Determination of the atmospheric refractive index structure parameter from refractivity measurements and amplitude scintillation measurements at 36 GHz

Results are presented of the determination of the atmospheric refractive index structure parameter C_n², deduced from amplitude scintillation measurements made on a 36 GHz radio link and also from the refractive index frequency spectrum computed from refractivity measurements using a radio refractometer. The two methods of measurement give acceptable agreement and provide further experimental evidence of the applicability of Tatarski's wave propagation theory in a turbulent medium to millimetre wavelengths.     

Weak scattering theory applied to equatorial ionospheric scintillation for a wide range of parametric values

The model of ionospheric fluctuations used by Booker and Ferguson (1978) to describe spread-F is applied to ionospheric scintillation in the band from 100 MHz to 10 GHz in equatorial regions. Calculations are based on long Isotropie field-aligned irregularities possessing an inverse power-law spectrum extending from an outer scale [wavelength/(2π)]linked to the properties of the neutral atmosphere down to an inner scale of the order of the ionic gyroradius. Spectral indices from 0 to 6 are considered, with special attention to the range from 1 to 4. The r.m.s. fluctuation of ionization density is assumed to be proportional to the ambient ionization density throughout the plasmasphere, but the effect is shown of removing the fluctuations at heights above 500, 750 and 1000 km. Using a height-distribution of phase-changing screens, calculations are made, for evening and presunrise conditions, of the mean square fluctuations both of phase and of fractional amplitude for situations in which an Earth terminal and a stationary satellite are both in the magnetic equatorial plane. Heights of equivalent single phase-changing screens are deduced for both phase and amplitude fluctuations; they are different from each other and from the height of maximum ionization density. It is concluded that the weak scattering theory can satisfactorily explain weak scintillation, but that amplitude scintillation at strengths of practical importance for radio communications requires the inclusion of refractive scattering in addition to diffractive scattering.     

Power spectra of VHF intensity scintillations from F2- and E-region ionospheric irregularities

An experiment is described for the routine study of scintillations and ionospheric irregularities at high-latitudes using NNSS satellites with additional coordinated observations by means of the EISCAT ionospheric radar facility. Early results, obtained during the development phase of the experiment, are presented of the power spectra of intensity fluctuations at 150 MHz observed at the equatorwards edge of the high-latitude irregularity zone. The spectra of 165 samples of night-time scintillation recorded during October 1982 to May 1983 show a spectral index with a mean value of −3.58 and a steepening of the spectral slope with increasing S₄. Some examples of scintillation arising from irregularities at E-layer height show spectral indices of magnitude generally smaller than for F-region cases. A few spectra have been found with a clear break in spectral slope at around 10 Hz, suggesting two regimes for irregularities of different scale sizes.     

Application of refractive scintillation theory to radio transmission through the ionosphere and the solar wind,and to reflection from a rough ocean

The results of Booker and Majidiahi (1981) concerning refractive scattering by large-scale irregularities in a phase-changing screen are combined with the theory of diffractive scattering by small-scale irregularities in order to study three intensity scintillation phenomena. The first is the reflection of radio and optical waves from an ocean surface disturbed by a spectrum of water waves. The second is the scintillation of VHP, UHF and SHF radio waves traversing the ionospheric F-region. The third is the scintillation of VHF, UHF and SHF radio waves traversing the solar wind. In each case appropriate values are chosen for the mean square fluctuation of phase, for the outer scale, for the inner scale and for the spectral index. Spectral diagrams are drawn to show how the outer scale, the inner scale, the Fresnel scale, the focal scale, the lens scale and the peak scale vary with a relevant parameter (electromagnetic wave-frequency for the ocean, RMS fractional fluctuation of ionization density for the ionosphere, and distance of closest approach to the Sun for the solar wind). For the ionosphere and the solar wind, multiple refractive scattering by weak irregularities occurs in practice whereas it is strong single scattering that is assumed in the thin-screen theory ; potential consequences of this are discussed qualitatively.     

Study of equatorial plasma bubble dynamics using GHz scintillation observations in the Indian sector

To study equatorial plasma bubble dynamics, telemetry signals (4 GHz) were recorded simultaneously from two geostationary satellites. INSAT-1B (74°E) and INSAT-1C (94°E) at Sikandarabad satellite Earth station (dip 42.0°) from January to December 1989 and at the Chenglepet satellite Earth station (dip 10.5°) during September–October 1989 along the same geomagnetic meridian. The characteristics and occurrence pattern of the scintillations suggest that these are equatorial plasma bubble induced events. Observations from the two satellites recorded simultaneously at each of these locations were utilized to estimate the east-west plasma bubble irregularity motion. Plasma bubble rise velocities over the magnetic equator were calculated from the systematic onset time differences observed between an equatorial and a low latitude station. The east-west plasma bubble velocity estimated at Sikandarabad, corresponding to 1200 km altitude in the equatorial plane, shows a night time variation pattern with a peak at around 2100 LT. The mean values over Chenglepet, which correspond to 400 km altitude, start decreasing right from 1900 LT and seem to be influenced by the plasma bubble rise velocities. The differences in magnitude observed between the present results and those reported elsewhere by other techniques are interpreted in terms of vertical shears in the plasma zonal flow over the equator. The near alignment of the two observing stations along a common geomagnetic meridian and the simultaneous use of two satellites located twenty degrees apart in longitude provided an excellent data base to study plasma bubble dynamics.     

Assessing the preservation potential of temperate,lowland alluvial sediments using airborne lidar intensity

Archaeological applications of airborne lidar topographic data are now well established and documented. However, less well explored by archaeologists and palaeoenvironmentalists are the potential applications of lidar intensity data. Here we explore this potential to remotely determine the differential preservation potential of valley floor sediments, within a temperate, lowland environment, especially where preserved within palaeochannels. We compare airborne lidar intensity data with simultaneously collected terrestrial records of sediment organic content, moisture and stratigraphy. Results suggest that while a correlation exists between lidar intensity values and sediment properties, it is neither linear nor robustly predictable. Nevertheless it is suggested that examination of lidar intensity data serve a useful purpose when assessing valley floor alluvial sediments.     

Alterations of Na,K and Rb concentrations in Mycenaean pottery and a proposed explanation using X‐ray diffraction

One of the most important reference groups for Mycenaean pottery is the Mycenae/Berbati (MB). In several studies, a second group has been identified (MBKR). The chemical compositions were similar to MB, but with important differences in the Na, K and Rb contents. The present study suggests that these differences are due to selective alteration and contamination processes that are indirectly determined by the original firing temperature. Therefore, groups MB and MBKR should be considered as a single reference group.     

Identification of Pottery‐Forming Techniques using Quantitative Analysis of the Orientation of Inclusions and Voids in Thin Sections

The relationship between forming techniques and orientation of the components of ceramic materials in the context of pottery technology has long been recognized and used for identification of the techniques employed to create archaeological ceramics. The alignment of inclusions is usually characterized by qualitative categories or ordinal scales. This paper evaluates the potential of quantitative orientation analysis for identification of pottery‐forming techniques. A collection of experimental pottery formed using five forming techniques and combined techniques was analysed. The forming techniques were characterized by the intervals of the mean direction and the circular standard deviations of the orientation of the major axes of the ellipses equivalent to the objects representing random sections of inclusions and voids. The most informative criteria for the identification of pottery techniques are the circular standard deviation of the inclusion/void orientations in the core area of the vessel wall in a perpendicular thin section and the mean direction in a tangential section.