IRREGULARITY STRUCTURES IN THE CUSP CLEFT AND POLAR-CAP REGIONS

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 Sondrestrom 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 Sondrestrom, 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 Sondrestrom 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 Sondrestrom 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.