Stability of toroidal magnetic fields in the radiation zone of a star

The stability of a toroidal magnetic field in the rotating radiation zone of a star is analyzed to estimate the maximum possible magnitude of relic fields. Equations for small perturbations are obtained taking into account the finite diffusivity and the stabilizing effect of the subadiabatic stratification. The numerical solution of the eigenvalue problem indicates that the threshold field strength for the onset of instability in the radiation zone of the Sun is about 600 G. This figure sets an upper bound for the strength of the relic field. The assumption that magnetic instabilities are present in the solar radiation zone disagrees with the observed abundance of lithium. Our analysis of joint stability of toroidal field and nonuniform rotation shows that two-dimensional MHD solutions for the solar tachocline are stable against three-dimensional perturbations.     

Rotational and magnetic corrections to the g-mode frequencies in a one-dimensional MHD model

The central magnetic field and rotation of the solar radiative zone are responsible for corrections to the g-mode frequencies. Magnetogravitational spectra are calculated analytically in a simple one-dimensional MHD model that goes beyond the WKB approximation and avoid any cusp resonances that trap the wave within the radiative zone in the presence of a weak magnetic background. The calculations are compared with spacecraft observations of the 1% frequency shifts for candidate g-modes found in the SOHO GOLF experiment. The magnetic correction is the main contribution for a strong magnetic field satisfying the approximation used. It is shown that a constant magnetic field of 700 kG in the radiative zone provides the required frequency shift for the n = ?10 g-mode. The rotational correction, which is due to the Coriolis force in the one-dimensional model used, is much less than a percent (α_Ω ≤ 0.003).     

关于频率电磁测深几个问题的探讨(五)——频率电磁测深中电磁场波型及意义

引出水平分层大地面上水平电偶极源及垂直磁偶极源的电磁场分量表达式,对式中核函数进行了变换,分析所含的波型及其与场区的关系,指出了TE型场、TM型场和TEM型场穿透地层的能力。根据波型与地层作用特点,优化电磁场分量,以提高频率电磁测深的探测效果。     

Properties of the distribution of azimuthal magnetic field in a plasma flow during laboratory simulations of astrophysical jets in a plasma-focus installation

The results of laboratory simulations of astrophysical jets are presented. Plasma flows generated in the PF-3 plasma-focus installation of the NRC “Kurchatov Institute” and propagating to distances substantially exceeding their transverse dimensions are studied. It is shown usingmagnetic probes that the plasma flow propagates with a frozen-in magnetic field. The resulting radial distribution of the azimuthal magnetic field corresponds well to the distribution created by a longitudinal current of ~10 kA flowing in a region with a radius of 1–2 cm near the axis. Structures associated with return currents are observed at the periphery of the flow. The magnetic field decays rapidly as the flow propagates along the axis. Nevertheless, the leading lobe of the plasma flow is preserved to substantial distances in a neon discharge, possibly due to radiative cooling of the plasma.     

Intercombinational line profiles in the UV spectra of T Tauri stars and analysis of the accretion zone

We have analyzed for the first time profiles of the SiIII 1892 Å and CIII 1909 Å intercombinational lines in HST spectra of the stars RY Tau and RU Lup. The widths of these optically thin lines exceeded 400 km/s, ruling out formation in the stellar chromosphere. Since the intensity of the Si line exceeds that of the C line, it is unlikely that a large fraction of the observed line flux is formed in a stellar wind. The observed profiles can be reproduced in the framework of an accretion shock model if the velocity field in the accretion zone is appreciably nonaxisymmetric. In this case, the line profiles should display periodic variations, which can be used to determine the accretion zone geometry and the topology of the magnetic field near the stellar surface; corresponding formulas are presented. In addition, periodic variations of the 0.3–0.7 keV X-ray flux should be observed.     

Solar forcing of the terrestrial atmosphere

The Sun provides the main energy input to the terrestrial atmosphere, and yet the impact of solar variability on long-term changes remains a controversial issue. Direct radiative forcing is the most studied mechanism. Other much weaker mechanisms, however, can have a significant leverage, but the underlying physics is often poorly known. We review the main mechanisms by which solar variability may impact the terrestrial atmosphere on time scales ranging from days to millennia. This includes radiative forcing, but also the effect of interplanetary perturbations and energetic particle fluxes, all of which are eventually driven by the solar magnetic field.     

