A wave guide model of lightning currents

In this paper, a lightning channel is simulated by a vertical wire of finite length and of moderate electric conductivity. At low frequencies, such a wire behaves like a resonant wave guide cavity in which only discrete resonant wave modes can be generated. The theory of resonant waves in a wire of finite diameter is outlined and applied to the return stroke with electric contact to the earth and to intracloud K-strokes without electric contact to the earth. Two types of current wave forms exist: aperiodic waves of the Bruce-Golde type and damped oscillations. The configurations of the electric charge density and the electric current in the wire are presented for the various wave modes as functions of height and time. From the observations of lightning current and of sferic wave forms, it is apparent that the first mode dominates, and that both types of wave forms can be identified. From these wave forms, the channel parameters-length and diameter can be derived. For the higher order modes, the resonance model loses its validity and freely propagating waves are expected during the whole lightning flash. These waves contribute to the observed continuous spectrum. The skin effect becomes important increasing the effective resistance of the wire with frequency. Since these waves interfere destructively, they do not contribute significantly to an effective charge transport.     

Observational evidence of a saturated gravity wave spectrum in the mesosphere

Five vertical profiles of scalar horizontal winds have been measured at high resolution (25m) in the range from 80–95 km during the last salvo of the MAC/SINE campaign in the summer 1987 at Andenes, Northern Norway (69.3°N). Our purpose in this study is to examine the consistency of the motion spectrum with the saturated spectrum of gravity waves proposed by Smith S. A., Fritts D. C. and Van Zandt T.E., (1987, J. atmos. Sci. 44, 1404). An analysis of vertical wavenumber spectra of the five horizontal wind profiles is presented and it is found that (a) the average slope of the five vertical wavenumber spectra is −3.0 ± 0.2 for wavelengths in the range from 6.4 km to 100 m. The slope is considerably steeper than the vertical wavenumber spectra of the horizontal velocity discussed in the literature, (b) the average vertical wavenumber spectrum shows that there is excellent agreement between the observed spectrum and the saturated spectrum in both slope and amplitude, suggesting that saturation processes do indeed act to control spectral amplitudes at large wavenumbers, and (c) a dominant vertical wavelength of 6.4 km is found in the mesosphere. Taken together, our observations provide further support for the saturated spectrum theory.     

Association of whistlers with lightning discharges on the Earth and on Jupiter

The association between whistlers and lightning discharges has been reviewed on the basis of terrestrial ionospheric satellite observations of VLF radio noise. Evidence indicating that the observed low-latitude radio noise is associated with thunderstorms includes (1) amplitude distribution and noise properties, (2) geographical location, (3) diurnal variation in activity, and (4) diurnal variation of frequency spectrum. Corresponding studies on the propagation of sferics in the ionosphere and the excitation of whistlers recently carried out for Jupiter are presented here and compared with the terrestrial studies.     

Observational evidence of local turbulence production due to gravity wave saturation in the summer mesosphere

Five foil chaff and two falling sphere rockets flown during the MAC/SINE Campaign on 15 July 1987 at Andenes, Northern Norway (69°17′N). From these rocket measurements, turbulent energy dissipation rates, vertical wind shears and Richardson numbers as functions of height were derived in the range from 82 to 92km. Turbulent energy dissipation rates generally range from 1.4 × 10⁻⁵ to 2.0 × 10⁻²W/kg and are consistent with other experiments performed at the same latitude. Strong wind shears of the order of 50–90 m/s/km are observed at various heights. Good correspondence between turbulence intensity peaks, regions of strong wind shear and low Richardson number is found. Vertical wavenumber spectra of the five scalar winds measured by the foil chaff rockets indicate that there is an excellent agreement with the saturation hypothesis, suggesting that the turbulence intensity peaks measured in this salvo are linked directly to the saturation of gravity wave motions via dynamical instabilities.     

Notes on the diversity of the properties of radio bursts observed on the nightside of Venus

We report on further studies of radio wave bursts detected by the Orbiting Electric Field Detector (OEFD) on PVO in the nightside ionosphere of Venus. We have tested a total of 25 cases of wave burst activity for evidence of whistler-mode propagation to the spacecraft from impulsive subionospheric sources. As in a previous study of 11 of these cases (Sonwalkar et al., 1991), we find at least two distinct classes of events, one, mostly involving bursts at 100 Hz only, that passes certain tests for whistler-mode propagation, and another, mostly involving bursts in two or more of the four PVO narrowband channels (at 100 Hz, 730 Hz, 5.4 kHz, and 30 kHz), that fails to pass the tests. The subionospheric lightning hypothesis continues to be tenable as a candidate explanation for many of the 100 Hz-only events, but its plausibility could be better evaluated if mechanisms could be found to explain the existence of a significant number of 100 Hz-only cases that do not pass all the applicable whistler-mode tests, as well as the existence at a wide range of altitudes of multichannel cases that are clearly not propagating whistler-mode waves. The wideband bursts are often observed at altitudes above 1000 km and frequently occur in regions of locally reduced electron density. Those observed at high altitude (and possibly at low altitude as well) are believed to be generated near the spacecraft, possibly by an as yet unknown mechanism responsible for similar burst observations made near Earth and other planets.     

New methods for the identification of evidence for bitting on horse remains from archaeological sites

This article describes alterations to equid lower second premolars and diastemata from a series of known life history equids and a number of archaeological horse specimens from the British Iron Age. Two new methods for recording bit wear are proposed involving the analysis of the extent and morphology of enamel/dentine exposure on the anterior edge of LP2s and analysis of the extent of new bone formation and bone loss to the diastema of the mandible. It is suggested that when a bit is used on a horse it acts more frequently on the anterior margin of the LP2 than has previously been thought and that repeated contact between the bit and the LP2s and diastemata results in recognisable damage to these areas of the mouth.     

