Irregularity structures in the cusp/cleft and polar cap regions

Abstract

A case study of the temporal behavior of ionospheric scintillations and their frequency spectra in the cusp/cleft and polar cap regions is presented. These measurements were made at Søndrestrom and Thule, Greenland, using the 243‐MHz transmissions from quasi‐stationary satellites during a coupling energetic and dynamics of atmospheric regions (CEDAR) high‐latitude plasma structure (HLPS)/solar terrestrial energy program (STEP) global aspects of plasma structures (GAPS) campaign. During this campaign, the incoherent scatter radar (ISR) observations were also performed at Søndrestrom, which defined the dynamic ionospheric environment in the cusp/cleft region. The availability of the radar results has enhanced this case study. It is found that scintillations at Søndrestrom are abruptly enhanced about an hour before magnetic noon when the propagation path to the satellite entered the cusp/cleft region. Subsequently, a series of enhanced and reduced scintillation activity was detected. The enhanced scintillation structures were found to be asymmetric, with sharp leading edges and diffuse trailing edges. Spaced‐antenna scintillation measurements at Søndrestrom detected considerable velocity shear, and the frequency spectra showed flat low‐frequency portions, implying the presence of turbulent plasma flows. A comparison with the ISR observations indicates that the temporal variation of scintillation was caused by the poleward convection of alternate regions with high‐ and low‐ionization density, the density depletions being caused by channels of high zonal flows associated with velocity shear. The level of scintillation observed in the low‐density regions imply the presence of small‐scale irregularities with considerable irregularity amplitude. In contrast to the above behavior, the polar cap scintillations exhibit deep minima between the transit of successive “patches” of ionization, and their frequency spectra imply the absence of turbulent plasma flows. It is postulated that in the cusp/cleft and polar cap regions, the gradient‐drift instability mechanism generates the observed small‐scale irregularities associated with discrete density enhancements, whereas a shear‐driven instability, such as the nonlinear collisional Kelvin‐Helmholtz (K‐H) instability mechanism, may generate the irregularities in the intervening low‐density regions.

Wave power studies of cusp crossings with the Polar satellite

“…Other instabilities such as the gradient drift instability [Cerisier et al, 1985], shear-driven turbulence [Basu et al, 1988[Basu et al, , 1994 …”

Simultaneous optical, CUTLASS HF radar, and FAST spacecraft observations: signatures of boundary layer processes in the cusp

“…The latter is a subset of the gradient drift instability (e.g. Tsunoda, 1988;Basu et al, 1994). For the active cusp geometry, with poleward flow up a gradient in the electron density, the gradient drift instability has been suggested as the dominant mode for driving the F-region plasma unstable.…”

On the collocation between dayside auroral activity and coherent HF radar backscatter

“…Sojka et al, 1994) in looking for the source of density gradients to feed irregularity formation processes here. Based on Sondrestromfjord ISR observations of plasma dynamics, combined with scintillation measurements of plasma irregularities, Basu et al (1994) postulated that the gradient drift plasma instability mechanism generates irregularities associated with discrete plasma density enhancements in the cusp polar cap region, and that the shear-driven instability may generate irregularities in the intervening low-density regions. They calculated growth time for the gradient instability process operating in weakly collisional plasma at 400 km altitude to be of the order of 30 s. This likely explains why the radar cusp boundary is oset north of the precipitation boundary by a few tens of kilometres.…”

“…The generation mechanism of backscatter targets has not yet been agreed upon, but the literature has identi®ed candidate processes including: gradient drift instability, shear instability,``stirring'' or¯ux tube interchange, and current convective instability which can be viewed as a subset of gradient drift instabilities (e.g. Tsunoda, 1988;Basu et al, 1994). The latter mode includes ®eld-aligned currents as an extra source of free energy in the context of interchange instabilities (Chaturvedi and Ossakow, 1981).…”

“…Under conditions for which plasma¯ow has a component in the direction of a density gradient, gradient drift instability is regarded as the dominant mode for driving the plasma unstable in the F-region auroral ionosphere (e.g. Ossakow and Chaturvedi, 1979;Basu et al, 1994). The typical geometry for gradient drift instabilities in the Northern Hemisphere cusp, is a density gradient towards north, a convection electric ®eld pointing eastward and a background magnetic ®eld pointing down.…”