A model for the constant-head pumping test conducted in vertically fractured media

An analytical model for the constant-head pumping test is developed for a partially penetrating well that has a finite thickness skin, and intersects a single vertical fracture. In the model, the fracture is fully confined and flow occurs only in the fracture. The model is developed using Laplace transform and finite Fourier transform methods. The model is to be used for analysing well test data from vertically fractured media and for verifying numerical models. Dimensionless curves are used to study the effects of a finite thickness skin and a partially penetrating wellbore. In the presence of a finite thickness skin, or a partially penetrating wellbore, a typical flow response for the constant-head pumping test has three distinct periods of flow corresponding to small-, intermediate- and large-time. Small- and large-time approximations are presented for the model. For tests where the wellbore is fully penetrating, or the partial penetration ratio is known, these approximations can be used to analyse field data.     

Full 3D MHD calculations of accretion flow structure in magnetic cataclysmic variables with strong,complex magnetic fields

We have performed three-dimensional magnetohydrodynamical calculations of stream accretion in cataclysmic variable stars for which the white dwarf primary possesses a strong, complex magnetic field. These calculations were motivated by observations of polars: cataclysmic variables containing white dwarfs with magnetic fields sufficiently strong to prevent the formation of an accretion disk. In this case, an accretion stream flows from the L1 point and impacts directly onto one or more spots on the surface of the white dwarf. Observations indicate that the white dwarfs in some binaries possess complex (non-dipolar) magnetic fields. We performed simulations of ten polars, with the only variable being the azimuthal angle of the secondary with respect to the white dwarf. These calculations are also applicable to asynchronous polars, where the spin period of the white dwarf differs by a few percent from the orbital period. Our results are equivalent to calculating the structure of one asynchronous polar at ten different spin-orbit beat phases. Our models have an aligned dipolar plus quadrupolar magnetic field centered on the whitedwarf primary. We find that, with a sufficiently strong quadrupolar component, an accretion spot arises near the magnetic equator for slightly less than half our simulations, while a polar accretion zone is active for most of the remaining simulations. For two configurations, accretion at a dominant polar region and in an equatorial zone occurs simultaneously. Most polar studies assume that the magnetic field is dipolar, especially for single-pole accretors. We demonstrate that, with the orbital parameters and magnetic-field strengths typical of polars, the accretion flow patterns can vary widely in the case of a complex magnetic field. This may make it difficult formany polars to determine observationally whether the field is pure dipolar or is more complex, but there shoulid be indications for some systems. In particular, a complex magnetic field should be suspected if there is an accretion zone near the white dwarf’s equator (assumed to be in the orbital plane) or if there are two or more accretion regions that cannot be fitted by dipolar magnetic field. Magnetic-field constraints are expected to be substantially stronger for asynchronous polars, with clearer signs of complex field geometry due to changes in the accretion flow structure as a function of azimuthal angle. These indications become clearer in asynchronous polars because each azimuthal angle corresponds to a different spin-orbit beat phase.     

不同重力场下静力触探试验离散元数值分析

采用离散元商业软件PFC2D对7种重力场下静力触探试验进行了对比分析试验,以研究高、低重力场下静力触探试验的异同点。数值试验结果表明：不同重力场下土体受静力触探贯入主要影响区域不同,低重力场下主要是上部土体受到影响,高重力场下主要是下部土体受到影响;归一化贯入阻力的最大值和稳定值、归一化球应力和偏应力的最大值均与重力加速度的倒数呈线性关系,均随重力加速度倒数的增加而增大;土体经历了明显的加载、卸载过程,重力加速度越小,加载、卸载现象越明显,同一重力加速度下上部土体加载、卸载程度大于下部土体。     

