Measurements of the angular distribution of auroral-zone bremsstrahlung in the middle atmosphere

Measurements of auroral-zone X-rays during rocket flights over Alaska in March 1978 have been analyzed to obtain angular distributions of electron bremsstrahlung in the atmosphere at altitudes of 45–65 km. The rockets carried passively collimated sodium iodide scintillation detectors which recorded X-rays in four energy ranges: > 5 keV, > 10 keV, > 20 keV and > 40 keV. Widespread precipitation events typical of post-breakup auroral activity have been examined. These measurements were made possible on two rocket flights: for one, large amplitude oscillations of the payload with respect to the vertical following parachute deployment allowed sampling of the angular distribution in the upper hemisphere (downward propagating X-rays), while on the other rocket, failure of the parachute system resulted in a tumbling motion of the payload which permitted measurements in both hemispheres (nadir/zenith). The observations reveal an angular distribution for X-rays in the atmosphere at depths (45–65 km) well below the production region, which is approximately isotropic in both hemispheres but with energy dependent ratios of the up/down components; the upward (backscattered) component is a small fraction (1–10%) of the total X-ray flux for 5–40 keV at the greatest altitude examined (65 km). At energies below 40 keV the energy spectrum for downward propagating X-rays hardens rapidly with increasing atmospheric depth, due to the photoelectric absorption at low energies, whereas the backscattered spectrum hardens only slightly in the middle atmosphere, maintaining an equilibrium-like form of diminishing intensity with depth. The present experimental results, apparently unique, are compared with theoretical calculations concerning the angular distribution of atmospheric bremsstrahlung.     

Written on the heavens: Recent studies in archaeoastronomy

Aveni, Anthony F., ed. The Lines of Nazca. Philadelphia: American Philosophical Society, 1990. xii + 343 pp. including references, appendices, and index. $60.00 cloth.

Aveni, Anthony F., ed. World Archaeoastronomy. Cambridge: Cambridge University Press, 1989. xiii + 504 pp. including chapter references and index. $125.00 cloth.     

Ancient Greece: Recent developments in Aegean archaeology and regional studies

This review begins by defining the diverse field of Greek archaeology. Based on our own expertise, we focus on recent advances in the study of ancient Greece, especially the prehistoric Aegean, and on regional approaches, primarily those associated with archaeological surface survey. General developments in method and theory are addressed as they relate to several major topics: social complexity, Aegean chronology, writing systems, exchange, and regional studies.     

Recent techniques of observations and results from the magnetopause regions

The problems encountered in making measurements in the magnetopause regions are caused mainly by the variability of the phenomena and the space/time ambiguity of most satellite observations. The physical quantities of interest for magnetopause studies are listed, together with the range of values observed. As an example of a satellite equipped for magnetopause observations the HEOS-2 payload is described. Data mainly from this and from the OGO-5 spacecraft are used to bring out characteristic features of the magnetopause and show how the experimental problems are attacked.     

A user's guide to scintillation

During the past four decades scintillation methods have been used for remote-sensing distant plasmas and for providing high angular resolution in radioastronomy. This brief review illustrates some of the techniques employed and explains the underlying theory in simple physical terms; it is not intended to be a complete survey of all applications of scintillation.     

Elongation of midlatitude scintillation irregularities

This paper reports a study of the length of the midlatitude F-region irregularities which cause scintillations. The length of scintillation irregularities is usually measured by the Full Correlation Analysis (FCA) method. If observations from a typical 3-receiver array, with small spacing, are used the length is always found to he only a few kilometers. Basic ionospheric theory indicates that the irregularities should he much longer than this. In this experiment scintillations of beacon transmissions from polar-orbiting satellites were observed on a 3-receiver array with 310 m spacing, and a fourth receiver 4.6 km away was used to check the lengths measured. Analysis of the scintillation observations from the 310 m array used the FCA method. In all cases the FCA lengths, from the small array, were much shorter than those measured by the distant receiver. Measurements using the 4.6 km receiver gave an average ratio of 44.5 and a half-length (in ionosphere) of 9.4 km. It appears that random errors in the correlation functions cause the FCA to underestimate the length of the ground pattern of irregularities when the spacing of the receivers is much less than the pattern size.     

