Showing papers by "University of St Andrews published in 1983"

Wave propagation in a magnetic cylinder

Abstract: The nature of oscillations in a magnetic cylinder embedded in a magnetic environment is investigated. It is shown that the standard slender flux tube analysis of a kink mode in a cylinder excludes the possibility of a second mode, which arises under photospheric conditions. Under coronal conditions, two widely separated classes of oscillation can be freely sustained, one on an acoustic time-scale and the other on an Alfvenic time-scale. The acoustic-type oscillations are always present, but the much shorter period, Alfvenic-type, oscillations arise only in high density (strictly, low Alfven velocity) loops. An application to waves in fibrils is also given, and suggests (following Wentzel, 1979) that they are fast kink waves propagating in a density enhancement.

Anisotropies in the perception of three-dimensional surfaces

Abstract: The appearance of certain three-dimensional surfaces was found to depend on the orientation of the depth contours forming the surface. This was true both when the depth was specified by motion parallax and when it was specified by binocular disparities. Slowly changing depth surfaces that generated a pattern of relative motions or disparities characterized by a one-dimensional expansion-compression were perceived differently from those that produced a shear transformation.

Fast pulsations in the solar corona

Abstract: Pulsations in radio emission from the solar coronal plasma have been detected for over a decade1–9. The oscillations are quasiperiodic, with periods of typically a second or so. Recently, sub-second time structures have been found in hard X rays10, and simultaneously in hard X rays and microwaves11. Here we examine whether magnetohydrodynamical oscillations in a density enhancement, treated for simplicity as a straight magnetic slab, can explain the observed short periods. A dense region in the corona (for example, a loop) can act as a wave trap, and symmetrical oscillations within that trap must be of short wavelength with correspondingly short period. An impulsive source (such as a flare) naturally gives rise to a quasiperiodic disturbance. Such oscillations are closely akin to the Pekeris modes of oceanography, the Love waves of seismology and the dielectric waves of fibre optics.

A numerical experiment relevant to line-tied reconnection in two-ribbon flares

Abstract: The nonlinear evolution of a reconnecting magnetic field configuration similar to that occurring just before the onset of ‘post’-flare loops in two-ribbon flares is determined. The evolution, which is obtained by numerically solving the resistive MHD equations, shows two new features that have not yet been incorporated into contemporary models of ‘post’-flare loops. The first of these new features is the formation of a nearly stationary fast-mode shock above the region corresponding to the top of the loops. This fast-mode shock occurs just below the magnetic neutral line and between the slow-mode shocks associated with fast magnetic reconnection at the neutral line. The second new feature is the creation and annihilation of large-scale magnetic islands in the current sheet above the loops. The annihilation of the islands occurs very rapidly and appears to be a manifestation of the coalescence instability. The creation and annihilation of magnetic islands could be important in understanding the energetics of ‘post’-flare loops since the coalescence instability can produce an intermittent energy release more than an order of magnitude faster than that predicted by steady-state reconnection theories.

Identification and probable role of a single neurone containing the neuropeptide Helix FMRFamide.

Abstract: Recently, there has been intense interest in the possibility that peptides might function as neurotransmitters1,2. Despite much progress, there remains no clear-cut example in which the production of a chemically characterized peptide may be ascribed to individual identifiable neurones of proven physiological role/Invertebrate systems have proved to be particularly valuable for the study of identified neurones, including those producing peptides3,4. We have now identified a neurone in ganglia of Helix that is associated with a peptide of the Phe-Met-Arg-Phe-NH2 (FMRFamide) group, and which influences tentacle contraction. This system offers for the first time the capacity to study peptidergic transmission in a system in which both the cell soma and the postsynaptic target may be readily and reproducibly identified.

Magnetic flux tubes on the Sun

Abstract: Magnetic fields are the cause of almost all forms of solar activity. Near the solar surface, and possibly in the entire convection zone, these fields occur in the form of isolated flux tubes. In recent years, new views have been developed (and older ones revived) in which this property plays a central role. Here we review these ideas, dealing with the nature of the solar cycle, sunspot structure, the origin of spicules and the source of mechanical heating in the solar atmosphere. The ideas are illustrated with the aid of a simple mathematical model for the behaviour of thin magnetic flux tubes. The properties of inhomogeneities in the corona (coronal loops) are also discussed.