射流泵流场的PIV测量

利用粒子图像速度场仪(PIV)对射流泵渐缩锥形入口、等径直管内有限空间水射流进行测量。该射流泵喉管面积与喷嘴面积之比为4.75,基于射流泵喷嘴直径D和喷嘴出口流速计算的雷诺数为3.68×105。通过流量比在0.20～0.80之间的变化来研究流量比对流场的影响。获得了射流泵对称面流场的速度矢量、轴心速度分布和轴向速度等值线图。结果表明:当射流泵的面积比确定后,射流泵内有限空间射流结构只与射流泵的流量比有关,流量比愈小,其轴心速度衰减得愈快,高速射流区愈短。测量结果为射流泵理论研究和优化设计提供可靠依据。     

碳酸盐岩内断裂带结构及其与油气成藏

以野外观察描述为手段,系统研究了碳酸盐岩断裂变形机制的影响因素及断裂带结构演化过程,剖析了碳酸盐岩地层中断裂带结构与流体运移的关系。研究表明,影响碳酸盐岩内断裂变形机制的因素包括岩性、孔隙度、变形深度、温度、胶结作用、先存裂缝等,控制断裂带结构形成的因素包括滑动位移和破裂模式等。低孔隙度碳酸盐岩以裂缝发育为主,高孔隙度碳酸盐岩变形早期产生变形带,带内裂缝联接逐渐发育成断层带。随着埋藏深度的增加,断裂带结构不同：埋藏深度小于3 km,断层核主要发育无内聚力的断层角砾岩和断层泥;埋藏深度大于3 km,断层核普遍发育有内聚力的断层角砾岩和碎裂岩,破碎带发育多种成因的裂缝。随着位移的增加,破裂模式从早期的破裂作用变为后期的碎裂作用,最终形成碎裂流。断裂带演化是一个四维过程,断层核和破碎带发育情况直接影响断层对油气的运移和封闭的作用。断裂变形机制、断裂带内部结构以及与流体运移关系的研究,都可为封闭性提供重要的理论依据。     

断层破碎带变形破坏失稳过程模拟

为了研究断层破碎带在采动影响下的变形破坏失稳过程,采用电镜、薄片分析及蠕变力学试验方法分别对范各庄矿F₀断层物质的原样和3种含水量、5组样品的复制样进行了分析研究。在力学实验的基础上,借助数值仿真软件FLAC^3D完成了对断层破碎带变形破坏失稳过程的模拟计算。模拟结果较好地刻画出了承压水对上覆岩层及断层带的影响;开采活动对底板岩层的影响;断层破碎带受开采影响的大小和范围;承压水影响的时间效应和特定水头作用下承压水沿断层导升的高度。     

松散承压含水层水文地质参数分区及水流场数值模拟

为了揭示华北型煤田松散承压含水层水文地质参数及其对地下水流数值模拟的意义,以安徽淮北煤田宿南矿区祁东煤矿松散层承压第四含水层(简称“四含”)为研究示范,对多个影响因素综合分析,采用层次分析-模糊综合评价法,对研究区四含水文地质参数进行分区,合理确定各分区的水文地质参数,并用于采煤情景下地下水流动态数值模拟。数值模拟结果表明：祁东煤矿2008-2012年浅部煤层开采过程中四含地下水流场没有明显改变,但南北分区水头差逐年增大,地下水流向始终从南向北,而且在井田北部水力梯度逐渐减小,在井田南部水力梯度逐渐增大,南北区以倾角为60°~70°、断距为10~320 m的魏庙断层为分界线,基岩中大倾角与大落差断层的出现是引起上覆松散承压含水层水力梯度异常的根本原因。     

基于过程模拟的李官堡水源地地下水污染预警

基于过程模拟的地下水污染预警是基于包气带和饱和带过程模拟的耦合实现的,且融合了涵盖整个地下水系统的四个预警指标。选取浑河冲洪积扇的李官堡水源地为例,基于Hydrus-1D和Visual Modflow分别进行包气带过程模拟和饱和带过程模拟,针对潜层和承压层的复杂程度分别制定了各自的预警临界值;经分析可知,随着时间的推移,地表污染物持续进入到地下水中,污染晕范围不断扩大,地下水中最大浓度值和水源井浓度值也持续增加,且承压含水层的预警级别要高于潜水含水层;同时基于潜层和承压层中污染物最大浓度值与时间分别近似呈线性关系和指数关系可进行长时间尺度的预警;并提出了零级预警区和一级预警区管理措施以监测和预防为主,二级预警区、三级预警区和四级预警区管理措施以控制和监测为主。     