Survey of diffractive-refractive scintillation theory

This article describes the basic features of diffractive-refractive scintillation theory. The theory deals with both diffractive scattering caused by irregularities whose scale is less than the Fresnel scale and refractive scattering caused by irregularities whose scale is greater than the Fresnel scale. This theory can be applied to almost all scintillation phenomena in nature. Some of these applications are briefly discussed.     

Ionospheric scintillation observations with radio interferometry

Radio astronomical interferometric observations are affected by atmospheric refraction, being particularly sensitive to inhomogeneities in the atmosphere. At frequencies below 2 GHz the influences of the ionosphere are significant in radio astronomy, especially for single dish observations and for connected element interferometry.Analytical expressions for the manifestations of weak ionospheric scintillation in radio interferometric observations, are derived. We indicate which ionospheric scintillation parameters can be derived from radio interferometric measurements. It is shown that the baseline dependence of the observed amplitude scintillation index implies a direct determination of the height of the region of random irregular electron distribution. Furthermore, the linear scale of the irregularities causing scintillation can be determined directly from the baseline dependence of the scintillation index S₄. From the mean square phase fluctuations as a function of interferometer baseline, the spatial scale of the irregularities responsible for this effect can also be determined. From a comparison with observational mid-latitude data we find indications that scintillation irregularities occur in the lower parts of the F2-layer. The spatial scale of irregularities causing amplitude scintillation is of the order of about 25 to about 500 metres. Phase scintillations are caused by irregularities with dimensions which are an order of magnitude larger.     

Behaviour of HILAT scintillation over spitsbergen

Results of the amplitude scintillation morphology of the HILAT satellite 137 MHz beacon transmission as measured at the Polish Polar Station at Hornsund, Spitsbergen (Δ = 73.4°) are presented. Seasonal, diurnal and latitudinal dependencies of scintillation intensity on magnetic activity were analyzed from over 2250 satellite passes recorded at solar minimum between April 1985 and March 1986. Regions with strong scintillation intensity appear to follow the auroral oval expansion and to move sunward with increasing level of magnetic activity. Maximum amplitude scintillation region coincides with the dayside cusp/cleft position during high magnetic activity. The dawn-dusk asymmetry in scintillation intensity is more distinct in winter than other months. The estimated summer/winter ratio of scintillation intensity is 1.4: 1. Numerical simulations compared with the observational results indicate that high latitude irregularities < 1 km are field-aligned and rod-like rather than sheet-like.     

Post-midnight equatorial scintillation activity in relation to geomagnetic disturbances

VHF amplitude scintillation measurements made during the period April 1978 through December 1982 at Calcutta (23°N, 88.5°E; 32°N dip), situated near the northern crest of the Appleton Anomaly in the Indian sector, have been used to study the association of post-midnight (as well as post-sunrise) scintillations with the occurrences of the maximum negative excursion in the variation of the Earth's horizontal magnetic intensity. The post-midnight scintillation has been found to be related to the maximum negative excursion occurring in the 0000–0600 LT interval. No such relation is observed with the pre-midnight excursions. Scintillation with onset between 0000 and 0300 LT shows remarkable correspondence with the occurrence of negative excursion (18 out of 20 available cases). Magnetic conditions with Dst < −150 nT have been found to be most effective in producing the above scintillation activity. From the present observations, a threshold value of the maximum negative excursion of Dst for producing scintillation may be obtained, Dst < −75 nT being significantly associated with the post-midnight scintillation occurrences. The results are interpreted in terms of the reversal of the equatorial horizontal electric field, under magnetically disturbed conditions, due to a coupling of the high latitude and magnetospheric current systems with the equatorial electric field.     