The in-crystal polarizability of the fluoride ion

Abstract: An ab initio value of the polarizability of the F- ion in the LiF crystal has been found by coupled Hartree-Fock theory with Moller-Plesset correlation corrections. The calculation separates the effects on the polarizability of the electrostatic crystal field and overlap with the six nearest Li+ neighbours. We find α = 0·92 A3, in excellent agreement with experiment. The correlation contribution is very sensitive to the crystal environment.

Utilization of the Ethanol Pathway in Carp Following Exposure to Anoxia

Abstract: 1.Crucian carp ( Carassius carassius L.) and Common carp ( Cyprinus carpio L.) were subjected to 2h progressive hypoxia followed by up to 6 h anoxia in closed respirometers at 15°C. 2.The concentrations of glycogen, glucose, phosphoryl creatine, alanine, succinate, lactate, ethanol and ammonia were determined in whole Crucian carp following exposure to both hypoxia and anoxia. 3.Ethanol and lactate were the main anaerobic end products. Glycogen utilization during anoxia amounted to 2mmol glucose equivalents.kg wet weight −1 h −1 . Around 85% of the ethanol produced (2.8mmolkg −1 h −1 ) was excreted. Lactate accumulation (0.7 mmol kg −1 h −1 ) was only sufficient to account for 18.5% of the glycogen stores utilized. Ammonia production(0.2 mmol.kg body wt −1 h −1 ) was independent of the environmental oxygen availability. 4.Ethanol concentrations (μmol.g wetwt −1 ) were 7 in red myotomal muscle, 4 in white myotomal muscle, and 2 in liver after 6h anoxia. 5.In contrast to Crucian carp, Common carp ( Cyprinus carpio L.) did not produce ethanol at 15°C even after near lethal exposure to anoxia (5–6 h).

Wave propagation in intense flux tubes

Abstract: The nature of non-adiabatic wave propagation in a slender magnetic flux tube is explored. The results of the theory are compared with the observations of Giovanelli et al. (1978), and found to be in general agreement. Those observations, of tubes in the photosphere-chromosphere, show outwardly propagating waves, with periods of 300 s, which take some 19 s to propagate from one level of line formation to another level higher in the atmosphere. In sharp contrast to this, is the time of 7 s for a similar disturbance outside the tube to propagate between the same two levels of line formation, estimated to be some 600 km apart in the field-free atmosphere.

Temporal and interference fringe analysis of TEM 01 * laser modes

Abstract: The properties of TEM01* doughnut modes have been examined by frequency analysis and by two-beam interference. Interpretation in terms of an evolving helix, made up to two orthogonal TEM01 modes of different frequency, is supported by computer simulation of fringe patterns. These patterns are shown to correspond closely with photographic recordings; the implications of the phenomena for the crossed-beam technique in laser velocimetry are outlined. Finally, the possibility of developing a beam of pure helical cophasal surface is discussed and its interference patterns analyzed.

On a common abstraction of de Morgan algebras and Stone algebras

Abstract: We consider a common abstraction of de Morgan algebras and Stone algebras which we call an MS-algebra. The variety of MS-algebras is easily described by adjoining only three simple equations to the axioms for a bounded distributive lattice. We first investigate the elementary properties of these algebras, then we characterise the least congruence which collapses all the elements of an ideal, and those ideals which are congruence kernels. We introduce a congruence which is similar to the Glivenko congruence in a p-algebra and show that the location of this congruence in the lattice of congruences is closely related to the subdirect irreducibility of the algebra. Finally, we give a complete description of the subdirectly irreducible MS-algebras.

Common anion heterojunctions: CdTe-CdHgTe

Abstract: The problem of band distortions at heterojunctions between semiconductors with common ionisation energies is solved numerically. The case of CdTe-CdHgTe heterojunctions is treated specifically, including effects of interdiffusion of alloy constituents and dopants. Analytic approximations are derived for conditions controlling the occurrence or abscence of maxima and minima in the conduction band profile at the interface. The calculations are applied to show the range of suitability of CdTe-CdHgTe heterojunctions for photodiode production, and for CdTe passivation of inter-diode regions.

