Induced Magnetization of Magnetite-titanomagnetite in Alternating Fields Ranging from 400 A/m to 80,000 A/m; Low-field Susceptibility (100 – 400 A/m) and Beyond

For remanence-bearing minerals (RBM) such as magnetite-titanomagnetite, susceptibility to induced magnetism (M) measured in alternating fields (H _AC ) is field-dependent. However, for fields ≤ 400 A/m, measured in an AC induction coil instrument (at 19,100 Hz), susceptibility k ₀ = M/H _AC is sufficiently linear to provide a reproducible rock (or mineral) magnetic characteristic and its anisotropy may be related to arrangements of minerals in rock, or for single mineral grains to their crystalline or shape anisotropy. For any remanence-bearing mineral at higher fields k _HF (= M/H _AC ) is not constant and the term susceptibility is not normally used. This study bridges the responses between traditional low-field susceptibility measurements and those due to high applied fields, for example when studying hysteresis or saturation magnetization of RBM. Where |k _HF | is measured in alternating fields that peak significantly above 400 A/m the M(H _AC ) relation is forced to follow a hysteresis loop in which |k _HF | > k ₀ for small |H _AC | and where |k _HF | decreases to zero for very large fields that achieve saturation magnetization. Hysteresis nonlinearity is due to remanence acquired with one field direction requiring a reverse field for its cancellation. We investigate the transition from initial, traditional “low-field” susceptibility (k ₀ ) measurements at 60 A/m, through 24 different fields from 400 A/m to 40,000 A/m (for very high k ₀ to 80,000 A/m). This reveals M(H _AC ) dependence beyond from conventional k ₀ through the range of hysteresis behavior in fields equal to and exceeding that required to achieve saturation magnetization (M _S ). We show k _HF increases with peak H _AC until the peak field is slightly less than saturation magnetization in natural rock samples rich in magnetite (TM₀ = Fe₃O₄) and TM₆₀ (Fe_2.4Ti_0.6O₄). All sample suites predominantly contain multidomain grains with subordinate pseudo-single domain and single-domain grains. k/k ₀ increases by ≤ 5% for fields up to 2 kA/m. Above 4 kA/m k/ k ₀ increases steeply and peaks, usually between 24 kA/m and 30 kA/m where all grains magnetic moments are activated by H _AC since this exceeds the coercive force of most grains. For higher peak H _AC , k/k ₀ declines sharply as increased H _AC values more effectively flip M with each field-direction switch, leading to the low gradient at distal portions of the hysteresis loop. For M₀-TM₆₀ bearing rocks, susceptibility peaks for fields ~12 kA/m and for magnetite rich rocks up to 24 kA/m. These values are approximately 10% of saturation magnetizations (M _S ) reported for the pure minerals from hysteresis DC field measurements. Both the field at peak k/k ₀ and the peak k/k ₀ value appear to be controlled by the dominant domain structure; multidomain behavior has larger k/k ₀ peaks at lower H _AC . Stacked k/k ₀ versus H _AC curves for each sample suite (n = 12 to n = 39) were successfully characterized at the 95% level by a polynomial fit that requires the cubic form k/k ₀ = a + bH + cH ² + dH ³. Thus, for most M-TM bearing rocks, susceptibility and anisotropy of susceptibility (AMS) measurements made on different instruments would be sufficiently precise for most geological applications, if peak alternating fields are ≤700 A/m.     

Standing Conference of Astronomy Professors

The Chair Mike Bode and the Secretary Phil Charles bring the minutes of the 13 October 1999 meeting of the Standing Conference of Astronomy Professors, attended by representatives of 11 universities.     

Seasonal vegetation and management influence overland flow velocity and roughness in upland grasslands

