Polarization of the Crab Nebula with disordered magnetic components

In this paper, we present an expanding disc model to derive the polarization properties of the Crab nebula. The distribution function of the plasma and the energy density of the magnetic field are prescribed as functions of the distance from the pulsar using the model derived by Kennel and Coroniti with  σ= 0.003  , where σ is the ratio of the Poynting flux to the kinetic energy flux in the bulk motion just before the termination shock. Unlike the case for previous models, we introduce a disordered magnetic field, which is parametrized by the fractional energy density of the disordered component. The flow dynamics are not solved, and the mean field is toroidal.
The averaged degree of polarization over the disc is obtained as a function of the inclination angle and fractional energy density of the disordered magnetic field. It is found for the Crab Nebula that the disordered component contains about 60 per cent of the magnetic field energy. This value is supported by the facts that the disc appears not as 'lip-shaped' but as 'rings' in the observed intensity map, and that the highest degree of polarization of ∼40 per cent is reproduced for rings, which is consistent with the observations.
We suggest that, because the disordered field contributes to the pressure rather than to the tension, the pinch force may have been overestimated in previous relativistic magnetohydrodynamic simulations. The disruption of the oppositely directed magnetic fields, which is proposed by Lyuvarsky, may actually take place. The relativistic flow speed, which is indicated by the front–back contrast, can be detected in the asymmetry of distributions of the position angle and depolarization.     

Trench triple junction off Central Japan—preliminary results of French-Japanese 1984 Kaiko cruise, Leg 2

Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the trench triple junction off central Japan, where the Japan, Izu-Bonin and Sagami Trenches intersect. The Izu-Bonin Trench is deeper than the Japan Trench and filled by a thick turbiditic series. Its anomalous depth is explained by the westward retreat of the edge of the northwestward moving Philippine Sea plate. On the contrary to what happens in the Japan Trench, horst and graben structures of the Pacific plate obliquely enters the Izu-Bonin Trench, suggesting that the actual boundary between these two trenches is located to the north of the triple junction. The inner wall of the Izu-Bonin Trench is characterized in the triple junction area by a series of slope basins whose occurrence is related to the dynamics of this area. The northernmost basin is overthrust by the edge of the fore-arc area of the Northeast Japan plate. The plate boundary is hardly discernible further east, which makes it impossible to locate precisely the triple junction itself. These features suggest that large intra-plate deformation occurs there due to the interaction of the plates involved in the triple junction and the weak mechanical strength of the wedge-shaped margin of the overriding plates.     

Recycled crustal melt injection into lithospheric mantle: implication from cumulative composite and pyroxenite xenoliths

A rare composite xenolith and abundant cumulative pyroxenites obtained from the Mesozoic Fangcheng basalts on the eastern North China Craton record a complex history of melt percolation and circulation in the subcontinental lithospheric mantle. The composite xenolith has a dunite core and an olivine clinopyroxenite rim. The dunite is of cumulative origin and has a granular recrystallized texture and extremely low Mg# [100 Mg/(Mg + Fe) = 81–82] contents in olivines. The olivine clinopyroxenite contains larger clinopyroxene and/or orthopyroxene with a few fine-grained olivine and tiny phlogopite, feldspar, and/or carbonate minerals interstitial to clinopyroxene. The clinopyroxene has low Mg# (83–85). Compositional similarity between dunitic olivine and pyroxenitic one indicates a sequential crystallization of dunite and pyroxenite from a precursor melt. Pyroxenite xenoliths include olivine websterites and clinopyroxenites, both are of cumulative origin. Estimation of the melt from major oxides in olivines and REE concentrations in clinopyroxenes in these composite and pyroxenite xenoliths suggests a derivation from subducted crustal materials, consistent with the highly enriched EMII-like Sr and Nd isotopic ratios observed in the pyroxenites. Occurrence of phlogopite, feldspar and carbonate minerals in some xenoliths requires the melt rich in alkalis (K, Na), silica and volatiles (water and CO₂) at the latest stage as well, similar to highly silicic and potassic melts. Thus, the occurrence of these composite and pyroxenite xenoliths provides an evidence for voluminous injection of recycled crustal melts into the lithosphere beneath the southeastern North China Craton at the Late Mesozoic, a reason for the rapid lithospheric enrichment in both elemental and isotopic compositions.     

A study of intermittent turbulence with cases-99 tower measurments

Characteristics of intermittent turbulence events in the stably stratified nocturnal boundary layer are investigated with data collected in the CASES-99 tower array of 300-m radius. The array consists of a central 60-m tower with eddy covariance measurements at eight levels and six satellite towers with eddy covariance measurements at 5 m. A significant increase in the magnitude of vertical wind velocity () and spectral information are used to define the onset of an intermittent turbulence event. Normally, only a subset of 5 m-levels in the tower network experience an intermittent turbulence event concurrent with one at the 5 m-level on the main tower. This behaviour reveals the small horizontal extent of most events. Intermittent turbulence events at the main tower 5-m level are normally confined to a layer much thinner than the 60-m tower height. The turbulent kinetic energy budget is evaluated for intermittent turbulence events observed at the 5-m level on the main tower. Generally, the onset of an intermittent turbulence event is not closely related to the reduction of the gradient Richardson number below 0.25, the critical Richardson number of turbulence generation for linear instability. Possible explanations including the influence of advected turbulence patches are discussed.     