A deconvolution technique for measuring D-region radio wave backscatter

A new deconvolution technique similar to that used by Jones and Thiele (1991) was used to determine the positions and Doppler shifts of D-region scatteres at Bribie Island (152°E, 27°S). The new technique performs analysis in the frequency domain. It is introduced and measurements of the aspect sensitivity and angular spread of D-region backscatter at 1.98 MHz are shown. These results are similar to those obtained by others.     

Patching up evidence for ironworking in the horn

Archaeological research on ironworking in the Horn of Africa is meager and patchy. This paper offers hypotheses for the origins of ironworking in the region. Two conventional schools of thought are analyzed. The first school favors an external origin and the second advocates local invention. The two viewpoints are critically examined and their weaknesses exposed. A third alternative, which, in principle, combines the two, is provided in their place. This looks at the history of the Horn in a broad spatial and integrating perspective. It attempts to show that the current physical barriers such as the Red Sea, the Nile Valley, and the cataracts of the Nile River have not always been as impenetrable as we tend to think. There is evidence that sometimes in the past, people from the opposite sides of these “barriers” shared language, commerce, religion, and politics. The paper also discusses the impact of ironworking in the region and the technological influence of the Horn on sub-Saharan Africa. Finally, the paper offers some suggestions for future directions toward a more comprehensive understanding of Iron Age culture and the socioeconomic history of the region.     

A numerical model for gravity wave dissipation in the thermosphere

Two simplified models of internal gravity wave dissipation due to viscosity, thermal conduction and ion-drag, in a multilayered, isothermal thermosphere are developed. Each of these models uses the WKB approximation, ray theory and the time-averaged equation of energy conservation, but whereas one of the models (A) employs all of the gravity wave equations appropriate to a dissipative atmosphere, the other (B) does not. Results derived from these models for one particular wave are compared to each other and also to some previously published results of Klostermeyer, which employed a full-wave, model. A breakdown of the WKB approximation in the lower, non-isothermal thermosphere leads to models A and B underestimating the total dissipation there. In the middle thermosphere model A estimates the dissipation reasonably well, while model B grossly overestimates the dissipation. In the upper thermosphere model A underestimates the total upward energy flux, probably as a result of the neglect of coupling into the dissipative waves at these levels, while no energy remains in model B. Results from model A show that when dissipation due to viscosity and thermal conduction are included correctly and simultaneously, the dissipation due to viscosity can exceed that due to thermal conduction by a factor of three. It is argued that ray theory may either overestimate or underestimate the energy flux reaching the upper boundary of a thermospheric model depending on both its height and the particular thermospheric model used.     

The effect of multi-duct structure on whistler-mode wave propagation

Ducting of whistler-mode waves is investigated by ray-tracing in a magnetospheric model in which multiple and complex duct structures are superimposed on a smooth magnetospheric plasma distribution. When two or more ducts are present, propagation through a duct in which the ray does not become trapped is found to result in little deviation of the ray path, showing that upgoing waves can traverse several ducts before becoming trapped in one that is suitably positioned.The presence of two ducts situated in the same meridian plane and close together in L-value (ΔL ~ 0.07) is found to enable a double-duct trapping mode. This has the special property of guiding waves with frequency above half the local electron gyrofrequency across the equatorial plane in such a way that they become retrapped in the duct from which they had previously escaped at its local detrapping frequency. This may explain the observation of whistlers with a particularly high ratio of cut-off frequency to nose frequency (Bernhardt, 1979).Ducting is also investigated for a more complex duct structure in which fine structure is superimposed on a broader larger enhancement main duct. Here, it is found that rays which are first trapped to propagate in the main duct at low altitude can be further trapped to be ducted inside fine structure enhancements at higher altitude. This can result in certain components of multipath whistlers always being excited together and also having a common exit-point in the lower ionosphere. This is shown to be consistent with experimental observations.     

Weathering evidence for an additional glacial stage in Tasmania

Quantitative studies of relative dating parameters, including subsurface dolerite weathering rind thickness and weathering front penetration, permit the differentiation of three broad groupings of till deposits in several valleys in north-central, western, western-central and south-western Tasmania. An early cold climatic phase–the Linda Glacial Stage–is recognised on the basis of this weathering evidence which also indicates that there have been at least two more recent Glacial Stages of decreasing severity, termed here the Henty and Margaret Glacial Stages respectively. This three-fold model bears a striking similarity to that of Lewis (1945) which was rejected nearly three decades ago. However, whereas Lewis' model was founded upon erosional evidence, the present study is based entirely upon depositional and weathering evidence.     

Anatomical evidence for the antiquity of human footwear use

Archeological evidence suggests that footwear was in use by at least the middle Upper Paleolithic (Gravettian) in portions of Europe, but the frequency of use and the mechanical protection provided are unclear from these data. A comparative biomechanical analysis of the proximal pedal phalanges of western Eurasian Middle Paleolithic and middle Upper Paleolithic humans, in the context of those of variably shod recent humans, indicates that supportive footwear was rare in the Middle Paleolithic, but that it became frequent by the middle Upper Paleolithic. This interpretation is based principally on the marked reduction in the robusticity of the lesser toes in the context of little or no reduction in overall lower limb locomotor robusticity by the time of the middle Upper Paleolithic.