Model computations of power spectra for ionospheric scintillations at GHz frequencies

High-latitude scintillation data show that the strength and spectral index of intensity scintillation are dependent on the propagation geometry. It will be shown that anisotropic irregularity spectra, with different indices along and across the magnetic field, lead to geometrical effects similar to those observed. In general, the spectrum along the magnetic field is steeper than that across the field, and the difference is more pronounced for nighttime conditions. Spectral anisotropy can be interpreted as a size-dependent irregularity anisotropy. Our data indicate that large-scale irregularities in the daytime and nighttime ionosphere are almost isotropic, while small-scale irregularities are anisotropic and considerably more so at night than during the day. It will be shown that anisotropic irregularity spectra could account for the observed scintillation and in situ temporal spectra with frequency-dependent slope. The effect depends strongly on the geometry. All lJmes ß ! ß , ß ß ß ß ß ß ß , ! ß ! 60 62 64 66 68 70 72 74 76 78 80 82 84 Geomagnetic Latitude Fig. 3. The same as in Figure 2 but for the scintillation index S 4.

Section: Introductionmentioning

confidence: 99%

High‐latitude irregularity spectra deduced from scintillation measurements

Wernik¹,

Liu²,

Franke³

et al. 1990

“…Some spectra exhibited notches that he elected to regard as due to the oscillatory feature of Fresnel filtering by a sufficiently thin scintillation layer, thereby deriving space-time conversion velocities. Other spectra showed no notches, which he correctly attributed to the great thickness of the scintillation layer [see Liu and Yeh, 1977]. However, one of the events showing a notch that he attributed to Fresnel filtering was his strongest event--the one least likely to exhibit the oscillatory feature of the filtering mechanism.…”

mentioning

confidence: 99%

Use of refractive scattering to explain SHF scintillations

Crain¹,

Booker²,

Ferguson³

1979

Ionization structure perpendicular to the line of sight can cause total internal reflection at glancing incidence, leading to refractive scattering that can dominate diffractive scattering. The long-wave cutoff in the spectrum of amplitude fluctuations then occurs, not at the Fresnel scale but at a larger scale that increases as the percentage fluctuation of ionization density increases. The additional amplitude fluctuation arising from the band of scales extending from the usual diffractive cutoff scale up to the refractive cutoff scale is capable of explaining strong SHF scintillation. It also lowers the estimates of the fluctuation of ionization density needed to explain strong VHF scintillation.The concept of refractive scattering is also likely to be applicable to SHF-EHF scintillation phenomena in the troposphere.

“…All these spectra covering a range of S4 index values varying between 0.5 and 0.9 do not show much variation of PSD at frequencies below the Fresnel frequency. Crane [1976] and Liu and Yeh [1977] have shown that the low-frequency spectral slope may change with variations of the anisotropy of the irregularities and the angle (• between the velocity vector and the normal to the plane containing the geomagnetic field and the propagation vector. At high latitudes, for moderate to strong levels of scintillations, the axial ratio is of the order of 10 and the average value of the angle (• during the period of our measurements is 16 ø.…”

Section: Figures 2a and 2b Illustrate The Scintillation Parameters Dementioning

confidence: 99%

Irregularity structures in the cusp/cleft and polar cap regions

Basu¹,

Basu²,

Chaturvedi³

et al. 1994

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.