European tectonic features observed by Magsat

Regional three-dimensional magnetic models have been developed to characterize the principal European long-wavelength magnetic anomalies represented on the improved magnetic anomaly map of Europe. The magnetic models were constrained by regional variations in geology and geophysical parameters (e.g., geologic boundaries, crustal thickness, heat flow). Because only limited measurements of magnetization are available on lower crustal and uppermost mantle rock samples, our results are useful in constraining and understanding the overall magnetization of these regions. Illustrations of these include: (1) geologic provinces across the Tornquist-Teisseyre tectonic zone; (2) regions of thin crust and high mantle heat flow in south-central Europe; (3) the Kursk-Voronezh magnetic anomaly; and (4) the Ladoga-Gulf of Bothnia zone. The region of the Tornquist-Teisseyre tectonic zone, that marks the boundary between the Fennoscandian-Baltic Shield and metastable Europe, is a major magnetic discontinuity. In south-central Europe, the regional magnetic variations appear to be directly related to variations in the lower crustal thickness and possibly also to heat flow. In addition, the famous Kursk (Ukraine) iron-ore deposit produces a prominent bullseye anomaly at satellite altitude. The Kiruna anomaly is modelled as having a large, deep body as its source. The high P-wave velocity, basal crustal layers encountered in rift (e.g., the Tornquist-Teisseyre tectonic zone itself) and continental arc (e.g., the Ladoga-Gulf of Bothnia zone) settings of Europe appear to be nearly non-magnetic.     

Confined groundwater near the rockhead in igneous rocks in the Mid-Levels area, Hong Kong, China

It is customary in Hong Kong to assume that the hydraulic conductivity of weathered igneous rocks decreases with depth or as the rock mass becomes less weathered. Such a hydraulic conductivity pattern can only lead to an unconfined aquifer. This paper presents a case study in the regions in and around the Mid-Levels area in Hong Kong regarding a possible relatively high hydraulic conductivity (K) zone and confined groundwater along the rockhead. The Mid-Levels area is located at the lower part of the north-facing slopes of Victoria Peak on Hong Kong Island and is prone to landslides. Although this site has a long history of geotechnical studies because of extensive urban development along the coast and public concern on slope stability, hydrogeology of the site remains poorly understood. This paper reexamined the hydraulic conductivity data in 7 boreholes conducted in the 1970s and found that 4 of them indicate an increase in K at the rockhead. Groundwater conditions revealed by tunnel construction at the coast suggest that K close to the rockhead is about 10 times greater than above rockhead. A careful analysis of storm response of a piezometer group with tips in different depths indicates that there was an upward flow from the bedrock to the colluvium. A field study of two overflow standpipes conducted by the authors showed that the water level can be 0.64 and 3.73 m above the ground surface, which illustrates that the deep groundwater is significantly artesian. A search of the archived site investigation reports from the government and private companies has led to an identification of about 24 sites with overflow boreholes, which suggests that overflow phenomenon is quite common in the study area. The paper then concludes that in the study area there is a relatively high K zone along the rockhead and the groundwater in the zone is confined. It is recommended that geotechnical engineers should carry out a more careful field study on an overflow borehole because such a borehole indicates a confined groundwater condition important for slope stability study and foundation design.     

Non-Darcian flow to a partially penetrating well in a confined aquifer with a finite-thickness skin

Non-Darcian flow to a partially penetrating well in a confined aquifer with a finite-thickness skin was investigated. The Izbash equation is used to describe the non-Darcian flow in the horizontal direction, and the vertical flow is described as Darcian. The solution for the newly developed non-Darcian flow model can be obtained by applying the linearization procedure in conjunction with the Laplace transform and the finite Fourier cosine transform. The flow model combines the effects of the non-Darcian flow, partial penetration of the well, and the finite thickness of the well skin. The results show that the depression cone spread is larger for the Darcian flow than for the non-Darcian flow. The drawdowns within the skin zone for a fully penetrating well are smaller than those for the partially penetrating well. The skin type and skin thickness have great impact on the drawdown in the skin zone, while they have little influence on drawdown in the formation zone. The sensitivity analysis indicates that the drawdown in the formation zone is sensitive to the power index (n), the length of well screen (w), the apparent radial hydraulic conductivity of the formation zone (K _r2), and the specific storage of the formation zone (S _s2) at early times, and it is very sensitive to the parameters n, w and K _r2 at late times, especially to n, while it is not sensitive to the skin thickness (r _s).     