There is considerable interest in how headwater management may influence downstream flood peaks in temperate humid regions. However, there is a dearth of data on flow velocities across headwater hillslopes and limited understanding of whether surface flow velocity is influenced by seasonal changes in roughness through vegetation cycles or management. A portable hillslope flume was used to investigate overland flow velocities for four common headwater grassland habitats in northern England: Low-density Grazing, Hay Meadow, Rank Grassland and Juncus effusus Rush pasture. Overland flow velocity was measured in replicate plots for each habitat, in response to three applied flow rates, with the experiments repeated during five different periods of the annual grassland cycle. Mean annual overland flow velocity was significantly lower for the Rank Grassland habitat (0.026 m/s) followed by Low-density Grazing and Rushes (0.032 and 0.029 m/s), then Hay Meadows (0.041 m/s), which had the greatest mean annual velocity (examples from 12 L/min flow rate). Applying our mean overland flow velocities to a theoretical 100 m hillslope suggests overland flow is delayed by >1 hr on Rank Grassland when compared to Hay Meadows in an 18 mm storm. Thus grassland management is important for slowing overland flow and delaying peak flows across upland headwaters. Surface roughness was also strongly controlled by annual cycles of vegetation growth, decay, grazing and cutting. Winter overland flow velocities were significantly higher than in summer, varying between 0.004 m/s (Rushes, November) and 0.034 m/s (Rushes, June); and velocities significantly increased after cutting varying between 0.006 m/s (Hay meadows, July) and 0.054 m/s (Hay meadows, September). These results show that seasonal vegetation change should be incorporated into flood modelling, as cycles of surface roughness in grasslands strongly modify overland flow, potentially having a large impact on downstream flood peak and timing. Our data also showed that Darcy-Weisbach roughness approximations greatly over-estimated measured flow velocities.     

The massive scalar field in a closed Friedmann universe model — new rigorous results

For the minimally coupled scalar field in Einstein's theory of gravitation we look for the space of solutions within the class of closed Friedmann universe models. We prove D ≥ 1, where D ≥ is the dimension of the set of solutions which can be integrated up to t → ∞ (D > 0 was conjectured by PAGE (1984)). We discuss concepts like “the probability of the appearance of a sufficiently long inflationary phase” and argue that it is primarily a probability measure μ in the space V of solutions (and not in the space of initial conditions) which has to be applied. μ is naturally defined for Bianchi-type I cosmological models because V is a compact cube. The problems with the closed Friedmann model (which led to controversial claims in the literature) will be shown to originate from the fact that V has a complicated non-compact non-Hausdorff Geroch topology: no natural definition of μ can be given. We conclude: the present state of our universe can be explained by models of the type discussed, but thereby the anthropic principle cannot be fully circumvented.     

Nearby galaxies. II. Luminosity function and spatial distribution

A sample of nearby galaxies (Schmidt and Boller 1992, paper I) has been analysed with regard to the luminosity function and the spatial distribution. The main results are as follows: (i) a relation between the slope of the luminosity function of the members of a group of galaxies and the earliest morphological type in this group has been detected which possibly is a new kind of environmental effects; (ii) the virial masses of the groups of galaxies are, on the average, by a factor 3 to 4 greater, only, than the luminous masses derived from the individual galaxy masses; (iii) the nearby galaxies are concentrated in a thin disk-like layer around the supergalactic plane the thickness of which is a few hundred kiloparsecs, only; (iv) a population of field galaxies with nearly constant density (about 3 per cent of the number density in the supergalactic plane) is extended into the voids on both the supergalactic hemispheres.     

Stalagmite growth perturbations from the Kumaun Himalaya as potential earthquake recorders

The central part of the Himalaya (Kumaun and Garhwal Provinces of India) is noted for its prolonged seismic quiescence, and therefore, developing a longer-term time series of past earthquakes to understand their recurrence pattern in this segment assumes importance. In addition to direct observations of offsets in stratigraphic exposures or other proxies like paleoliquefaction, deformation preserved within stalagmites (speleothems) in karst system can be analyzed to obtain continuous millennial scale time series of earthquakes. The Central Indian Himalaya hosts natural caves between major active thrusts forming potential storehouses for paleoseismological records. Here, we present results from the limestone caves in the Kumaun Himalaya and discuss the implications of growth perturbations identified in the stalagmites as possible earthquake recorders. This article focuses on three stalagmites from the Dharamjali Cave located in the eastern Kumaun Himalaya, although two other caves, one of them located in the foothills, were also examined for their suitability. The growth anomalies in stalagmites include abrupt tilting or rotation of growth axes, growth termination, and breakage followed by regrowth. The U-Th age data from three specimens allow us to constrain the intervals of growth anomalies, and these were dated at 4273?±?410 years BP (2673–1853 BC), 2782?±?79 years BP (851–693 BC), 2498?±?117 years BP (605–371 BC), 1503?±?245 years BP (262–752 AD), 1346?±?101 years BP (563–765 AD), and 687?±?147 years BP (1176–1470 AD). The dates may correspond to the timings of major/great earthquakes in the region and the youngest event (1176–1470 AD) shows chronological correspondence with either one of the great medieval earthquakes (1050–1250 and 1259–1433 AD) evident from trench excavations across the Himalayan Frontal Thrust.