IRREGULARITY STRUCTURES IN THE CUSP CLEFT AND POLAR-CAP REGIONS

A case study of the temporal behavior of ionospheric scintillations an d their frequency spectra in the cusp/cleft and polar cap regions is p resented. These measurements were made at Sondrestrom and Thule, Green land, using the 243-MHz transmissions from quasi-stationary satellites during a coupling energetic and dynamics of atmospheric regions (CEDA R) high-latitude plasma structure (HLPS)/solar terrestrial energy prog ram (STEP) global aspects of plasma structures (GAPS) campaign. During this campaign, the incoherent scatter radar (ISR) observations were a lso performed at Sondrestrom, which defined the dynamic ionospheric en vironment in the cusp/cleft region. The availability of the radar resu lts has enhanced this case study. It is found that scintillations at S ondrestrom are abruptly enhanced about an hour before magnetic noon wh en the propagation path to the satellite entered the cusp/cleft region . Subsequently, a series of enhanced and reduced scintillation activit y was detected. The enhanced scintillation structures were found to be asymmetric, with sharp leading edges and diffuse trailing edges. Spac ed-antenna scintillation measurements at Sondrestrom detected consider able velocity shear, and the frequency spectra showed flat low-frequen cy portions, implying the presence of turbulent plasma flows. A compar ison with the ISR observations indicates that the temporal variation o f scintillation was caused by the poleward convection of alternate reg ions with high- and low-ionization density, the density depletions bei ng caused by channels of high zonal flows associated with velocity she ar. The level of scintillation observed in the low-density regions imp ly the presence of small-scale irregularities with considerable irregu larity amplitude. In contrast to the above behavior, the polar cap sci ntillations exhibit deep minima between the transit of successive ''pa tches'' of ionization, and their frequency spectra imply the absence o f 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 densi ty enhancements, whereas a shear-driven instability, such as the nonli near collisional Kelvin-Helmholtz (K-H) instability mechanism, may gen erate the irregularities in the intervening low-density regions.