Quartz microfabric of the Laxfordian Canisp Shear Zone,NW Scotland

The Canisp Shear Zone transects layered Lewisian gneisses near Lochinver, NW Scotland. It is a vertical ductile shear zone with a dextral shear sense, formed during Laxfordian amphibolite facies metamorphism, transposing the layering to new foliation and linear structures. Minerals in the layered gneisses show little or no shape fabric, while a strong shape fabric defines the foliation. For quartz, this shape fabric is accompanied by development of a preferred crystal orientation with fabric patterns reflecting the geometry of the shear deformation. The quartz fabric shows a pole-free area around the lineation with the c-axes concentrated in an asymmetric cross-girdle or a point maximum perpendicular to the shear plane, and a monoclinic symmetry consistent with the shear sense.     

On the occurrence and characteristics of intense low-altitude electric fields observed by Freja

High-resolution measurements by the double probe electric field instrument on the Freja satellite are presented. The observations show that extremely intense (up to 1 V m⁻¹) and fine-structured (<1 km) electric fields exist at auroral latitudes within the altitude regime explored by Freja (up to 1700 km). The intense field events typically occur within the early morning sector of the auroral oval (01-07 MLT) during times of geomagnetic activity. In contrast to the observations within the auroral acceleration region characterized by intense converging electric fields associated with electron precipitation, upward ion beams and upward field-aligned currents, the intense electric fields observed by Freja are often found to be diverging and located within regions of downward field-aligned currents outside the electron aurora. Moreover, the intense fields are observed in conjunction with precipitating and transversely energized ions of energies 0.5-1 keV and may play an important role in the ion heating. The observations suggest that the intense electric field events are associated with small-scale low-conductivity ionospheric regions void of auroral emissions such as east-west aligned dark filaments or vortex streets of black auroral curls located between or adjacent to auroral arcs within the morningside diffuse auroral region. We suggest that these intense fields also exist at ionospheric altitudes although no such observations have yet been made. This is possible since the height-integrated conductivity associated with the dark filaments may be as low as 0.1 S or less. In addition, Freja electric field data collected outside the auroral region are discussed with particular emphasis on subauroral electric fields which are observed within the 19–01 MLT sector between the equatorward edge of the auroral oval and the inner edge of the ring current.     

Organic Remains in Finnish Subglacial Sediments

Many sites in Fennoscandia contain pre-Late Weichselian beds of organic matter, located mostly in the flanks of eskers. It is a matter of debate whether these fragmentary beds were deposited in situ, or whether they were deposited elsewhere and then picked up and moved by glacial ice. The till-mantled esker of Harrinkangas includes a shallow depression filled with sand and silt containing, for example, several tightly packed laminar sheets of brown moss (Bryales) remains. It is argued that these thin peat sheets were transported at the base of the ice sheet, or englacially, and were deposited together with the silt and sand on the side of a subglacial meltwater tunnel. Subglacial meltout till subsequently covered the flanks of the esker near the receding ice margin. Information about the depositional and climatic environments was obtained from biostratigraphic analysis of the organic matter. Pollen spectra for the peat represent an open birch forest close to the tundra zone. A thin diamicton beneath the peat contains charred pine wood, recording the former presence of pine forests in western Finland. The unhumified, extremely well-preserved peat evidently originated during the final phase of an ice-free period, most probably the end of the Eemian Interglaciation. It was redeposited in the esker by the last ice sheet. Reconstructions of the Pleistocene chronology and stratigraphy of central Fennoscandia that rely on such redeposited organic matter should be viewed with caution.     

The dynamical evolution of massive black hole binaries - II. Self-consistent N-body integrations

We use a hybrid N-body program to study the evolution of massive black hole binaries in the centers of galaxies, mainly to understand the factors affecting the binary eccentricity, the response of the galaxy to the binary merger, and the effect of loss-cone depletion on the merger time. The scattering experiments from paper I (Quinlan, 1996)[NewA, 1, 35] showed that the merger time is not sensitive to the eccentricity growth unless a binary forms with at least a moderate eccentricity. We find here that the eccentricity can become large under some conditions if a binary forms in a galaxy with a flat core or with a radial bias in its velocity distribution, especially if the dynamical friction is enhanced by resonances as suggested by Rauch & Tremaine (1996)[NewA, 1, 149]. But the necessary conditions seem unlikely, and our prediction from paper I remains unchanged: in most cases the eccentricity will start and remain small. The ejection of stars caused by the hardening of a binary may explain why large elliptical galaxies have weaker density cusps than smaller galaxies. If so, the central velocity distributions in those galaxies should have strong tangential anisotropies. The wandering of a binary from the center of a galaxy counteracts the effects of loss-cone depletion and helps the binary merge.     

Exploiting Illusory Grid Lines for Object-Location Memory Performance in Urban Topographic Maps

In order to be successful in spatial orientation tasks, people need to recall locations and configurations of spatial objects from their memory. This understanding of geographic space often arises from experience with cartographic media representing topographic and topological information by graphic symbols. Learning spatial information from graphic media is influenced by different perception-based grouping effects distorting the accuracy of spatial object-positions and their relations. Such geometric inaccuracies can be softened by adding a grid layer, which regionalizes the map and can be used as an additional orientation pattern. This grid layer usually consists of solid lines and overlays semantic information. The present paper reports the results of two empirical studies on object-location memory (OLM) performance. In these studies, the amount of visual detail of the grid layer was reduced. By positioning the grid layer below specific urban topographic objects (study 1), the grid pattern was graphically interrupted. These interrupted grid lines were completed by cognitive completion mechanisms (illusory grid lines) described in the Gestalt principles of closure and continuation. The second experiment examined the maximum grid line gap that is closed by cognitive line completion and keeps an advantage for OLM (study 2).     

Almost analytic models of ultramagnetized neutron star envelopes

Recent ROSAT measurements show that the X-ray emission from isolated neutron stars is modulated at the stellar rotation period. To interpret these measurements, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. We present nearly analytic models of the thermal structure of the envelopes of ultramagnetized neutron stars. Specifically, we examine the limit in which only the ground Landau level is filled. We use the models to estimate the amplitude of modulation expected from non-uniformities in the surface temperatures of strongly magnetized neutron stars. In addition, we estimate cooling rates for stars with fields B  ∼ 10¹⁵ − 10¹⁶ G, which are relevant to models that invoke 'magnetars' to account for soft γ-ray emission from some repeating sources.