities with scale‐size‐dependent magnetospheric coupling

“…During the last few years a large number of experimental [Curtis et al, 1982;Vickrey et al, 1980Vickrey et al, , 1986Basu et al, 1984Basu et al, , 1988Gurnett et al, 1984;Weimer et al, 1985;Cerisier et al, 1985;Baker et al, 1986;Kintner et al, 1987] and gradient and an electric field perpendicular to the geomagnetic field. The electric field will result in a relative velocity between convecting ionospheric ions and the neutral thermosphere.…”