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.     

Charlemagne's involvement in central and southern Italy: power and the limits of authority

This article is an overview of political developments in southern Italy during Charlemagne's reign. Traditionally the historiography has approached this topic from a Carolingian or papal perspective. Without denying the reality of both papal and Carolingian influence, the article argues that neither of these institutions exercised predominant influence in southern Italy in this period, much as they may have wished to. Rather the pattern of political (and to an extent ideological) development in the area was determined by a series of compromises dictated by self-interest and the limits of power. This article therefore deals in turn with the evidence concerning the main protagonists in the south: the abbey of Farfa, the dukes of Spoleto, the monasteries of Monte Cassino and San Vincenzo al Volturno and the princes of Benevento. The article goes on to argue that the activities of these institutions are driven by self-interest. Finally the paper concludes that in the 790s there is a change in the way Carolingian government worked, at least in Spoleto.     

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.     

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.     

Recent results in auroral-zone scintillation studies

Nighttime data from the Defense Nuclear Agency's Wideband satellite have consistently shown a pronounced scintillation enhancement at the point where the propagation vector lies within the local L-shell. Simultaneous observations at two well-separated stations and spaced-receiver measurements have shown that this feature is caused by a latitudinally narrow, unstable F-region ionization enhancement that produces sheet-like intermediate-scale structures. A more detailed characterization of the source region has been derived from simultaneous measurements from the TRIAD satellite and the Chatanika radar.The source region is typically convecting southward, whereby the poleward gradient at its southern boundary is stable to the gradient-drift instability. Thus, Ossakow and Chaturvedi (1979) postulated the current-convective instability as a source mechanism. Birkeland currents, when sufficiently intense, can destabilize a region that is otherwise stable to the gradient-drift instability.The sheet-like anisotropy of the irregularities is perhaps the most intriguing feature of the instability. Two mechanisms have been postulated to explain it. This paper reviews the development and current status of our understanding of this recently discovered high-latitude instability.     

The accuracy of computing ionospheric radio-wave scintillation by the thin-phase-screen approximation

In the calculation of the diffraction effects produced by a thick medium containing spatial irregularities in refractive index, a commonly-used approximate method is to represent the thick medium by a thin phase-changing screen. It is sometimes assumed that this gives a poor approximation when the medium is thick enough to produce appreciable amplitude fluctuations in the emergent wave. It is shown here that under practical ionospheric conditions the thin screen method still gives results in very good agreement with those obtained from a single-scattering calculation, even at gigahertz frequencies.     

Double structure of ionospheric conductivity in the midnight auroral oval during a substorm

Measurements of precipitating particles on board DMSP F7 spacecraft are used to analyze the distribution of ionospheric conductance in the midnight auroral zone during substorms. The distribution is compared with the meridional profile of ionospheric currents calculated from magnetic data from the Kara meridional chain. Two regions of high Hall conductance are found; one of them is the traditional auroral zone, at latitudes 64–68°, and the other is a narrow band at latitudes 70–73°. The position of high conductance zones is in agreement with the location of the intense westward currents. The accelerated particle population is typical of electrons E_e > 5 keV in the high conductance region.     

Magnetospheric tail structure: concepts,problems and storm-time development of the auroral oval

This paper reviews current knowledge on links between the Earth's magnetic tail and the auroral oval, and identifies some problems remaining. It considers electrons as tracers of the geomagnetic field, boundaries between different regions, plasma flows, and pressure balance conditions. The auroral arc is considered as a standing discontinuity in the flow of central plasma sheet (CPS) plasma field-aligned current systems are also proposed. The plasma instability responsible for the breakup phase of an auroral substorm is also discussed.     

The Biafran secession and the limits of self-determination

The Biafran secession crisis raised a series of profound and unanswerable questions about the nature and limits of self-determination, state sovereignty and African decolonization. A wide range of actors—both supporters and opponents of Biafra—viewed the Nigerian civil war and Biafra's attempt to carve out a new state as an important moment in the history of self-determination as a political and legal principle. Likewise, the collapse of Biafra seemed to offer a series of lessons for movements asserting their right to self-determination, as well as for those seeking to limit its application, suggesting the open-ended and contested nature of the concept even as it was institutionalized in the fabric of international human rights law. Ultimately, however, Biafra's failed secession attempt could only highlight the ambiguity and contested nature of sovereignty and self-determination in the international system, and the ability of groups such as the Igbos to exploit their indeterminacy in an effort to achieve their aims.     

Fabry-Perot spectrometer observations of the auroral oval/polar cap boundary above Mawson,Antarctica

Zenith observations of the oxygen λ1630 nm auroral/airglow emission (produced at an altitude of ∼220 to ∼250 km) were obtained with the Mawson Fabry-Perot Spectrometer (FPS) during three ‘zenith direction only’ observing campaigns in 1993. The data show many instances of strong (50 to 100 m s⁻¹) upwellings in the vertical wind, when the auroral oval is located equatorward of the zenith. Our data appear consistent with the existence of a region of upwelling up to ∼ 4° poleward of the poleward boundary of the visible auroral oval, rather than short duration, explosive heating events. The upwellings are probably the vertical component of wind shear produced by reversal of the zonal thermospheric winds, which occurs near the poleward boundary of the visible auroral oval. Zenith temperature was also seen to increase when the oval was equatorward of Mawson, showing rises of up to 300 K or more. However, this increase is at times unrelated to the upwellings, and seems to be caused by the expansion of the warm polar cap over the observing site.On a number of nights the boundary between the polar cap and the auroral oval was observed to pass over our site several times, occasionally showing a quasi-periodic expansion and contraction. We speculate that this quasi-periodic movement may be related to periodic auroral activity that is known to generate large-scale gravity waves.     

The limits of individualism,the ideology of the west

Louis Dumont. From Mandeville to Marx. Chicago: The University of Chicago Press. 236 pp. Appendix, notes, and bibliography. $17.50.     

The limits of objective interests

Jack L. Goldsmith and Eric A. Posner are influential academics in American debates on international law. Their recent book, The limits of international law, is appraised in this review article. It discusses the exclusion in their analysis of the possibility that states have a real or perceived interest in complying with international law per se. It explains that this is at odds with the authors' subjective approach to state interests, and essentially guarantees their conclusion that international law compliance is solely instrumental. Furthermore, the article discusses the problem that the authors fail to engage with relevant scholarship that provides a counterpoint to theirs, in particular with regard to constructivism. On the basis of these critiques the review concludes that the book is flawed and cannot be considered to have made its case.