Dennis Sciama, 1926–1999

Fellow of the RAS, Fellow of the Royal Society, inspring cosmologist and leader.
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Quantitative particle size,microtextural and outline shape analyses of glacigenic sediment reworked by paraglacial debris flows

Particle size analysis and scanning electron microscopy have been used with some success to differentiate sediments sampled from varying geomorphological environments. In the paraglacial environment, however, discrimination of paraglacially reworked deposits from in situ glacigenic deposits has proven to be problematic. We consider till sediments that have been reworked by paraglacial debris flows and in situ, unreworked parent material, and apply a series of quantitative techniques in an attempt to discriminate samples from each environment. Initial analysis of particle size suggests some eluviation of fines from the debris‐flow‐reworked deposits but is of insufficient significance to allow reliable differentiation of geomorphological environment. Similarly, although slight differences are identified in terms of quartz grain surface microtextures and quartz grain outline shape, subsequent PCA, cluster and Fourier analyses fail to reliably differentiate quartz grains from each geomorphological environment, owing to high levels of intra‐sample heterogeneity. This lack of apparent difference may arise from a lack of paraglacial process operation of significant magnitude or duration for a characteristic process imprint to emerge at this scale of observation. Accordingly, further assessment of particle‐size and microtextural characteristics of glacial and paraglacial sediments may be most fruitful where sediments have been reworked over long distances. Moreover, differentiating glacial from paraglacial deposits is likely to yield the most reliable results where rigorous statistical analyses are combined with a wide range of sedimentological and geomorphological techniques. Copyright © 2008 John Wiley and Sons, Ltd.     

Response of estuarine meio- and macrofauna to in situ bioremediation of oil-contaminated sediment

Numerous studies have demonstrated the efficacy of bioremediation for enhancing oil removal but the ecological effect on shoreline biota is unclear. Therefore, a field experiment was designed at an intertidal sandflat in SW England to assess the effects of nutrient addition to oiled sediments on meio- and macrofauna for a period of up to 45 weeks. Natural assemblages were exposed to different types of experimental treatments (no oil, oil alone, oil treated with slow-release fertiliser or liquid fertiliser). Bioremediation stimulated the microbial population and increased oil biodegradation. This, however, did not result in faster recolonisation rates of fertilised versus non-fertilised oiled sediments. Mild effects of oil and bioremediation treatments on benthic fauna were observed, including short-term shifts in dominance patterns. Decreased abundance of dominant species in the oiled compared to unoiled sediments resulted in significantly higher evenness of benthic assemblages within the first 11 weeks of the experiment.     

Helium escape from the Earth''s atmosphere: The charge exchange mechanism revisited

We have studied the escape of neutral helium from the terrestrial atmosphere through exothermic charge exchange reactions between He⁺ ions and the major atmospheric constituents N₂, O₂, and O. Elastic collisions with the neutral background particles were treated quantitatively using a recently developed kinetic theory approach. An interhemispheric plasma transport model was employed to provide a global distribution of He⁺ ions as a function of altitude, latitude and local solar time and for different levels of solar ionization. Combining these ion densities with neutral densities from an MSIS model and best estimates for the reaction rate coefficients of the charge exchange reactions, we computed the global distribution of the neutral He escape flux. The escape rates show large diurnal and latitudinal variations, while the global average does not vary by more than a factor of three over a solar cycle. We find that this escape mechanism is potentially important for the overall balance of helium in the Earth's atmosphere. However, more accurate values for the reaction rate coefficients of the charge exchange reactions are required to make a definitive assessment of its importance.     

The role of neutral winds and ionospheric electric field in forming stable sporadic E-layers

Using the combined measurements from a rocket flight through a stable intense sporadic E-layer, we examine the shortcomings of conventional wind shear theory of ion layer formation, principally in underestimating the role of the ambient ionospheric electric field. Our results imply that the ionospheric electric field may control the stability and precise location of such ionisation layers within a region of convergent ion flow.     

On the interaction of auroral protons with the earth's atmosphere

The interaction of energetic auroral protons with the atmosphere is investigated. The results of a random number algorithm that describes the proton-hydrogen interconversion reactions as the beam loses energy are adopted to construct an energy deposition curve applicable over a wide range of initial proton energies. lonization rates and production rates of ejected electrons are computed and emission rates of hydrogen Balmer alpha and beta lines are evaluated using recently available low energy cross-sections.     

Limits from rapid TeV variability of Mrk 421

The extreme variability event in the TeV emission of Mrk 421, recently reported by the Whipple team, imposes the tightest limits on the typical size of the TeV emitting regions in active galactic nuclei (AGN). We examine the consequences that this imposes on the bulk Lorentz factor of the emitting plasma and on the radiation fields present in the central region of this active nucleus. No strong evidence is found for extreme Lorentz factors. However, energetics arguments suggest that any accretion in Mrk 421 has to take place at low rates, compatible with an advection-dominated regime.     

A magnetic fabric study of Late Whitcliffian (U. Silurian) siltstones from the Welsh Borderland

The uppermost Whitcliffian strata of the Welsh Borderlands record the final phase of marine sedimentation in the Silurian basin and shelf environments. They give way transitionally to the succeeding non-marine Lower Old Red Sandstone. As a contribution to the understanding of the change from marine to continental facies, the magnetic fabric method was used to obtain palaeocurrent data for the uppermost Whitcliffian. The results confirm that the uppermost Whitcliffian palaeocurrent pattern is essentially “Silurian” in character and does not foreshadow the deltaic and littoral patterns of sediment distribution observed in the basal Old Red Sandstone.