中国及相邻海域地温场分布规律的研究

引言地温场是地球物理场的组成部份,地温场信息从一个侧面反映了地球的物理特性。近年来,不少学者从不同方面,不同程度地对我国陆地和海域,以及东部邻区的地温场性质和地质意义进行过研究,已积累了一些地温场资料。为了对中国及相邻海域的地温场分布特征有一总体的认识,我们试图对目前所获得的各种地热信息进行综合分析。为了便于讨论,参考上述发表的国内外文献,收集了某些煤田、油气田和部份省市的地温资     

工作面无线电波透视实测场强成像分析及应用

为提高坑透数据解释效果,通过对工作面无线电波透视磁场强度的理论分析与公式推导,表明在正常煤层范围,场强值的变化主要受观测点几何位置控制;而在观测点场强路径穿过地质异常区时,主要影响因素为地质异常区内路径长及电磁波能量吸收系数值。将场强值H与观测点路径长R的乘积命名为M,则正常煤层段M可视为常量,而在地质异常存在范围,M值显著降低。将工作面划分为若干小单元,进行M值层析成像反演,可求取工作面各单元M值,再除以工作面平均宽度,得到各单元格场强值,从而获得工作面实测场强成像图。张集矿探测表明,该方法较好地反映了工作面内地质异常区的平面分布情况,回采验证探测结果可靠。作为新的解释手段,正在实际坑透探测中广泛应用。     

Dissipation of diamagnetic currents and plasma heating in coronal magnetic loops

SOHO and TRACE data have shown that the coronal plasma is heated most actively near sunspots, in magnetic loops that issue from the penumbral region. The source of heating is nonuniform in height, and its power is maximum near the footpoints of the magnetic loops. The heating process is typically accompanied by the injection of dense chromospheric plasma into the coronal parts of the magnetic loops. It is important that the radiative losses cannot be compensated for via electron thermal conduction in the loops, which have temperatures of 1.0–1.5 MK; therefore, some heating source must operate throughout the entire length of the loop, balancing radiative losses and maintaining a quasi-steady state of the loop over at least several hours. As observations show, the plasma density inside the loops exceeds the density of the ambient plasma by more than an order of magnitude. It is supposed that the enhanced plasma density inside the loops results from the development of the ballooning mode of a flute-type instability in the sunspot penumbra, where the plasma of the inner sunspot region, with β _i ? 1, comes into contact with the dense chromospheric plasma, which has β _e ? β _i (β is the gas-to-magnetic pressure ratio). As the chromospheric plasma penetrates into the potential field of the sunspot, the generated diamagnetic currents balance the excess gas pressure. These currents efficiently decay due to the Cowling conductivity. Even if neutrals are few in number in the plasma (accounting for less than 10^?5 of the total mass density), this conductivity ensures a heating rate that exceeds the rate of the normal Joule dissipation of diamagnetic currents by 7–8 orders of magnitude. Helium is an important factor in the context of plasma heating in magnetic loops. Its relatively high ionization potential, while not forbidding dielectronic recombination, ensures a sufficiently high number of neutrals in the coronal plasma and maintains a high heating rate due to the Cowling conductivity, even at coronal temperatures. The heating results from the “burning-out” of the nonpotential component of the magnetic field of the coronal magnetic loops. This mechanism provides the necessary heating rate for the plasma inside the loops if the loops are thin enough (with thickness of the order of 10⁵–10⁶ cm). This may imply that the observed (1–5) × 10⁸-cm-thick loops consist of numerous hot, thin threads. For magnetic loops in hydrostatic equilibrium, the calculated heating function exponentially decreases with height on characteristic scales a factor of 1.8 smaller than the total-pressure scale height, since the scale heights for the total pressure and for the ⁴He partial pressure are different. The heating rate is proportional to the square of the plasma pressure in the loop, in agreement with observational data.