Physiology of Laminaria

Abstract: . Sporophytes of Laminaria digitata and L. saccharina were collected from the shore at different times of year. Intact sporophytes of both species, and discs cut from L. digitata lamina tissue were maintained for several weeks in laboratory cultures under various nitrate, phosphate, temperature and daylength regimes. Substrate-saturated uptake rates of approximately 24 μgN g dry weight-1 h-1 from 20 μM nitrate and 8 μgP g dry weight-1 h-1 from 10 μM phosphate were more than sufficient to account for internal nitrogen and phosphorus accumulation. Other nitrogen sources - nitrite, ammonium and urea - were also taken up, independently of each other, and supported growth. During the late spring decline of ambient nutrient levels, when growth rates on the shore also decline markedly, enrichment with nitrate (15 μM) and phosphate (3μ) together prevented this decline and also maintained peak photosynthetic rates (net photosynthesis = 10.4 μlO2 cm-2 h-1) which otherwise dropped to 4.5 μO2 cm-2 h-1 over 47 days at ambient nutrient levels (0.5 μM nitrate and phosphate). Slow summer growth rates in June/July were enhanced to greater than spring peak values by combined nitrate (7.5 μM) and phosphate (1.8 μM) enrichment; neither was sufficient alone, although the individual nutrients were accumulated internally. A lesser, but significant enhancement was also achieved in September. In midwinter, nutrient enhancement itself (10.5 μM nitrate and 3.0 μM phosphate) had a small effect on growth rates only if summer water temperature (15°C) was used, but the dramatic effect of an increased photoperiod (7.5 to 17.5 h) was in excess of that expected for the increased radiant energy available. This was found to be due, at least in L. digitaia discs, to the re-establishment of a surface meristoderm in the dormant winter tissue; this was particularly active with high phosphate supply. Growth of the new lamina in January on the shore was much reduced if the old lamina was removed, although the time of initiation of new growth remained unchanged. In the laboratory, the old lamina was found to be a source of nitrogen for new lamina but not of fixed carbon or phosphorus. New lamina photosynthesis, manifest as mannitol accumulation, was directly proportional to phosphate supply up to at least 7.5 μM phosphate. Mannitol accumulation was then suppressed by a nitrate supply above 12 um, presumably by diversion of fixed carbon to other biosynthetic pathways.

Nasotemporal overlap in the human retina investigated by means of simple reaction time to lateralized light flash

Abstract: When light flashes are presented laterally simple vocal and manual responses are faster to stimuli in the visual half-field having direct access to the responding hemisphere (an “uncrossed” reaction) than stimuli which go initially to the nonresponding hemisphere (a “crossed” reaction). In the latter case an interhemispheric crossing is presumably necessary and so the crossed-minus-uncrossed difference (CUD) can be identified with interhemispheric transmission time. This paradigm was used to investigate the problem of whether or not there is an overlap of ipsi- and contralaterally projecting ganglion cells at the border between nasal and temporal areas of the human retina, resulting in dual representation of the midline in the brain. If such an overlap does exist then presenting stimuli on this region ought to result in an abolition of the CUD since information would be equally available to both hemispheres. Accordingly vocal and manual reaction times to flashes presented at 1/2, 1, 2, and 4 deg of visual angle were measured. In both cases a consistent CUD was found and this was present at all four points. These results are interpreted as arguing against the existence of overlap in man though some alternative possibilities are discussed.

Compositional range of Mn-garnet in zoned granitic pegmatites

Abstract: A relationship has been established between garnet composition (determined by gravimetric analysis) and zones in large complex granitic pegmatites. Mn-rich garnet (MnO 4l-270/o) occurs in the inner and replacement zones, garnet with Mn - Fe in intermediate mineral assemblages and garnet with Fe > Mn in the wall and contact zones. Pure spessartine garnet is typical of replacement and quartz-rich core zones of pegmatites containing lithium minerals such. as spodumene, Iepidolite, petalite and amblygonite.