Equatorwards limits of the southern scintillation oval

Radio signals in the VHF range were recorded and compared with ionograms over a wide range of southern latitudes during a few equinoctial months for which a large variation in the magnetic disturbance level was observed. It is evident that the equatorwards edge of the auroral scintillation oval extends well into mid-latitudes for high values of magnetic K-index. The range-spreading type of spread-F and scintillation-producing irregularities show a high degree of spatial coincidence from the polar cap to mid-latitudes. It is suggested that the inhomogeneities responsible for both ionospheric phenomena are associated with the equatorwards propagation of travelling ionospheric disturbances (T1Ds) generated in the auroral zone.     

Quasi-periodic scintillation events at southern auroral latitudes

Quasi-periodic (QP) radio scintillations were observed during (1987) on 244 MHz and 1.5 GHz geostationary satellite transmissions in the southern auroral zone from Davis station (68.6°S, 78.0°E geographic, 74.6°S Aλ) in Antarctica. Three distinct types of OP events were identified, with occurrence times mainly restricted to the period 18-00 MLT. The substantial loss of signal associated with these events appears to be an important factor in determining the reliability of satellite links on 1.5 GHz in auroral regions. Previous observations at mid-latitudes of QP scintillations have noted a preference for large zenith angles and equatorward azimuths. It is demonstrated that a height transition in a densely ionized layer can produce QP scintillations in a manner analogous to a dense column of ionization but at lower ionization densities, as well as demonstrating a zenith angle and azimuthal dependence that is more consistent with observations than a column of ionization. At the occurrence times noted, the raypath may be intersecting the poleward edge of the trough where sporadic-E is a regular feature. QP scintillation events may result when the E_s-layer is height modulated by the passage of acoustic-gravity waves originating in the auroral zone.     

New world complex societies: Recent economic,social, and political studies

Archaeologists working on the complex societies of Latin America have made impressive empirical advances on a number of fronts during the past five years. This article focuses on economic, social, and political issues, using the following topics for organization: intensive agriculture, demography, exchange, households, urbanism, chiefdoms, and state-level polities. For each topic, I review recent archaeological research and summarize pertinent theoretical issues. Unfortunately, the heavy accumulation of new archaeological results is not matched by the conceptual advances that are needed to explore the analytical significance of the data.     

A scintillation theory of fading in long distance HF ionospheric communications

Scintillation theory is used to study the fading of radio waves returned from the ionospheric F-region in the HF band at oblique incidence. For both low and high penetration frequencies calculations are made as a function of wave frequency, but emphasis is given to behaviour near the maximum usable frequency for one-hop F-transmission over distances up to 4000 km. Estimates are made of

• 1.(i) the fluctuations of phase both for long-term (~ 1h) and for short-term (~ the fading correlation time),
• 2.(ii) the fading correlation distance,
• 3.(iii) the quasi-period of fading,
• 4.(iv) the twinkling correlation distance,
• 5.(v) the quasi-period of twinkling,
• 6.(vi) the angular departure of the arrival azimuth from the mean and
• 7.(vii) the correlation bandwidth.

The slow fluctuations often experienced in long distance HF radio communications are a manifestation of twinkling, rather than fading. Under normal conditions in the F-region the correlation bandwidth is so large that the bandwidth of transmission at HF is controlled by the dispersive properties of the ionosphere, rather than by the scattering properties. The reverse is true in the presence of spread-F. For sufficiently strong spread-F the correlation bandwidth for long distance HF radio communications is of the order of only 10 Hz, thereby creating the phenomenon known as flutter fading. Theoretical estimates of the scintillation parameters are compared with experience in long distance HF ionospheric communications over the last 60 years. Agreement is promising, but future experiments should be designed in the light of theory; existing observational data do not provide some of the information needed.