Timescales of magma differentiation from basalt to andesite beneath Hekla Volcano, Iceland: Constraints from U-series disequilibria in lavas from the last quarter-millennium flows

Measurements of ²³⁸U-²³⁰Th-²²⁶Ra disequilibria, Sr-Nd-Pb-Hf isotopes and major-trace elements have been conducted for lavas erupted in the last quarter-millennium at Hekla volcano, Iceland. The volcanic rocks range from basalt to dacite. Most of the lavas (excluding dacitic samples) display limited compositional variations in radiogenic Sr-Nd-Pb-Hf isotopes (⁸⁷Sr/⁸⁶Sr = 0.70319-0.70322; ¹⁴³Nd/¹⁴⁴Nd = 0.51302-0.51305; ²⁰⁶Pb/²⁰⁴Pb = 19.04-19.06; ²⁰⁷Pb/²⁰⁴Pb = 15.53-15.54; ²⁰⁸Pb/²⁰⁴Pb = 38.61-38.65; ¹⁷⁶Hf/¹⁷⁷Hf = 0.28311-0.28312). All the samples possess (²³⁰Th/²³⁸U) disequilibrium with ²³⁰Th excesses, and they show systematic variations in (²³⁰Th/²³²Th) and (²³⁸U/²³²Th) ratios. The highest ²²⁶Ra excesses occur in the basalt and most differentiated andesite lavas, while some basaltic-andesite lavas have (²²⁶Ra/²³⁰Th) ratio that are close to equilibrium. The ²³⁸U-²³⁰Th-²²⁶Ra disequilibria variations cannot be produced by simple closed-system fractional crystallization with radioactive decay of ²³⁰Th and ²²⁶Ra in a magma chamber. A closed-system fractional crystallization model and assimilation and fractional crystallization (AFC) model indicate that the least differentiated basaltic andesites were derived from basalt by fractional crystallization with a differentiation age of ∼24 ± 11 kyr, whereas the andesites were formed by assimilation of crustal material and fractionation of the basaltic-andesites within 2 kyr. Apatite is inferred to play a key role in fractionating the parent-daughter nuclides in ²³⁰Th-²³⁸U and ²²⁶Ra-²³⁰Th to make the observed variations. Our proposed model is that several batches of basaltic-andesite magmas that formed by fractional crystallization of a basaltic melt from a deeper reservoir, were periodically injected into the shallow crust to form individual magma pockets, and subsequently modifying the original magma compositions via simultaneous assimilation and fractional crystallization. The assimilant is the dacitic melt, which formed by partial melting of the crust.     

Accurate ocean tide modeling in southeast Alaska and large tidal dissipation around Glacier Bay

An accurate prediction of ocean tides in southeast Alaska is developed using a regional, barotropic ocean model with a finite difference scheme. The model skill is verified by the observational tidal harmonics in southeast Alaska including Glacier Bay. The result is particularly improved in Glacier Bay compared to the previous model described by Foreman et al. (2000). The model bathymetry dominates the model skill. We re-estimate tidal energy dissipation in the Alaska Panhandle and suggest a value for tidal energy dissipation of 3.4 GW associated with the M₂ constituent which is 1.5 times the estimation of Foreman et al. (2000). A large portion of the M₂ energy budget entering through Chatham Strait is dissipated in the vicinity of Glacier Bay. Moreover, it is shown that the developed model has the potential to correct the ocean tide loading effect in geodetic data more efficiently than the model of Foreman et al. (2000), especially around Glacier Bay.     

A diagnostic study of future evaporation changes projected in CMIP5 climate models

It has been pointed out that climatological-mean precipitation-evaporation difference (P–E) should increase under global warming mainly through the increasing saturation level of moisture. This study focuses on evaporation changes under global warming and their dependency on the direct warming effect, on the basis of future projections from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Over most of the tropical, subtropical and midlatitude regions, the direct contribution from surface temperature increase is found to dominate the projected increase in evaporation. This contribution is nevertheless offset partially, especially over the oceans, by contributions from weakening surface winds and increasing near-surface relative humidity. Greater warming of surface air than of the sea surface also acts to reduce surface evaporation, by reducing both the exchange coefficient and humidity contrast at the surface. Though generally of secondary importance, this contribution is the dominant factor over the subpolar oceans. Over the polar oceans, the effect of sea-ice retreat dominantly contributes to the evaporation increase in winter, whereas the reduced exchange coefficient and surface humidity contrast coupled with the sea-ice retreat account for most of the response during summertime. Over the continents, changes in the surface exchange coefficient, reflecting changes in soil moisture and vegetation among other factors, are important to modulate the direct effects of the warming and the generally reduced surface air relative humidity.     

A practical method to transform frequency dependent impedance to time domain

In order to perform time history earthquake response analyses with consideration to both the dynamic soil–structure interaction and the non‐linear behaviour of the structure, it is important to transform the soil impedance in the frequency domain to the impulse response in the time domain. In this paper, a new transform method with high practicality is proposed. First, the formulation of the proposed transform method is described. Next, the validity of the method is examined using an example problem whose impulse response is analytically obtained. Then, the impedance of the rigid foundation on 2‐layered soil is transformed to the time domain, and the characteristics of the impulse response are investigated. Finally, time history earthquake response analyses of a structure on the soil using the obtained impulse response are carried out. The validity and the efficiency of the proposed method are confirmed through these investigations. Copyright © 2005 John Wiley & Sons, Ltd.     

Practical causal hysteretic damping

A number of experiments indicate that the internal damping corresponding to the energy dissipation of many materials is essentially frequency independent. Accordingly, an analysis model that can express such characteristics (called a hysteretic damping model) in the time domain is needed. Although a great number of investigations into this subject have been carried out, there are a few practical methods. In this paper, a simple hysteretic damping model which satisfies the causality condition is presented using an extension of the complex stiffness transfer method that the author has proposed. Compared with the energy proportional damping model and the Biot model, the applicability and the efficiency of this model to time history response analyses were confirmed well by example problems. Copyright © 2006 John Wiley & Sons, Ltd.